Range of Purposes Behind Codes

Codes of ethics in the professions serve numerous purposes. At the level of the profession itself, codes document the standards according to which the profession can be held accountable for the conduct of its members. Further, because society relegates control for many of the professions to the professional organizations themselves, the professions have developed codes to provide the basis of self-regulation (Soskolne 1989). At the level of the individual professional, codes can provide a practical guide to members of the profession who might be experiencing a moral or ethical dilemma concerning their professional conduct in a particular circumstance. Where a professional finds himself or herself in a state of moral or ethical tension, it is self-evident that codes can be helpful in providing counsel.

The existence of a code provides the basis of a profession’s ethics programme of activity designed to instil ethical standards among its membership (Gellermann, Frankel and Ladenson 1990; Hall 1993). Revisions to the code can be considered through grass-roots individual membership input at organizational meetings, workshops and conferences. This ongoing discussion of issues and concerns constitutes a review process ensuring that any code remains sensitive to changing social values. Professions depending for their sustenance on public support thereby improve their likelihood of remaining publicly accountable and relevant (Glick and Shamoo 1993).

Codes could assist professionals being charged with malpractice and perhaps even in litigation. Demonstrated adherence to one’s professional code would likely be deemed indicative of adherence to standards of practice consistent with professional norms. If such practice were to have resulted in harm, the code-abiding individual professional would be less likely to be found guilty of having committed a wrong. However, based on the principle of trust (Pellegrino, Veatch and Langan 1991), the public has the expectation that the best possible professional judgement will be exercised in the public interest. Where the physician-patient relationship is concerned, the patient has the right under the principle of trust to expect that his or her interests will be best served. However, an ethical tension arises when the public good is potentially harmed in circumstances where the individual patient’s best interests are being served. In such circumstances, it is the public good that will usually need to take precedence over that of the individual. Regardless, codes provide no substitute for legal liability dimensions of conduct for which government has enacted laws to protect the public interest (Cohen 1982).

Weight and Intent of Codes

Codes do have associated with them the notion of statutory force, implying the ability for their enforcement through the administering of some type of disciplinary action. Indeed, the notions of accountability and self-regulation referred to above have associated with them some sense of control (minimally, peer pressure; maximally, the removal of licence to practice) that can be exercised over the members of the profession by the professional organization itself. Because of this, some professional organizations have preferred to avoid these connotations associated with codes and opt rather for “guidelines”. The latter emphasize guidance with fewer implications for enforcement associated with them. Other groups have preferred to avoid all connotations associated with codes or guidelines; instead, they have preferred to develop “declarations on ethics” for their specific organizations (Jowell 1986). Throughout this chapter the term code will imply “guidelines”.

It should be apparent that codes (and also guidelines) do not carry the force of law. In essence, codes and guidelines are intended to provide guidance for professionals, collectively and individually, in their relationships with their clients (including patients and research subjects), with their colleagues and co-workers (including their students), and with the public (including stakeholder groups). In addition, codes require that the quality of professional work and hence the stature of the profession itself is advanced. In general, codes associated with the physician-patient relationship will require that the patient’s interests take precedence over any other interests; the physician is placed firmly in the position of “patient advocate”. One exception to this would arise in the context of infectious diseases, where the patient’s rights may have to be considered second to the public welfare. In contrast, however, it can generally be stated that codes associated with scientific research will require that the public interest take precedence over any individual or other interests. One exception would be where a researcher discovers child abuse in a research subject; here the researcher would have the obligation to report this to the child welfare authorities.

Code Development, Review and Revision

The process by which codes are developed has consequences for their application. By including members of the profession and students of the profession in code development, as well as in code review and revision, ownership of the resultant document by a greater number of individuals is believed more likely. With broad-based ownership, increased compliance by a greater number is believed more assured.

Content and Structure of Codes

The content of a code should be user friendly to maximize its utility. Codes can be of varying length. Some are brief, while some are substantial. The more substantial that a code is, the more specific it is possible for it to be. Codes can be made to be user friendly by virtue of their structure and content. For example, a summary set of the principles upon which the code is based could be presented first, followed by expanded aspirational or prescriptive statements, which constitute the code itself. These can be followed by a commentary that explicates each statement in turn, perhaps noting special circumstances in the form of case studies that might serve as useful examples. The principles and their interpretation(s), however, are highly dependent on the values recognized as inherent to the pursuits of a profession. While these values may be universal, interpretations as well as practices at the local and regional levels may differ. Thus, while a statement of the profession’s core values can provide an anchor for its statements on ethics and should appear as a preface to the guidelines (Gellermann, Frankel and Ladenson 1990), sensitivity to regional differences can be demonstrated through the more detailed commentary and case study materials.

The commentary should incorporate, or could be followed by or complemented with, case study materials that derive from real-life instances of ethical dilemmas or tensions. The case study materials could be ethically analysed in either sanitized (i.e., anonymous) form, or can be made to reflect the parties involved (of course, only with the approval of the parties for their names to be included) (for example, Soskolne 1991). The objective behind case studies is not to seek retribution, but rather to provide examples for teaching purposes. Learning is enhanced by real-life situations.

It is from an understanding of the code that it becomes possible for a profession to develop more detailed standards of practice. These address more specific areas of activity associated with professional conduct, including a broad range of activity from interpersonal behaviours to both how research is conducted and how the results of that research are communicated. Standards of practice for the profession could be included in an ethics package; they reflect on each profession’s skill set and therefore add particular considerations that go beyond the statement of its ethics principles.

Scope of Codes

The development of a code by any profession has almost invariably tended to be driven by issues having a direct bearing on that profession. Consequently, codes tend to have a focus narrowly defined by each profession’s own concerns. However, codes also need to take broader social issues into account (Fawcett 1993). In fact, in a recent analysis of several codes, Summers et al. (1995) found that guidelines on specific social issues, such as environmental effects or conflict resolution, are scarcely mentioned at all in existing codes. Because the professions manifest substantial influence, if their codes indeed were to take broader social issues into account, then a great confluence and concurrence of effort would be brought to bear on those areas of human endeavour that currently fall between the cracks in promoting the common social good. Such pressure could help reduce dangers to human welfare, such as militarism and ecological destruction.

Ethics Training

It should be recognized that there exist two schools of thought for ethics training: one is based in a principle-driven approach while the other is case based, also known as casuistry. It is this author’s view, which remains to be tested, that a balance between the two is essential for successful applied ethics training in the professions (Soskolne 1991/92). However, it is well known that ethically analyzed case study material has an invaluable role to play in the education process. Cases provide a context for applying principles.

Because graduate ethics training in the professions is becoming more recognized as an essential place for students to gain awareness of the values, ethical principles and standards of practice of the profession, a model curriculum might ideally be included as part of a code; this will facilitate the training of students intent on entering the profession. The need for this is demonstrated through a recent survey that identified inconsistencies and shortcomings regarding the ethics components in graduate training programmes across the United States (Swazey, Anderson and Seashore 1993).

Recent History of Codes in Selected Professions

In western cultures, the medical profession has had the advantage of discussions about ethics since the time of Socrates (470–399 B.C.), Plato (427–347 B.C.) and Aristotle (384–322 B.C.) (Johnson 1965). Since then, codes have been developed and periodically revised in response to newly recognized issues arising, for example, from human value shifts and, more recently, from technological advances (Declaration of Helsinki 1975; Ad hoc Committee on Medical Ethics 1984; Russel and Westrin 1992). Since the 1960s, other professions have become involved in code development for their own professional organizations. The area of professional codes in fact has become a cottage industry since the 1980s. The American Association for the Advancement of Science (AAAS) has been instrumental in this movement. Under the auspices of its Committee on Scientific Freedom and Responsibility, AAAS initiated a seminal professional ethics project designed to examine the features of and activities associated with codes in the science and engineering professions. The report arising from this effort subsequently generated renewed interest in discussing both code development and revision with many of the professions (Chalk, Frankel and Chafer 1980).

The health/caring professions have long been engaged in discussions of ethical tensions arising from the nature of their professional pursuits. The codes that have evolved have tended, however, to focus on the physician-patient relationship, with concerns about confidentiality being pre-eminent. More recently, perhaps motivated by the growth of applied health research, codes have expanded their attention to include issues pertaining to researcher-patient relationships. Because of population-based research, codes now are addressing concerns for researcher-population relationships. The latter has been aided by the experience of other professions such as sociology, anthropology and statistics.

Many of the caring professions related to the practice of occupational health have been engaged in the discussion of professional ethics. These include: industrial hygiene (Yoder 1982; LaDou 1986); epidemiology (Beauchamp et al. 1991; IEA Workshop on Ethics, Health Policy and Epidemiology 1990; Chemical Manufacturers Association’s Epidemiology Task Group 1991; Council for International Organizations of Medical Sciences 1991, 1993); medicine and numerous of its subspeciality areas, including occupational medicine (Coye 1982; American Occupational Medical Association 1986; International Commission on Occupational Health 1992; Standing Committee of Doctors of the EEC 1980); nursing; toxicology; statistics (International Statistical Institute 1986); psychology; engineering and risk analysis.

In the occupationally specific areas of health services (Guidotti et al. 1989), medicine (Samuels 1992) and health and safety (LaDou 1986), as well as in occupational and environmental health (Rest 1995), relevant portions of professional codes have been abstracted. These presentations serve well the need for furthering discussion in these areas with a view to revising extant codes.

The importance of integrating ethics into the day-to-day activities of professionals is exemplified by these recent texts, which contain appropriately detailed sections on ethics. The professional thereby is reminded that in all aspects of professional practice, all decisions and recommendations have consequences with associated ethical underpinnings.

More recent work on the subject of misconduct in science requires integration into newer texts (Dale 1993; Grandjean and Andersen 1993; Office of the Assistant Secretary for Health 1992; Price 1993; Reed 1989; Sharphorn 1993; Soskolne 1993a; Soskolne 1993b; Soskolne and Macfarlane, 1995; Teich and Frankel 1992). Because one of the fundamental goals of science is the pursuit of truth through objectivity, plagiarism and the fabrication or the falsification of data are counter to the scientific ethic. As the scientific enterprise expands to include more and more scientists, misconduct in science is coming to the attention of the public more frequently. However, it is believed that even in the face of increasing competition and the potential for conflicting interests, the vast majority of those engaged in science do adhere to the principles of truth and objectivity. The frequency of misconduct does, however, remain difficult to assess (Goldberg and Greenberg 1993; Greenberg and Martell 1992; Frankel 1992).

The potential harm to particular scientific efforts as a result of misconduct is one concern. Another concern is the loss of faith by the public in scientists, with consequent reductions in support for the scientific enterprise. The latter has such potentially dire consequences for both science and society that all scientists, and especially students of science, need to be trained in the scientific ethic and reminded of these principles from time to time.

Several case studies serve to demonstrate misconduct (Broad and Wade 1982; Office of Research Integrity 1993; Price 1993; Needleman, Geiger and Frank 1985; Soskolne and Macfarlane, 1995; Swazey, Anderson and Seashore 1993; Soskolne 1991). The determinants of ethical dilemmas are numerous, but one survey among risk analysts in New Jersey (Goldberg and Greenberg 1993) suggests that the two most important causes are “on the job pressure” and “pressure caused by economic implications of result”. The authors of this study noted that possible causes of misconduct include “conflicts of interest, competition with unregulated and unscrupulous competitors, and general lack of individual or societal ethics”. While some codes do address the need for honesty and objectivity in science, the seriousness of current pressures to perform in the presence of apparently declining awareness of societal ethics would dictate that training at all levels include the subject of ethics, values and philosophy. Indeed, the United States Public Health Service requires that universities seeking to obtain research grant support have procedures in place for dealing with and reporting misconduct in science (Reed 1989). Furthermore, university training programmes in public health disciplines must include ethics teaching to qualify for federal funding (Office of the Assistant Secretary for Health 1992).

Normative Nature of Codes

Codes of professional conduct tend to be a narrative description of an assemblage of normative practices. These practices pertain to the moral and ethical standards of a group, be it a professional organization, association or society, having a common skill set in the service of people.

The basis of respective codes has been the so-called Golden Rule, which prescribes that one should do to others what one would have others do to oneself, do one’s level best, and call to the attention of others any act of misconduct.

Approaches to Developing Codes

Most professional organizations have produced codes through the top-down approach, where the elected officials of the profession have undertaken the task. However, as noted earlier (see “Code development, review and revision”), the bottom-up approach is more likely to result in compliance with codes, because grass-roots participation in the process results in a feeling of ownership of the outcome and hence a greater likelihood of compliance. The view that the power brokers of the profession have major influence over the specification of what constitutes appropriate professional conduct could detract from the credibility associated with any resultant code. The more that the “final” code is reflective of community norms, the greater the likelihood that it will be adhered to.

Codes developed by international organizations do have the power of influencing regional groups of people to consider the concerns and statements included in international codes. In this way, regions that have not given attention to the development of codes might be stimulated to do so. Presumably, provided the intent of international codes is limited to a function of providing stimulus, ongoing interaction could serve to iteratively modify and update international codes so that ultimately the international code could well reflect transnational concerns. Care must be exercised to respect regional cultural norms that are not in conflict with, for example, accepted declarations on human rights. Hence, code makers should be sensitive to cultural differences, and not allow their work to homogenize human behaviour; cultural diversity must rather be encouraged.

Mechanisms for Enforcement

Noted earlier was the fact that codes do imply some degree of self-regulation if the expectation of accountability is to have meaning. This would suggest the existence of procedures for investigating allegations of misconduct (or malpractice) of any type, and for correcting conduct deemed professionally inappropriate (Price 1993; Dale 1993; Grandjean and Andersen 1993). In addition, some remedy might be provided for any harms that might have derived from professional misconduct.

The procedures to be invoked in investigations of allegations of misconduct or malpractice must be pre-specified. The maxim of “innocent until proven guilty” should be evident and be seen to be applied. However, because public confidence rests on professional self-regulation, investigations should be dealt with as efficiently as possible with respect for due process at all times (Sharphorn 1993; Soskolne 1993a, b).

The threat of revoking professional licence to practice is one way that the profession has leverage to maximize among its members adherence to any codes. Many professions have no such leverage; their membership is made up of dues-paying individuals with a wide range of qualifications for which regional legislatures have not required licensure as a requirement of membership in the profession. The loss of the right to practice one’s profession therefore is not applicable in many professions as a penalty for misconduct. The only recourse in such instances is that of peer pressure.

Current Issues of Concern to Occupational Health Professionals

It is not within the scope of this article to develop a comprehensive code, but rather to present the process by which codes are developed. It is the intent in so doing to provide motivation for the ongoing discussion of codes (as a component of a broader-based professional ethics programme) and to alert the reader to current issues about which further discussion is needed for the possible inclusion of such resolved matters into revised codes.

As noted by Guidotti et al. (1989), certain issues had been overlooked in codes that existed at that time. These included the virtue of full access to accurate information, and that the burden of risk should not be taken by the worker in the presence of unproved but sound evidence. The question of accurate information and implied truth has associated with it issues of scientific integrity (as referred to in North America) or of scientific dishonesty (as referred to in Denmark) (Andersen et al. 1992; Grandjean and Andersen 1993). Clearly, the pursuit of truth as the main target of scientific endeavour must be reinforced at every opportunity, including its full integration into codes, case study materials and ethics programmes generally (Hall 1993).

With technological advances, the ability grows to more precisely measure biological parameters. For example, biomarkers is one area that opens up a Pandora’s box of ethical issues and resulting tensions that have yet to be addressed in codes. Several such issues have been identified by Ashford (1986) and by Grandjean (1991). Since existing codes were developed prior to the availability on a commercial scale of this technology, codes would serve the occupational health community better if they were updated to provide some guidance on related concerns. To achieve this, explication of such thorny questions as the workers’ right to work in the face of high-risk susceptibility identified through biomarker assays, requires extensive discussion in workshops and conferences specially convened for the purpose. Case study materials would assist in this effort. So profound are the ramifications of biomarker studies that their implications, as well as those related to other potential high technology breakthroughs, could be best addressed through the profession’s continual review of the code.

Because issues such as biomarkers can be difficult to resolve, it may be appropriate for like professions dealing with similar issues to consolidate their efforts and establish mechanisms for exchanging information to assist in the resolution of difficult and challenging related ethical issues. In particular, the need to address the timing for introducing high technology procedures for which ethical considerations have not yet been established also needs to be recognized and addressed by standing committees on ethics for the respective occupational safety and health professions. Other stakeholder groups probably should be included in such deliberations, including the community representatives themselves on whom such studies would be conducted.

In a researcher’s enthusiasm to implement new technological measures into studies for which the ramifications are not fully understood (in the belief that benefit would result), it should be recognized that greater harm than benefit to the subjects of these studies could, in fact, arise (e.g., job loss today is potentially more harmful than the possibility of premature death at some future date). Hence, great caution must be exercised in advance of the implementation of such technologies. Only after due discussion has been exercised by the professional groups having an interest in the use of such technologies, together with a broad range of stakeholder interest groups, should their implementation be considered.

Another current issue involves the notion of data privacy, which is one that returns to the public arena periodically. In the age of computers, the potential exists for linking records created for one purpose with records created for another purpose. Advocates of data privacy have been concerned that records so created could be potentially damaging to individuals. While individual rights to privacy must take precedence over the research needs of the community, the fact that population-based research is uninterested in the data at the individual level must be brought to the attention of the data privacy advocates. In so doing, it should be easy to demonstrate that the public good is better served by allowing appropriately qualified researchers, trained in data processing and confidentiality, access to individual data for population-based research purposes.

Concern about the extension of principlism applied in the physician-patient setting to that of the community-research situation has been noted above (see “Recent history of codes in selected professions”). Vineis and Soskolne (1993) have found that the established principles of autonomy, beneficence, non-maleficence and distributive justice are not easily applicable at the societal level. For example, available information about the safety of exposures often is too scanty to allow decisional autonomy; beneficence is considered from the societal viewpoint rather than from that of the individual; and equity is frequently violated. Ethics require careful consideration when defining what is acceptable to society; the simple mathematical formulations used for risk-benefit evaluations cannot be applied directly to individuals. Further development and integration of these ideas are necessary.

In conclusion, codes have a fundamental role to play in the professions. They also could play an important role in safeguarding the common good if they took broader social issues into account. They need to be developed with grass-roots and stakeholder input as part of a broad-based programme of ethics supported by each profession. Codes—including the profession’s core values, the commentary associated with a code and case study materials—must be subjected to a process of periodic review and revision. Now, more than ever, codes are needed not only for professional accountability and self-regulation purposes, but also to help practitioners with the moral and ethical challenges faced by constantly advancing technologies that have implications, amongst others, for the rights and duties of all affected individuals and interest groups. A substantial and challenging task lies ahead.

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Lung function may be measured in a number of ways. However, the aim of the measurements has to be clear before the examination, in order to interpret the results correctly. In this article we will discuss lung function examination with special regard to the occupational field. It is important to remember the limitations in different lung function measurements. Acute temporary lung function effects may not be discernible in case of exposure to fibrogenic dust like quartz and asbestos, but chronic effects on lung function after long-term (>20 years) exposure may be. This is due to the fact that chronic effects occur years after the dust is inhaled and deposited in the lungs. On the other hand, acute temporary effects of organic and inorganic dust, as well as mould, welding fumes and motor exhaust, are well suited to study. This is due to the fact that the irritative effect of these dusts will occur after a few hours of exposure. Acute or chronic lung function effects also may be discernible in cases of exposure to concentrations of irritating gases (nitrogen dioxide, aldehydes, acids and acid chlorides) in the vicinity of well documented exposure limit values, especially if the effect is potentiated by particulate air contamination.

Lung function measurements have to be safe for the examined subjects, and the lung function equipment has to be safe for the examiner. A summary of the specific requirements for different kinds of lung function equipment are available (e.g., Quanjer et al. 1993). Of course, the equipment must be calibrated according to independent standards. This may be difficult to achieve, especially when computerized equipment is being used. The result of the lung function test is dependent on both the subject and the examiner. To provide satisfactory results from the examination, technicians have to be well trained, and able to instruct the subject carefully and also encourage the subject to carry out the test properly. The examiner should also have knowledge about the airways and lungs in order to interpret the results from the recordings correctly.

It is recommended that the methods used have a fairly high reproducibility both between and within subjects. Reproducibility may be measured as the coefficient of variation, that is, the standard deviation multiplied by 100 divided by the mean value. Values below 10% in repeated measurements on the same subject are deemed acceptable.

In order to determine if the measured values are pathological or not, they must be compared with prediction equations. Usually the prediction equations for spirometric variables are based on age and height, stratified for sex. Men have on the average higher lung function values than women, of the same age and height. Lung function decreases with age and increases with height. A tall subject will therefore have higher lung volume than a short subject of the same age. The outcome from prediction equations may differ considerably between different reference populations. The variation in age and height in the reference population will also influence the predicted values. This means, for example, that a prediction equation must not be used if age and/or height for the examined subject are outside the ranges for the population that is the basis for the prediction equation.

Smoking will also diminish lung function, and the effect may be potentiated in subjects who are occupationally exposed to irritating agents. Lung function used to be considered as not being pathological if the obtained values are within 80% of the predicted value, derived from a prediction equation.

Measurements

Lung function measurements are carried out to judge the condition of the lungs. Measurements may either concern single or multiple measured lung volumes, or the dynamic properties in the airways and lungs. The latter is usually determined through effort-dependent manoeuvres. The conditions in the lungs may also be examined with regard to their physiological function, that is, diffusion capacity, airway resistance and compliance (see below).

Measurements concerning ventilatory capacity are obtained by spirometry. The breathing manoeuvre is usually performed as a maximal inspiration followed by a maximal expiration, vital capacity (VC, measured in litres). At least three technically satisfactory recordings (i.e., full inspiration and expiration effort and no observed leaks) should be done, and the highest value reported. The volume may be directly measured by a water-sealed or a low-resistive bell, or indirectly measured by pneumotachography (i.e., integration of a flow signal over time). It is important here to note that all measured lung volumes should be expressed in BTPS, that is, body temperature and ambient pressure saturated with water vapour.

Forced expired vital capacity (FVC, in litres) is defined as a VC measurement performed with a maximally forced expiratory effort. Due to the simplicity of the test and the relatively inexpensive equipment, the forced expirogram has become a useful test in the monitoring of lung function. However, this has resulted in many poor recordings, of which the practical value is debatable. In order to carry out satisfactory recordings, the updated guideline for the collection and use of the forced expirogram, published by the American Thoracic Society in 1987, may be useful.

Instantaneous flows may be measured on flow-volume or flow-time curves, while time average flows or times are derived from the spirogram. Associated variables which can be calculated from the forced expirogram are forced expired volume in one second (FEV₁, in litres per second), in percentage of FVC (FEV₁%), peak flow (PEF, l/s), maximal flows at 50% and 75% of forced vital capacity (MEF₅₀ and MEF₂₅, respectively). An illustration of the derivation of FEV₁ from the forced expirogram is outlined in figure 1. In healthy subjects, maximal flow rates at large lung volumes (i.e., at the beginning of expiration) reflect mainly the flow characteristics of the large airways while those at small lung volumes (i.e., the end of expiration) are usually held to reflect the characteristics of the small airways, figure 2. In the latter the flow is laminar, while in the large airways it may be turbulent.

Figure 1. Forced expiratory spirogram showing the derivation of FEV₁ and FVC according to the extrapolation principle.

Figure 2.  Flow-volume curve showing the derivation of peak expiratory flow (PEF), maximal flows at 50% and 75% of forced vital capacity (and , respectively).

PEF may also be measured by a small portable device such as the one developed by Wright in 1959. An advantage with this equipment is that the subject may carry out serial measurements—for example, at the workplace. To get useful recordings, however, it is necessary to instruct the subjects well. Moreover, one should keep in mind that measurements of PEF with, for example, a Wright meter and those measured by conventional spirometry should not be compared due to the different blow techniques.

The spirometric variables VC, FVC and FEV₁ show a reasonable variation between individuals where age, height and sex usually explain 60 to 70% of the variation. Restrictive lung function disorders will result in lower values for VC, FVC and FEV₁. Measurements of flows during expiration show a great individual variation, since the measured flows are both effort and time dependent. This means, for example, that a subject will have extremely high flow in case of diminished lung volume. On the other hand, the flow may be extremely low in case of very high lung volume. However, the flow is usually decreased in case of a chronic obstructive disease (e.g., asthma, chronic bronchitis).

Figure 3.  A principal outline of the equipment for determination of total lung capacity (TLC) according to the helium dilution technique.

The proportion of residual volume (RV), that is, the volume of air which still is in the lungs after a maximal expiration, can be determined by gas dilution or by body plethysmography. The gas dilution technique requires less complicated equipment and is therefore more convenient to use in studies carried out at the workplace. In figure 3, the principle for the gas dilution technique has been outlined. The technique is based on dilution of an indicator gas in a rebreathing circuit. The indicator gas must be sparingly soluble in biological tissues so that it is not taken up by the tissues and blood in the lung. Hydrogen was initially used, but because of its ability to form explosive mixtures with air it was replaced by helium, which is easily detected by means of the thermal conductivity principle.

The subject and the apparatus form a closed system, and the initial concentration of the gas is thus reduced when it is diluted into the gas volume in the lungs. After equilibration, the concentration of indicator gas is the same in the lungs as in the apparatus, and functional residual capacity (FRC) can be calculated by means of a simple dilution equation. The volume of the spirometer (including the addition of the gas mixture into the spirometer) is denoted by V_S, V_L is the volume of the lung, F_i is the initial gas concentration and F_f is the final concentration.

FRC = V_L = [(V_S · F_i) / F_f] – V_S

Two to three VC manoeuvres are carried out to provide a reliable base for the calculation of TLC (in litres). The subdivisions of the different lung volumes are outlined in figure 4.

Figure 4. Spirogram labelled to show the subdivisions of the total capacity.

Due to change in the elastic properties of the airways, RV and FRC increase with age. In chronic obstructive diseases, increased values of RV and FRC are usually observed, while VC is decreased. However, in subjects with badly ventilated lung areas—for example, subjects with emphysema—the gas dilution technique may underestimate RV, FRC and also TLC. This is due to the fact that the indicator gas will not communicate with closed-off airways, and therefore the decrease in the indicator gas concentration will give erroneously small values.

Figure 5. A principal outline of the recording of airway closure and the slope of the alveolar plateau (%).

Measures of airway closure and gas distribution in the lungs can be obtained in one and the same manoeuvre by the single breath wash-out technique, figure 5. The equipment consists of a spirometer connected to a bag-in-box system and a recorder for continuous measurements of nitrogen concentration. The manoeuvre is carried out by means of a maximal inspiration of pure oxygen from the bag. In the beginning of the expiration, the nitrogen concentration increases as a result of emptying the subject’s deadspace, containing pure oxygen. The expiration continues with the air from the airways and alveoli. Finally, air from the alveoli, containing 20 to 40% nitrogen, is expired. When the expiration from the basal parts of the lungs increases, the nitrogen concentration will rise abruptly in case of airway closure in dependent lung regions, figure 5. This volume above RV, at which airways close during an expiration, is usually expressed as closing volume (CV) in percentage of VC (CV%). Distribution of the inspired air in the lungs is expressed as the slope of the alveolar plateau (%N₂ or phase III, %N₂/l). It is obtained by taking the difference in nitrogen concentration between the point when 30% of the air is exhaled and the point for airway closure, and dividing this by the corresponding volume.

Ageing as well as chronic obstructive disorders will result in increased values for both CV% and phase III. However, not even healthy subjects have a uniform gas distribution in the lungs, resulting in slightly elevated values for phase III, that is, 1 to 2% N₂/l. The variables CV% and phase III are considered to reflect the conditions in the peripheral small airways with an internal diameter about 2 mm. Normally, the peripheral airways contribute to a small part (10 to 20%) of the total airway resistance. Quite extensive changes which are not detectable by conventional lung function tests like dynamic spirometry, may occur, for example, as a result of an exposure to irritating substances in the air in the peripheral airways. This suggests that airway obstruction begins in the small airways. Results from studies also have shown alterations in CV% and phase III before any changes from the dynamic and static spirometry have occurred. These early changes may go into remission when exposure to hazardous agents has ceased.

The transfer factor of the lung (mmol/min; kPa) is an expression of the diffusion capacity of oxygen transport into the pulmonary capillaries. The transfer factor can be determined using single or multiple breath techniques; the single breath technique is considered to be most suitable in studies at the workplace. Carbon monoxide (CO) is used since the back pressure of CO is very low in the peripheral blood, in contrast to that of oxygen. The uptake of CO is assumed to follow an exponential model, and this assumption can be used to determine the transfer factor for the lung.

Determination of TL_CO (transfer factor measured with CO) is carried out by means of a breathing manoeuvre including a maximal expiration, followed by a maximal inspiration of a gas mixture containing carbon monoxide, helium, oxygen and nitrogen. After a breath-holding period, a maximal exhalation is done, reflecting the content in the alveolar air, Figure 10. Helium is used for the determination of the alveolar volume (V_A). Assuming that the dilution of CO is the same as for helium, the initial concentration of CO, before the diffusion has started, can be calculated. TL_CO is calculated according to the equation outlined below, where k depends on the dimensions of the component terms, t is the effective time for breath-holding and log is base 10 logarithm. Inspired volume is denoted V_i and the fractions F of CO and helium are denoted by i and a for inspired and alveolar, respectively.

TL_CO = k V_i (F_a,He/F_i,He) log (F_i,CO F_a,He/F_a,CO F_i,He) (t)^-1

Figure 6. A principal outline of the recording of transfer factor

The size of TL_CO will depend on a variety of conditions—for example, the amount of available haemoglobin, the volume of ventilated alveoli and perfused lung capillaries and their relation to each other. Values for TL_CO decrease with age and increase with physical activity and increased lung volumes. Decreased TL_CO will be found in both restrictive and obstructive lung disorders.

Compliance (l/kPa) is a function, inter alia, of the elastic property of the lungs. The lungs have an intrinsic tendency to collaborate—that is, to collapse. The power to keep the lungs stretched will depend on the elastic lung tissue, the surface tension in the alveoli, and the bronchial musculature. On the other hand, the chest wall tends to expand at lung volumes 1 to 2 litres above the FRC level. At higher lung volumes, power has to be applied to further expand the chest wall. At the FRC level, the corresponding tendency in the lungs is balanced by the tendency to expand. The FRC level is therefore denoted by the resting level of the lung.

The compliance of the lung is defined as the change in volume divided by the change in transpulmonary pressure, that is, the difference between the pressures in the mouth (atmospheric) and in the lung, as the result of a breathing manoeuvre. Measurements of the pressure in the lung are not easily carried out and are therefore replaced by measurements of the pressure in the oesophagus. The pressure in the oesophagus is almost the same as the pressure in the lung, and it is measured with a thin polyethylene catheter with a balloon covering the distal 10 cm. During inspiratory and expiratory manoeuvres, the changes in volume and pressure are recorded by means of a spirometer and pressure transducer, respectively. When the measurements are performed during tidal breathing, dynamic compliance can be measured. Static compliance is obtained when a slow VC manoeuvre is carried out. In the latter case, the measurements are carried out in a body plethysmograph, and the expiration is intermittently interrupted by means of a shutter. However, measurements of compliance are cumbersome to perform when examining exposure effects on lung function at the worksite, and this technique is considered to be more appropriate in the laboratory.

A decreased compliance (increased elasticity) is observed in fibrosis. To cause a change in volume, large changes in pressure are required. On the other hand, a high compliance is observed, for example, in emphysema as the result of loss of elastic tissue and therefore also elasticity in the lung.

The resistance in the airways essentially depends on the radius and length of the airways but also on air viscosity. The airway resistance (R_L in (kPa/l) /s), can be determined by use of a spirometer, pressure transducer and a pneumotachograph (to measure the flow). The measurements may also be carried out using a body plethysmograph to record the changes in flow and pressure during panting manoeuvres. By administration of a drug intended to cause broncho-constriction, sensitive subjects, as a result of their hyperreactive airways, may be identified. Subjects with asthma usually have increased values for R_L.

Acute and Chronic Effects of Occupational Exposure on Pulmonary Function

Lung function measurement may be used to disclose an occupational exposure effect on the lungs. Pre-employment examination of lung function should not be used to exclude job-seeking subjects. This is because the lung function of healthy subjects varies within wide limits and it is difficult to draw a borderline below which it can safely be stated that the lung is pathological. Another reason is that the work environment should be good enough to allow even subjects with slight lung function impairment to work safely.

Chronic effects on the lungs in occupationally exposed subjects may be detected in a number of ways. The techniques are designed to determine historical effects, however, and are less suitable to serve as guidelines to prevent lung function impairment. A common study design is to compare the actual values in exposed subjects with the lung function values obtained in a reference population without occupational exposure. The reference subjects may be recruited from the same (or nearby) workplaces or from the same city.

Multivariate analysis has been used in some studies to assess differences between exposed subjects and matched unexposed referents. Lung function values in exposed subjects may also be standardized by means of a reference equation based on lung function values in the unexposed subjects.

Another approach is to study the difference between the lung function values in exposed and unexposed workers after adjustment for age and height with the use of external reference values, calculated by means of a prediction equation based on healthy subjects. The reference population may also be matched to the exposed subjects according to ethnic group, sex, age, height and smoking habits in order to further control for those influencing factors.

The problem is, however, to decide if a decrease is large enough to be classified as pathological, when external reference values are being used. Although the instruments in the studies have to be portable and simple, attention must be paid both to the sensitivity of the chosen method for detecting small anomalies in airways and lungs and the possibility of combining different methods. There are indications that subjects with respiratory symptoms, such as exertion dyspnoea, are at a higher risk of having an accelerated decline in lung function. This means that the presence of respiratory symptoms is important and so should not be neglected.

The subject may also be followed-up by spirometry, for example, once a year, for a number of years, in order to give a warning against the development of illness. There are limitations, however, since this will be very time-consuming and the lung function may have deteriorated permanently when the decrease can be observed. This approach therefore must not be an excuse for delay in carrying out measures in order to decrease harmful concentrations of air pollutants.

Finally, chronic effects on lung function may also be studied by examining the individual changes in lung function in exposed and unexposed subjects over a number of years. One advantage of the longitudinal study design is that the intersubject variability is eliminated; however, the design is considered to be time-consuming and expensive.

Susceptible subjects may also be identified by comparing their lung function with and without exposure during working shifts. In order to minimize possible effects of diurnal variations, lung function is measured at the same time of day on one unexposed and one exposed occasion. The unexposed condition can be obtained, for example, by occasionally moving the worker to an uncontaminated area or by use of a suitable respirator during a whole shift, or in some cases by performing lung function measurements in the afternoon of a worker’s day off.

One special concern is that repeated, temporary effects can result in chronic effects. An acute temporary lung function decrease may not only be a biological exposure indicator but also a predictor of a chronic lung function decrement. Exposure to air pollutants may result in discernible acute effects on lung function, although the mean values of the measured air pollutants are below the hygienic limit values. The question thus arises, whether these effects really are harmful in the long run. This question is hard to answer directly, especially since the air pollution in workplaces often has a complex composition and the exposure cannot be described in terms of mean concentrations of single compounds. The effect of an occupational exposure is also partly due to the sensitivity of the individual. This means that some subjects will react sooner or to a larger extent than others. The underlying pathophysiological ground for an acute, temporary decrease in lung function is not fully understood. The adverse reaction upon exposure to an irritating air contaminant is, however, an objective measurement, in contrast to subjective experiences like symptoms of different origin.

The advantage of detecting early changes in airways and lungs caused by hazardous air pollutants is obvious—the prevailing exposure may be reduced in order to prevent more severe illnesses. Therefore, an important aim in this respect is to use the measurements of acute temporary effects on lung function as a sensitive early warning system that can be used when studying groups of healthy working people.

Monitoring of Irritants

Irritation is one of the most frequent criteria for setting exposure limit values. It is, however, not certain that compliance with an exposure limit based on irritation will protect against irritation. It should be considered that an exposure limit for an air contaminant usually contains at least two parts—a time-weighted average limit (TWAL) and a short-term exposure limit (STEL), or at least rules for exceeding the time-weighted average limit, “excursion limits”. In the case of highly irritating substances, such as sulphur dioxide, acrolein and phosgene, it is important to limit the concentration even during very short periods, and it has therefore been common practice to fix occupational exposure limit values in the form of ceiling limits, with a sampling period that is kept as short as the measuring facilities will allow.

Time-weighted average limit values for an eight-hour day combined with rules for excursion above these values are given for most of the substances in the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value (TLV) list. The TLV list of 1993-94 contains the following statement concerning excursion limits for exceeding limit values:

“For the vast majority of substances with a TLV-TWA, there is not enough toxicological data available to warrant a STEL = short-term exposure limit). Nevertheless, excursions above the TLV-TWA should be controlled even where the eight-hour TWA is within recommended limits.”

Exposure measurements of known air contaminants and comparison with well documented exposure limit values should be carried out on a routine basis. There are, however, many situations when the determination of compliance with exposure limit values is not enough. This is the case in the following circumstances (inter alia):

1. when the limit value is too high to safeguard against irritation
2. when the irritant is unknown
3. when the irritant is a complex mixture and there is no suitable indicator known.

As advocated above, the measurement of acute, temporary effects on lung function can be used in these cases as a warning against over-exposure to irritants.

In cases (2) and (3), acute, temporary effects on lung function may be applicable also in testing the efficiency of control measures to decrease exposure to air contamination or in scientific investigations, for example, in attributing biological effects to components of air contaminants. A number of examples follow in which acute, temporary lung function effects have been successfully employed in occupational health investigations.

Studies of Acute, Temporary Lung Function Effects

Work-related, temporary decrease of lung function over a work shift was recorded in cotton workers at the end of 1950. Later, several authors reported work-related, acute, temporary changes of lung function in hemp and textile workers, coal miners, workers exposed to toluene di-isocyanate, fire-fighters, rubber processing workers, moulders and coremakers, welders, ski waxers, workers exposed to organic dust and irritants in water-based paints.

However, there are also several examples where measurements before and after exposure, usually during a shift, have failed to demonstrate any acute effects, despite a high exposure. This is probably due to the effect of normal circadian variation, mainly in lung function variables depending on the size of airway calibre. Thus the temporary decrease in these variables must exceed the normal circadian variation to be recognized. The problem may be circumvented, however, by measuring lung function at the same time of the day at each study occasion. By using the exposed employee as his or her own control, the interindividual variation is further decreased. Welders were studied in this way, and although the mean difference between unexposed and exposed FVC values was less than 3% in 15 examined welders, this difference was significant at the 95% confidence level with a power of more than 99%.

The reversible transient effects on the lungs can be used as an exposure indicator of complicated irritating components. In the study cited above, particles in the work environment were crucial for the irritating effects on the airways and lungs. The particles were removed by a respirator consisting of a filter combined with a welding helmet. The results indicated that the effects on the lungs were caused by the particles in welding fumes, and that the use of a particulate respirator might prevent this effect.

Exposure to diesel exhaust also gives measurable irritative effects on the lungs, shown as an acute, temporary lung function decrease. Mechanical filters mounted on the exhaust pipes of trucks used in loading operations by stevedores relieved subjective disorders and reduced the acute, temporary lung function decrease observed when no filtration was done. The results thus indicate that the presence of particles in the work environment does play a role in the irritative effect on airways and lungs, and that it is possible to assess the effect by measurements of acute changes in lung function.

A multiplicity of exposures and a continually changing work environment may present difficulties in discerning the causal relationship of the different agents existing in a work environment. The exposure scenario in sawmills is an illuminating example. It is not possible (e.g., for economical reasons) to carry out exposure measurements of all possible agents (terpenes, dust, mould, bacteria, endotoxin, mycotoxins, etc.) in this work environment. A feasible method may be to follow the development of lung function longitudinally. In a study of sawmill workers in the wood-trimming department, lung function was examined before and after a working week, and no statistically significant decrease was found. However, a follow-up study carried out a few years later disclosed that those workers who actually had a numerical decrease in lung function during a working week also had an accelerated long-term decline in lung function. This may indicate that vulnerable subjects can be detected by measuring changes in lung function during a working week.

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The respiratory system extends from the breathing zone just outside of the nose and mouth through the conductive airways in the head and thorax to the alveoli, where respiratory gas exchange takes place between the alveoli and the capillary blood flowing around them. Its prime function is to deliver oxygen (O₂) to the gas-exchange region of the lung, where it can diffuse to and through the walls of the alveoli to oxygenate the blood passing through the alveolar capillaries as needed over a wide range of work or activity levels. In addition, the system must also: (1) remove an equal volume of carbon dioxide entering the lungs from the alveolar capillaries; (2) maintain body temperature and water vapour saturation within the lung airways (in order to maintain the viability and functional capacities of the surface fluids and cells); (3) maintain sterility (to prevent infections and their adverse consequences); and (4) eliminate excess surface fluids and debris, such as inhaled particles and senescent phagocytic and epithelial cells. It must accomplish all of these demanding tasks continuously over a lifetime, and do so with high efficiency in terms of performance and energy utilization. The system can be abused and overwhelmed by severe insults such as high concentrations of cigarette smoke and industrial dust, or by low concentrations of specific pathogens which attack or destroy its defence mechanisms, or cause them to malfunction. Its ability to overcome or compensate for such insults as competently as it usually does is a testament to its elegant combination of structure and function.

Mass Transfer

The complex structure and numerous functions of the human respiratory tract have been summarized concisely by a Task Group of the International Commission on Radiological Protection (ICRP 1994), as shown in figure 1. The conductive airways, also known as the respiratory dead space, occupy about 0.2 litres. They condition the inhaled air and distribute it, by convective (bulk) flow, to the approximately 65,000 respiratory acini leading off the terminal bronchioles. As tidal volumes increase, convective flow dominates gas exchange deeper into the respiratory bronchioles. In any case, within the respiratory acinus, the distance from the convective tidal front to alveolar surfaces is short enough so that efficient CO₂-O₂ exchange takes place by molecular diffusion. By contrast, airborne particles, with diffusion coefficients smaller by orders of magnitude than those for gases, tend to remain suspended in the tidal air, and can be exhaled without deposition.

Figure 1.  Morphometry, cytology, histology, function and structure of the respiratory tract and regions used in the 1994 ICRP dosimetry model.

A significant fraction of the inhaled particles do deposit within the respiratory tract. The mechanisms accounting for particle deposition in the lung airways during the inspiratory phase of a tidal breath are summarized in figure 2. Particles larger than about 2 mm in aerodynamic diameter (diameter of a unit density sphere having the same terminal settling (Stokes) velocity) can have significant momentum and deposit by impaction at the relatively high velocities present in the larger airways. Particles larger than about 1 mm can deposit by sedimentation in the smaller conductive airways, where flow velocities are very low. Finally, particles with diameters between 0.1 and 1 mm, which have a very low probability of depositing during a single tidal breath, can be retained within the approximately 15% of the inspired tidal air that is exchanged with residual lung air during each tidal cycle. This volumetric exchange occurs because of the variable time-constants for airflow in the different segments of the lungs. Due to the much longer residence times of the residual air in the lungs, the low intrinsic particle displacements of 0.1 to 1 mm particles within such trapped volumes of inhaled tidal air become sufficient to cause their deposition by sedimentation and/or diffusion over the course of successive breaths.

Figure 2. Mechanisms for particle deposition in lung airways

The essentially particle-free residual lung air that accounts for about 15% of the expiratory tidal flow tends to act like a clean-air sheath around the axial core of distally moving tidal air, such that particle deposition in the respiratory acinus is concentrated on interior surfaces such as airway bifurcations, while interbranch airway walls have little deposition.

The number of particles deposited and their distribution along the respiratory tract surfaces are, along with the toxic properties of the material deposited, the critical determinants of pathogenic potential. The deposited particles can damage the epithelial and/or the mobile phagocytic cells at or near the deposition site, or can stimulate the secretion of fluids and cell-derived mediators that have secondary effects on the system. Soluble materials deposited as, on, or within particles can diffuse into and through surface fluids and cells and be rapidly transported by the bloodstream throughout the body.

Aqueous solubility of bulk materials is a poor guide to particle solubility in the respiratory tract. Solubility is generally greatly enhanced by the very large surface-to-volume ratio of particles small enough to enter the lungs. Furthermore, the ionic and lipid contents of surface fluids within the airways are complex and highly variable, and can lead to either enhanced solubility or to rapid precipitation of aqueous solutes. Furthermore, the clearance pathways and residence times for particles on airway surfaces are very different in the different functional parts of the respiratory tract.

The revised ICRP Task Group’s clearance model identifies the principal clearance pathways within the respiratory tract that are important in determining the retention of various radioactive materials, and thus the radiation doses received by respiratory tissues and other organs after translocation. The ICRP deposition model is used to estimate the amount of inhaled material that enters each clearance pathway. These discrete pathways are represented by the compartment model shown in figure 3. They correspond to the anatomic compartments illustrated in Figure 1, and are summarized in table 1, along with those of other groups providing guidance on the dosimetry of inhaled particles.

Figure 3. Compartment model to represent time-dependent particle transport from each region in 1994 ICRP model

Table 1. Respiratory tract regions as defined in particle deposition models

Extrathoracic airways

As shown in figure 1, the extrathoracic airways were partitioned by ICRP (1994) into two distinct clearance and dosimetric regions: the anterior nasal passages (ET₁) and all other extrathoracic airways (ET₂)—that is, the posterior nasal passages, the naso- and oropharynx, and the larynx. Particles deposited on the surface of the skin lining the anterior nasal passages (ET₁) are assumed to be subject only to removal by extrinsic means (nose blowing, wiping and so on). The bulk of material deposited in the naso-oropharynx or larynx (ET₂) is subject to fast clearance in the layer of fluid that covers these airways. The new model recognizes that diffusional deposition of ultrafine particles in the extrathoracic airways can be substantial, while the earlier models did not.

Thoracic airways

Radioactive material deposited in the thorax is generally divided between the tracheobronchial (TB) region, where deposited particles are subject to relatively fast mucociliary clearance, and the alveolar-interstitial (AI) region, where the particle clearance is much slower.

For dosimetry purposes, the ICRP (1994) divided deposition of inhaled material in the TB region between the trachea and bronchi (BB), and the more distal, small airways, the bronchioles (bb). However, the subsequent efficiency with which cilia in either type of airways are able to clear deposited particles is controversial. In order to be certain that doses to bronchial and bronchiolar epithelia would not be underestimated, the Task Group assumed that as much as half the number of particles deposited in these airways is subject to relatively “slow” mucociliary clearance. The likelihood that a particle is cleared relatively slowly by the mucociliary system appears to depend on its physical size.

Material deposited in the AI region is subdivided among three compartments (AI₁, AI₂ and AI₃) that are each cleared more slowly than TB deposition, with the subregions cleared at different characteristic rates.

Figure 4. Fractional deposition in each region of respiratory tract for reference light worker (normal nose breather) in 1994 ICRP model.

Figure 4 depicts the predictions of the ICRP (1994) model in terms of the fractional deposition in each region as a function of the size of the inhaled particles. It reflects the minimal lung deposition between 0.1 and 1 mm, where deposition is determined largely by the exchange, in the deep lung, between tidal and residual lung air. Deposition increases below 0.1 mm as diffusion becomes more efficient with decreasing particle size. Deposition increases with increasing particle size above 1 mm as sedimentation and impaction become increasingly effective.

Less complex models for size-selective deposition have been adopted by occupational health and community air pollution professionals and agencies, and these have been used to develop inhalation exposure limits within specific particle size ranges. Distinctions are made between:

1. those particles that are not aspirated into the nose or mouth and therefore represent no inhalation hazard
2. the inhalable (also known as inspirable) particulate mass (IPM)—those that are inhaled and are hazardous when deposited anywhere within the respiratory tract
3. the thoracic particulate mass (TPM)—those that penetrate the larynx and are hazardous when deposited anywhere within the thorax and
4. the respirable particulate mass (RPM)—those particles that penetrate through the terminal bronchioles and are hazardous when deposited within the gas-exchange region of the lungs.

In the early 1990s there has been an international harmonization of the quantitative definitions of IPM, TPM and RPM. The size-selective inlet specifications for air samplers meeting the criteria of the American Conference of Governmental Industrial Hygienists (ACGIH 1993), the International Organization for Standardization (ISO 1991) and the European Standardization Committee (CEN 1991) are enumerated in table 2. They differ from the deposition fractions of ICRP (1994), especially for larger particles, because they take the conservative position that protection should be provided for those engaged in oral inhalation, and thereby bypass the more efficient filtration efficiency of the nasal passages.

Table 2. Inhalable, thoracic and respirable dust criteria of ACGIH, ISO and CEN, and PM₁₀ criteria of US EPA

The US Environmental Protection Agency (EPA 1987) standard for ambient air particle concentration is known as PM₁₀, that is, particulate matter less than 10 mm in aerodynamic diameter. It has a sampler inlet criterion that is similar (functionally equivalent) to TPM but, as shown in Table 2, somewhat different numerical specifications.

Air Pollutants

Pollutants can be dispersed in air at normal ambient temperatures and pressures in gaseous, liquid and solid forms. The latter two represent suspensions of particles in air and were given the generic term aerosols by Gibbs (1924) on the basis of analogy to the term hydrosol, used to describe dispersed systems in water. Gases and vapours, which are present as discrete molecules, form true solutions in air. Particles consisting of moderate to high vapour pressure materials tend to evaporate rapidly, because those small enough to remain suspended in air for more than a few minutes (i.e., those smaller than about 10 mm) have large surface-to-volume ratios. Some materials with relatively low vapour pressures can have appreciable fractions in both vapour and aerosol forms simultaneously.

Gases and vapours

Once dispersed in air, contaminant gases and vapours generally form mixtures so dilute that their physical properties (such as density, viscosity, enthalpy and so on) are indistinguishable from those of clean air. Such mixtures may be considered to follow ideal gas law relationships. There is no practical difference between a gas and a vapour except that the latter is generally considered to be the gaseous phase of a substance that can exist as a solid or liquid at room temperature. While dispersed in air, all molecules of a given compound are essentially equivalent in their size and probabilities of capture by ambient surfaces, respiratory tract surfaces and contaminant collectors or samplers.

Aerosols

Aerosols, being dispersions of solid or liquid particles in air, have the very significant additional variable of particle size. Size affects particle motion and, hence, the probabilities of physical phenomena such as coagulation, dispersion, sedimentation, impaction onto surfaces, interfacial phenomena and light-scattering properties. It is not possible to characterize a given particle by a single size parameter. For example, a particle’s aerodynamic properties depend on density and shape as well as linear dimensions, and the effective size for light scattering is dependent on refractive index and shape.

In some special cases, all of the particles are essentially the same in size. Such aerosols are considered to be monodisperse. Examples are natural pollens and some laboratory-generated aerosols. More typically, aerosols are composed of particles of many different sizes and hence are called heterodisperse or polydisperse. Different aerosols have different degrees of size dispersion. It is, therefore, necessary to specify at least two parameters in characterizing aerosol size: a measure of central tendency, such as a mean or median, and a measure of dispersion, such as an arithmetic or geometric standard deviation.

Particles generated by a single source or process generally have diameters following a log-normal distribution; that is, the logarithms of their individual diameters have a Gaussian distribution. In this case, the measure of dispersion is the geometric standard deviation, which is the ratio of the 84.1 percentile size to the 50 percentile size. When more than one source of particles is significant, the resulting mixed aerosol will usually not follow a single log-normal distribution, and it may be necessary to describe it by the sum of several distributions.

Particle characteristics

There are many properties of particles other than their linear size that can greatly influence their airborne behaviour and their effects on the environment and health. These include:

Surface. For spherical particles, the surface varies as the square of the diameter. However, for an aerosol of given mass concentration, the total aerosol surface increases with decreasing particle size. For non-spherical or aggregate particles, and for particles with internal cracks or pores, the ratio of surface to volume can be much greater than for spheres.

Volume. Particle volume varies as the cube of the diameter; therefore, the few largest particles in an aerosol tend to dominate its volume (or mass) concentration.

Shape. A particle’s shape affects its aerodynamic drag as well as its surface area and therefore its motion and deposition probabilities.

Density. A particle’s velocity in response to gravitational or inertial forces increases as the square root of its density.

Aerodynamic diameter. The diameter of a unit-density sphere having the same terminal settling velocity as the particle under consideration is equal to its aerodynamic diameter. Terminal settling velocity is the equilibrium velocity of a particle that is falling under the influence of gravity and fluid resistance. Aerodynamic diameter is determined by the actual particle size, the particle density and an aerodynamic shape factor.

Types of aerosols

Aerosols are generally classified in terms of their processes of formation. Although the following classification is neither precise nor comprehensive, it is commonly used and accepted in the industrial hygiene and air pollution fields.

Dust. An aerosol formed by mechanical subdivision of bulk material into airborne fines having the same chemical composition. Dust particles are generally solid and irregular in shape and have diameters greater than 1 mm.

Fume. An aerosol of solid particles formed by condensation of vapours formed by combustion or sublimation at elevated temperatures. The primary particles are generally very small (less than 0.1 mm) and have spherical or characteristic crystalline shapes. They may be chemically identical to the parent material, or may be composed of an oxidation product such as metal oxide. Since they may be formed in high number concentrations, they often rapidly coagulate, forming aggregate clusters of low overall density.

Smoke. An aerosol formed by condensation of combustion products, generally of organic materials. The particles are generally liquid droplets with diameters less than 0.5 mm.

Mist. A droplet aerosol formed by mechanical shearing of a bulk liquid, for example, by atomization, nebulization, bubbling or spraying. The droplet size can cover a very large range, usually from about 2 mm to greater than 50 mm.

Fog. An aqueous aerosol formed by condensation of water vapour on atmospheric nuclei at high relative humidities. The droplet sizes are generally greater than 1 mm.

Smog. A popular term for a pollution aerosol derived from a combination of smoke and fog. It is now commonly used for any atmospheric pollution mixture.

Haze. A submicrometer-sized aerosol of hygroscopic particles that take up water vapour at relatively low relative humidities.

Aitken or condensation nuclei (CN). Very small atmospheric particles (mostly smaller than 0.1 mm) formed by combustion processes and by chemical conversion from gaseous precursors.

Accumulation mode. A term given to the particles in the ambient atmosphere ranging from 0.1 to about 1.0 mm in diameter. These particles generally are spherical (having liquid surfaces), and form by coagulation and condensation of smaller particles that derive from gaseous precursors. Being too large for rapid coagulation and too small for effective sedimentation, they tend to accumulate in the ambient air.

Coarse particle mode. Ambient air particles larger than about 2.5 mm in aerodynamic diameter and generally formed by mechanical processes and surface dust resuspension.

Biological Responses of the Respiratory System to Air Pollutants

Responses to air pollutants range from nuisance to tissue necrosis and death, from generalized systemic effects to highly specific attacks on single tissues. Host and environmental factors serve to modify the effects of inhaled chemicals, and the ultimate response is the result of their interaction. The main host factors are:

1. age—for example, older people, especially those with chronically reduced cardiovascular and respiratory function, who may not be able to cope with additional pulmonary stresses
2. state of health—for example, concurrent disease or dysfunction
3. nutritional status
4. immunological status
5. sex and other genetic factors—for example, enzyme-related differences in biotransformation mechanisms, such as deficient metabolic pathways, and inability to synthesize certain detoxification enzymes
6. psychological state—for example, stress, anxiety and
7. cultural factors—for example, cigarette smoking, which may affect normal defences, or may potentiate the effect of other chemicals.

The environmental factors include the concentration, stability and physicochemical properties of the agent in the exposure environment and the duration, frequency and route of exposure. Acute and chronic exposures to a chemical may result in different pathological manifestations.

Any organ can respond in only a limited number of ways, and there are numerous diagnostic labels for the resultant diseases. The following sections discuss the broad types of responses of the respiratory system which may occur following exposure to environmental pollutants.

Irritant response

Irritants produce a pattern of generalized, non-specific tissue inflammation, and destruction may result at the area of contaminant contact. Some irritants produce no systemic effect because the irritant response is much greater than any systemic effect, while some also have significant systemic effects following absorption—for example, hydrogen sulphide absorbed via the lungs.

At high concentrations, irritants may cause a burning sensation in the nose and throat (and usually also in the eyes), pain in the chest and coughing producing inflammation of the mucosa (tracheitis, bronchitis). Examples of irritants are gases such as chlorine, fluorine, sulphur dioxide, phosgene and oxides of nitrogen; mists of acids or alkali; fumes of cadmium; dusts of zinc chloride and vanadium pentoxide. High concentrations of chemical irritants may also penetrate deep into the lungs and cause lung oedema (the alveoli are filled with liquid) or inflammation (chemical pneumonitis).

Highly elevated concentrations of dusts which have no chemical irritative properties can also mechanically irritate bronchi and, after entering the gastrointestinal tract, may also contribute to stomach and colon cancer.

Exposure to irritants may result in death if critical organs are severely damaged. On the other hand, the damage may be reversible, or it may result in permanent loss of some degree of function, such as impaired gas-exchange capacity.

Fibrotic response

A number of dusts lead to the development of a group of chronic lung disorders termed pneumoconioses. This general term encompasses many fibrotic conditions of the lung, that is, diseases characterized by scar formation in the interstitial connective tissue. Pneumoconioses are due to the inhalation and subsequent selective retention of certain dusts in the alveoli, from which they are subject to interstitial sequestration.

Pneumoconioses are characterized by specific fibrotic lesions, which differ in type and pattern according to the dust involved. For example, silicosis, due to the deposition of crystalline-free silica, is characterized by a nodular type of fibrosis, while a diffuse fibrosis is found in asbestosis, due to asbestos-fibre exposure. Certain dusts, such as iron oxide, produce only altered radiology (siderosis) with no functional impairment, while the effects of others range from a minimal disability to death.

Allergic response

Allergic responses involve the phenomenon known as sensitization. Initial exposure to an allergen results in the induction of antibody formation; subsequent exposure of the now “sensitized” individual results in an immune response—that is, an antibody-antigen reaction (the antigen is the allergen in combination with an endogenous protein). This immune reaction may occur immediately following exposure to the allergen, or it may be a delayed response.

The primary respiratory allergic reactions are bronchial asthma, reactions in the upper respiratory tract which involve the release of histamine or histamine-like mediators following immune reactions in the mucosa, and a type of pneumonitis (lung inflammation) known as extrinsic allergic alveolitis. In addition to these local reactions, a systemic allergic reaction (anaphylactic shock) may follow exposure to some chemical allergens.

Infectious response

Infectious agents can cause tuberculosis, anthrax, ornithosis, brucellosis, histoplasmosis, Legionnaires’ disease and so on.

Carcinogenic response

Cancer is a general term for a group of related diseases characterized by the uncontrolled growth of tissues. Its development is due to a complex process of interacting multiple factors in the host and the environment.

One of the great difficulties in attempting to relate exposure to a specific agent to cancer development in humans is the long latent period, typically from 15 to 40 years, between onset of exposure and disease manifestation.

Examples of air pollutants that can produce cancer of the lungs are arsenic and its compounds, chromates, silica, particles containing polycyclic aromatic hydrocarbons and certain nickel-bearing dusts. Asbestos fibres can cause bronchial cancer and mesothelioma of the pleura and peritoneum. Deposited radioactive particles may expose lung tissue to high local doses of ionizing radiation and be the cause of cancer.

Systemic response

Many environmental chemicals produce a generalized systemic disease due to their effects upon a number of target sites. Lungs are not only the target for many harmful agents but the site of entry of toxic substances which pass through the lungs into the bloodstream without any damage to the lungs. However, when distributed by the blood circulation to various organs, they can damage them or cause general poisoning and have systemic effects. This role of the lungs in occupational pathology is not the subject of this article. However, the effect of finely dispersed particulates (fumes) of several metal oxides which are often associated with an acute systemic syndrome known as metal fume fever should be mentioned.

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Table 5. List of of technical products unlikely to present acute hazard in normal use (continued)

Source: WHO 1996.

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Table 5. List of technical products unlikely to present acute hazard in normal use

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Table 6. Technical products not included in the WHO Classification and believed to be obsolete or discontinued for use as pesticides

Source: WHO 1996.

Table 7. List of gaseous or volatile fumigants not classified under the WHO Recommended Classification of Pesticides by Hazard

Note: The WHO Classification does not set out any criteria for air concentrations on which classification could be based. Most of these compounds are of high hazard and recommended exposure limits for occupational exposure have been adopted by national authorities in many countries.

Source: WHO 1996.

The entries and abbrevations used in the tables’ various columns are explained here under the corresponding heading.

Name

The first column in the tables list the approved name of active ingredients. Trade names are not listed since there are many of these.

Status

The following abbreviations are used:

• ISO: Indicates the common name approved by the International Organization for Standardization (ISO). Such names are, when available, preferred by the WHO to all other common names. However, some of these names may not be acceptable for national use in some countries. If the letters ISO appear within parentheses (e.g., with fentin acetate), this indicates that ISO has standardized (or is in the process of standardizing) the name of the base, but not the name of the derivative listed in the “Name” column. (Fentin is an ISO name, but fentin acetate is not.)
• N( ): Indicates approval by a national ministry or other body, which is shown in parentheses as follows: A: United States Environmental Protection Agency (EPA), American National Standards Institute (ANSI), the Weed Science Society of America or the Entomological Society of America; B: British Standards Institution or the British Pharmacopoeia Commission; F: Association française de normalisation; J: Japanese Ministry of Agriculture and Forestry; U: Gosudarstvennyi Komitet Standartov, former USSR.
• C: Chemical, trivial or other common name.

Main use

In most cases only a single use is given. This is only for identification purposes and does not exclude other uses. The following abbreviations are used:

• AC: acaricide
• AP: aphicide
• B: bacteriostat (soil)
• FM: fumigant
• F: fungicide, other than for seed treatment
• FST: fungicide, for seed treatment
• H: herbicide
• I: insecticide
• IGR: insect growth regulator
• Ix: ixodicide (for tick control)
• L: larvicide
• M: molluscicide
• N: nematocide
• O: other use for plant pathogens
• PGR: plant growth regulator
• R: rodenticide
• RP( ): repellent (species)
• -S: applied to soil; not used with herbicides or PGRs
• SY: synergist.

Chemical type

A limited number of chemical types are shown in this column. Most have some significance in the sense that they may have a common antidote or may be confused in the nomenclature with other chemical types. For example, thiocarbamates are not cholinesterase inhibitors and do not have the same effects as carbamates. The following abbreviations are used:

• C: carbamate
• CNP: chloronitrophenol derivative
• OC: organochlorine compound
• OM: organomercury compound
• OP: organophosphorus compound
• OT: organotin compound
• P: pyridyl derivative
• PA: phenoxyacetic acid derivative
• PY: pyrethroid
• T: triazine derivative
• TC: thiocarbamate.

These chemical classification are included only for convenience and do not represent a recommendation on the part of the WHO as to the way in which pesticides should be classified. It should, furthermore, be understood that some pesticides may fall into more than one type.

Chemical type is not shown where it is apparent from the name.

Physical state

This refers only to the technical compound. The following are used:

• L: liquid, including solids with a melting point below 50C
• oil: oily liquid; refers to physical state only
• S: solid, includes waxes.

It may happen in a few cases that where the technical product is a solid, highly concentrated liquid formulations may need to be classified in a more hazardous class. In most cases, oils have been classified as liquids unless very viscous at ordinary temperatures.

Route

Oral route values are used unless the dermal route values place the compound in a hazardous class or the dermal values are significantly lower than the oral values, although in the same class. The following abbreviations are used:

• D: dermal
• O: oral.

LD₅₀ (mg/kg)

The LD₅₀ value is a statistical estimate of the number of mg of toxicant per kg of body weight required to kill 50% of a large population of test animals; the rat is used unless otherwise states. A single value is given: “c” preceding the value indicates that it is a value within a wider than usual range, adopted for classification purposes; “+” preceding the value indicates that the kill at the stated dose was less than 50% of the test animals.

The toxicity data for pyrethroids are highly variable according to isomer ratios, the vehicle for oral administration and the husbandry of the test animals. The variability is reflected in the prefix “c”. The single LD₅₀ value now chosen for classification purposes is based on administration in corn oil and is much lower than that in aqueous solutions. This has resulted in considerable changes in the classification of some products and also underlines the need for classification by formulation if labelling is to reflect true hazard.

The figures in this column are not median values; rather, a safety margin is incorporated by choosing the lower confidence limit in most cases. Where a sex difference occurs in LD₅₀ values, the value for the more sensitive sex is used. A number of classification adjustments have been made in respect of some pesticides and these are explained. A borderline case has been classified in the more or less hazardous class after consideration of its toxicology and use experience.

In table 5, a number of pesticides are listed as unlikely to present any acute hazard in normal use. The WHO Classification is open-ended but it is clear that there must be a point at which the acute hazard posed by the use of these compounds is so low as to be negligible provided that the necessary precautions are taken. For the purposes of this table, it has been assumed that this point is an oral LD₅₀ of 2,000 mg/kg for solids and 3,000 mg/kg for liquids. However, it should not be overlooked that in formulations of these technical products, solvents or vehicles may present a greater hazard than the actual pesticide and therefore classification of a formulation in one of the higher hazard classes may be necessary.

Biological pesticides are not included in the WHO Classification because the methods of the safety testing of live biological agents are not appropriate to classification procedures applied to chemical compounds.

Remarks

Where the classification of a technical product has been adjusted, the basis for this is indicated in this column. Major irritant properties are noted; these do not affect classification. Where the name of a technical product is cross-referenced, the referenced product will be found in the same table. Abbreviations are used to indicate that a WHO/FAO Data Sheet (DS) or an issue of International Programme on Chemical Safety (IPCS) Environmental Health Criteria (EHC) Series or Health and Safety Guide contains further information on the product; the relevant issue numbers follow the abbreviations.

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Table 4. List of technical products classified in Class III: "Slightly hazardous"