Hazard Surveillance and Survey Methods

Occupational surveillance involves active programmes to anticipate, observe, measure, evaluate and control exposures to potential health hazards in the workplace. Surveillance often involves a team of people that includes an occupational hygienist, occupational physician, occupational health nurse, safety officer, toxicologist and engineer. Depending upon the occupational environment and problem, three surveillance methods can be employed: medical, environmental and biological. Medical surveillance is used to detect the presence or absence of adverse health effects for an individual from occupational exposure to contaminants, by performing medical examinations and appropriate biological tests. Environmental surveillance is used to document potential exposure to contaminants for a group of employees, by measuring the concentration of contaminants in the air, in bulk samples of materials, and on surfaces. Biological surveillance is used to document the absorption of contaminants into the body and correlate with environmental contaminant levels, by measuring the concentration of hazardous substances or their metabolites in the blood, urine or exhaled breath of workers.

Medical Surveillance

Medical surveillance is performed because diseases can be caused or exacerbated by exposure to hazardous substances. It requires an active programme with professionals who are knowledgeable about occupational diseases, diagnoses and treatment. Medical surveillance programmes provide steps to protect, educate, monitor and, in some cases, compensate the employee. It can include pre-employment screening programmes, periodic medical examinations, specialized tests to detect early changes and impairment caused by hazardous substances, medical treatment and extensive record keeping. Pre-employment screening involves the evaluation of occupational and medical history questionnaires and results of physical examinations. Questionnaires provide information concerning past illnesses and chronic diseases (especially asthma, skin, lung and heart diseases) and past occupational exposures. There are ethical and legal implications of pre-employment screening programmes if they are used to determine employment eligibility. However, they are fundamentally important when used to (1) provide a record of previous employment and associated exposures, (2) establish a baseline of health for an employee and (3) test for hypersusceptibility. Medical examinations can include audiometric tests for hearing loss, vision tests, tests of organ function, evaluation of fitness for wearing respiratory protection equipment, and baseline urine and blood tests. Periodic medical examinations are essential for evaluating and detecting trends in the onset of adverse health effects and may include biological monitoring for specific contaminants and the use of other biomarkers.

Environmental and Biological Surveillance

Environmental and biological surveillance starts with an occupational hygiene survey of the work environment to identify potential hazards and contaminant sources, and determine the need for monitoring. For chemical agents, monitoring could involve air, bulk, surface and biological sampling. For physical agents, monitoring could include noise, temperature and radiation measurements. If monitoring is indicated, the occupational hygienist must develop a sampling strategy that includes which employees, processes, equipment or areas to sample, the number of samples, how long to sample, how often to sample, and the sampling method. Industrial hygiene surveys vary in complexity and focus depending upon the purpose of the investigation, type and size of establishment, and nature of the problem.

There are no rigid formulas for performing surveys; however, thorough preparation prior to the on-site inspection significantly increases effectiveness and efficiency. Investigations that are motivated by employee complaints and illnesses have an additional focus of identifying the cause of the health problems. Indoor air quality surveys focus on indoor as well as outdoor sources of contamination. Regardless of the occupational hazard, the overall approach to surveying and sampling workplaces is similar; therefore, this chapter will use chemical agents as a model for the methodology.

Routes of Exposure

The mere presence of occupational stresses in the workplace does not automatically imply that there is a significant potential for exposure; the agent must reach the worker. For chemicals, the liquid or vapour form of the agent must make contact with and/or be absorbed into the body to induce an adverse health effect. If the agent is isolated in an enclosure or captured by a local exhaust ventilation system, the exposure potential will be low, regardless of the chemical’s inherent toxicity.

The route of exposure can impact the type of monitoring performed as well as the hazard potential. For chemical and biological agents, workers are exposed through inhalation, skin contact, ingestion and injection; the most common routes of absorption in the occupational environment are through the respiratory tract and the skin. To assess inhalation, the occupational hygienist observes the potential for chemicals to become airborne as gases, vapours, dusts, fumes or mists.

Skin absorption of chemicals is important primarily when there is direct contact with the skin through splashing, spraying, wetting or immersion with fat-soluble hydrocarbons and other organic solvents. Immersion includes body contact with contaminated clothing, hand contact with contaminated gloves, and hand and arm contact with bulk liquids. For some substances, such as amines and phenols, skin absorption can be as rapid as absorption through the lungs for substances that are inhaled. For some contaminants such as pesticides and benzidine dyes, skin absorption is the primary route of absorption, and inhalation is a secondary route. Such chemicals can readily enter the body through the skin, increase body burden and cause systemic damage. When allergic reactions or repeated washing dries and cracks the skin, there is a dramatic increase in the number and type of chemicals that can be absorbed into the body. Ingestion, an uncommon route of absorption for gases and vapours, can be important for particulates, such as lead. Ingestion can occur from eating contaminated food, eating or smoking with contaminated hands, and coughing and then swallowing previously inhaled particulates.

Injection of materials directly into the bloodstream can occur from hypodermic needles inadvertently puncturing the skin of health care workers in hospitals, and from high-velocity projectiles released from high-pressure sources and directly contacting the skin. Airless paint sprayers and hydraulic systems have pressures high enough to puncture the skin and introduce substances directly into the body.

The Walk-Through Inspection

The purpose of the initial survey, called the walk-through inspection, is to systematically gather information to judge whether a potentially hazardous situation exists and whether monitoring is indicated. An occupational hygienist begins the walk-through survey with an opening meeting that can include representatives of management, employees, supervisors, occupational health nurses and union representatives. The occupational hygienist can powerfully impact the success of the survey and any subsequent monitoring initiatives by creating a team of people who communicate openly and honestly with one another and understand the goals and scope of the inspection. Workers must be involved and informed from the beginning to ensure that cooperation, not fear, dominates the investigation.

During the meeting, requests are made for process flow diagrams, plant layout drawings, past environmental inspection reports, production schedules, equipment maintenance schedules, documentation of personal protection programmes, and statistics concerning the number of employees, shifts and health complaints. All hazardous materials used and produced by an operation are identified and quantified. A chemical inventory of products, by-products, intermediates and impurities is assembled and all associated Material Safety Data Sheets are obtained. Equipment maintenance schedules, age and condition are documented because the use of older equipment may result in higher exposures due to the lack of controls.

After the meeting, the occupational hygienist performs a visual walk-through survey of the workplace, scrutinizing the operations and work practices, with the goal of identifying potential occupational stresses, ranking the potential for exposure, identifying the route of exposure and estimating the duration and frequency of exposure. Examples of occupational stresses are given in figure 1. The occupational hygienist uses the walk-through inspection to observe the workplace and have questions answered. Examples of observations and questions are given in figure 2.

Figure 1.  Occupational stresses. 

Figure 2.  Observations and questions to ask on a walk-through survey.

In addition to the questions shown in figure 5, questions should be asked that uncover what is not immediately obvious. Questions could address:

1. non-routine tasks and schedules for maintenance and cleaning activities
2. recent process changes and chemical substitutions
3. recent physical changes in the work environment
4. changes in job functions
5. recent renovations and repairs.

Non-routine tasks can result in significant peak exposures to chemicals that are difficult to predict and measure during a typical workday. Process changes and chemical substitutions may alter the release of substances into the air and affect subsequent exposure. Changes in the physical layout of a work area can alter the effectiveness of an existing ventilation system. Changes in job functions can result in tasks performed by inexperienced workers and increased exposures. Renovations and repairs may introduce new materials and chemicals into the work environment which off-gas volatile organic chemicals or are irritants.

Indoor Air Quality Surveys

Indoor air quality surveys are distinct from traditional occupational hygiene surveys because they are typically encountered in non-industrial workplaces and may involve exposures to mixtures of trace quantities of chemicals, none of which alone appears capable of causing illness (Ness 1991). The goal of indoor air quality surveys is similar to occupational hygiene surveys in terms of identifying sources of contamination and determining the need for monitoring. However, indoor air quality surveys are always motivated by employee health complaints. In many cases, the employees have a variety of symptoms including headaches, throat irritation, lethargy, coughing, itching, nausea and non-specific hypersensitivity reactions that disappear when they go home. When health complaints do not disappear after the employees leave work, non-occupational exposures should be considered as well. Non-occupational exposures include hobbies, other jobs, urban air pollution, passive smoking and indoor exposures in the home. Indoor air quality surveys frequently use questionnaires to document employee symptoms and complaints and link them to job location or job function within the building. The areas with the highest incidence of symptoms are then targeted for further inspection.

Sources of indoor air contaminants that have been documented in indoor air quality surveys include:

• inadequate ventilation (52%)
• contamination from inside of the building (17%)
• contamination from outside of the building (11%)
• microbial contamination (5%)
• contamination from the building materials (3%)
• unknown causes (12%).

For indoor air quality investigations, the walk-through inspection is essentially a building and environmental inspection to determine potential sources of contamination both inside and outside of the building. Inside building sources include:

1. building construction materials such as insulation, particleboard, adhesives and paints
2. human occupants that can release chemicals from metabolic activities
3. human activities such as smoking
4. equipment such as copy machines
5. ventilation systems that can be contaminated with micro-organisms.

Observations and questions that can be asked during the survey are listed in figure 3.

Figure 3. Observations and questions for an indoor air quality walk-through survey.

Sampling and Measurement Strategies

Occupational exposure limits

After the walk-through inspection is completed, the occupational hygienist must determine whether sampling is necessary; sampling should be performed only if the purpose is clear. The occupational hygienist must ask, “What will be made of the sampling results and what questions will the results answer?” It is relatively easy to sample and obtain numbers; it is far more difficult to interpret them.

Air and biological sampling data are usually compared to recommended or mandated occupational exposure limits (OELs). Occupational exposure limits have been developed in many countries for inhalation and biological exposure to chemical and physical agents. To date, out of a universe of over 60,000 commercially used chemicals, approximately 600 have been evaluated by a variety of organizations and countries. The philosophical bases for the limits are determined by the organizations that have developed them. The most widely used limits, called threshold limit values (TLVs), are those issued in the United States by the American Conference of Governmental Industrial Hygienists (ACGIH). Most of the OELs used by the Occupational Safety and Health Administration (OSHA) in the United States are based upon the TLVs. However, the National Institute for Occupational Safety and Health (NIOSH) of the US Department of Health and Human Services has suggested their own limits, called recommended exposure limits (RELs).

For airborne exposures, there are three types of TLVs: an eight-hour time-weighted-average exposure, TLV-TWA, to protect against chronic health effects; a fifteen-minute average short-term exposure limit, TLV-STEL, to protect against acute health effects; and an instantaneous ceiling value, TLV-C, to protect against asphyxiants or chemicals that are immediately irritating. Guidelines for biological exposure levels are called biological exposure indices (BEIs). These guidelines represent the concentration of chemicals in the body that would correspond to inhalation exposure of a healthy worker at a specific concentration in air. Outside of the United States as many as 50 countries or groups have established OELs, many of which are identical to the TLVs. In Britain, the limits are called the Health and Safety Executive Occupational Exposure Standards (OES), and in Germany OELs are called Maximum Workplace Concentrations (MAKs).

OELs have been set for airborne exposures to gases, vapours and particulates; they do not exist for airborne exposures to biological agents. Therefore, most investigations of bioaerosol exposure compare indoor with outdoor concentrations. If the indoor/outdoor profile and concentration of organisms is different, an exposure problem may exist. There are no OELs for skin and surface sampling, and each case must be evaluated separately. In the case of surface sampling, concentrations are usually compared with acceptable background concentrations that were measured in other studies or were determined in the current study. For skin sampling, acceptable concentrations are calculated based upon toxicity, rate of absorption, amount absorbed and total dose. In addition, biological monitoring of a worker may be used to investigate skin absorption.

Sampling strategy

An environmental and biological sampling strategy is an approach to obtaining exposure measurements that fulfils a purpose. A carefully designed and effective strategy is scientifically defensible, optimizes the number of samples obtained, is cost-effective and prioritizes needs. The goal of the sampling strategy guides decisions concerning what to sample (selection of chemical agents), where to sample (personal, area or source sample), whom to sample (which worker or group of workers), sample duration (real-time or integrated), how often to sample (how many days), how many samples, and how to sample (analytical method). Traditionally, sampling performed for regulatory purposes involves brief campaigns (one or two days) that concentrate on worst-case exposures. While this strategy requires a minimum expenditure of resources and time, it often captures the least amount of information and has little applicability to evaluating long-term occupational exposures. To evaluate chronic exposures so that they are useful for occupational physicians and epidemiological studies, sampling strategies must involve repeated sampling over time for large numbers of workers.

Purpose

The goal of environmental and biological sampling strategies is either to evaluate individual employee exposures or to evaluate contaminant sources. Employee monitoring may be performed to:

Gas chromatography is a technique used to separate and concentrate chemicals in mixtures for subsequent quantitative analysis. There are three main components to the system: the sample injection system, a column and a detector. A liquid or gaseous sample is injected using a syringe, into an air stream that carries the sample through a column where the components are separated. The column is packed with materials that interact differently with different chemicals, and slows down the movement of the chemicals. The differential interaction causes each chemical to travel through the column at a different rate. After separation, the chemicals go directly into a detector, such as a flame ionization detector (FID), photo-ionization detector (PID) or electron capture detector (ECD); a signal proportional to concentration is registered on a chart recorder. The FID is used for almost all organics including: aromatics, straight chain hydrocarbons, ketones and some chlorinated hydrocarbons. Concentration is measured by the increase in the number of ions produced as a volatile hydrocarbon is burned by a hydrogen flame. The PID is used for organics and some inorganics; it is especially useful for aromatic compounds such as benzene, and it can detect aliphatic, aromatic and halogenated hydrocarbons. Concentration is measured by the increase in the number of ions produced when the sample is bombarded by ultraviolet radiation. The ECD is primarily used for halogen-containing chemicals; it gives a minimal response to hydrocarbons, alcohols and ketones. Concentration is measured by the current flow between two electrodes caused by ionization of the gas by radioactivity.

The mass spectrophotometer is used to analyse complex mixtures of chemicals present in trace amounts. It is often coupled with a gas chromatograph for the separation and quantification of different contaminants.

Atomic absorption spectroscopy is primarily used for the quantification of metals such as mercury. Atomic absorption is the absorption of light of a particular wavelength by a free, ground-state atom; the quantity of light absorbed is related to concentration. The technique is highly specific, sensitive and fast, and is directly applicable to approximately 68 elements. Detection limits are in the sub-ppb to low-ppm range.

Infrared analysis is a powerful, sensitive, specific and versatile technique. It uses the absorption of infrared energy to measure many inorganic and organic chemicals; the amount of light absorbed is proportional to concentration. The absorption spectrum of a compound provides information enabling its identification and quantification.

UV absorption spectroscopy is used for analysis of aromatic hydrocarbons when interferences are known to be low. The amount of absorption of UV light is directly proportional to concentration.

Polarographic methods are based upon the electrolysis of a sample solution using an easily polarized electrode and a nonpolarizable electrode. They are used for qualitative and quantitative analysis of aldehydes, chlorinated hydrocarbons and metals.

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After a hazard has been recognized and evaluated, the most appropriate interventions (methods of control) for a particular hazard must be determined. Control methods usually fall into three categories:

1. engineering controls
2. administrative controls
3. personal protective equipment.

As with any change in work processes, training must be provided to ensure the success of the changes.

Engineering controls are changes to the process or equipment that reduce or eliminate exposures to an agent. For example, substituting a less toxic chemical in a process or installing exhaust ventilation to remove vapours generated during a process step, are examples of engineering controls. In the case of noise control, installing sound-absorbing materials, building enclosures and installing mufflers on air exhaust outlets are examples of engineering controls. Another type of engineering control might be changing the process itself. An example of this type of control would be removal of one or more degreasing steps in a process that originally required three degreasing steps. By removing the need for the task that produced the exposure, the overall exposure for the worker has been controlled. The advantage of engineering controls is the relatively small involvement of the worker, who can go about the job in a more controlled environment when, for instance, contaminants are automatically removed from the air. Contrast this to the situation where the selected method of control is a respirator to be worn by the worker while performing the task in an “uncontrolled” workplace. In addition to the employer actively installing engineering controls on existing equipment, new equipment can be purchased that contains the controls or other more effective controls. A combination approach has often been effective (i.e., installing some engineering controls now and requiring personal protective equipment until new equipment arrives with more effective controls that will eliminate the need for personal protective equipment). Some common examples of engineering controls are:

• ventilation (both general and local exhaust ventilation)
• isolation (place a barrier between the worker and the agent)
• substitution (substitute less toxic, less flammable material, etc.)
• change the process (eliminate hazardous steps).

The occupational hygienist must be sensitive to the worker’s job tasks and must solicit worker participation when designing or selecting engineering controls. Placing barriers in the workplace, for example, could significantly impair a worker’s ability to perform the job and may encourage “work arounds”. Engineering controls are the most effective methods of reducing exposures. They are also, often, the most expensive. Since engineering controls are effective and expensive it is important to maximize the involvement of the workers in the selection and design of the controls. This should result in a greater likelihood that the controls will reduce exposures.

Administrative controls involve changes in how a worker accomplishes the necessary job tasks—for example, how long they work in an area where exposures occur, or changes in work practices such as improvements in body positioning to reduce exposures. Administrative controls can add to the effectiveness of an intervention but have several drawbacks:

1. Rotation of workers may reduce overall average exposure for the workday but it provides periods of high short-term exposure for a larger number of workers. As more becomes known about toxicants and their modes of action, short-term peak exposures may represent a greater risk than would be calculated based on their contribution to average exposure.
2. Changing work practices of workers can present a significant enforcement and monitoring challenge. How work practices are enforced and monitored determines whether or not they will be effective. This constant management attention is a significant cost of administrative controls.

Personal protective equipment consists of devices provided to the worker and required to be worn while performing certain (or all) job tasks. Examples include respirators, chemical goggles, protective gloves and faceshields. Personal protective equipment is commonly used in cases where engineering controls have not been effective in controlling the exposure to acceptable levels or where engineering controls have not been found to be feasible (for cost or operational reasons). Personal protective equipment can provide significant protection to workers if worn and used correctly. In the case of respiratory protection, protection factors (ratio of concentration outside the respirator to that inside) can be 1,000 or more for positive-pressure supplied air respirators or ten for half-face air-purifying respirators. Gloves (if selected appropriately) can protect hands for hours from solvents. Goggles can provide effective protection from chemical splashes.

Intervention: Factors to Consider

Often a combination of controls is used to reduce the exposures to acceptable levels. Whatever methods are selected, the intervention must reduce the exposure and resulting hazard to an acceptable level. There are, however, many other factors that need to be considered when selecting an intervention. For example:

• effectiveness of the controls
• ease of use by the employee
• cost of the controls
• adequacy of the warning properties of the material
• acceptable level of exposure
• frequency of exposure
• route(s) of exposure
• regulatory requirements for specific controls.

Effectiveness of controls

Effectiveness of controls is obviously a prime consideration when taking action to reduce exposures. When comparing one type of intervention to another, the level of protection required must be appropriate for the challenge; too much control is a waste of resources. Those resources could be used to reduce other exposures or exposures of other employees. On the other hand, too little control leaves the worker exposed to unhealthy conditions. A useful first step is to rank the interventions according to their effectiveness, then use this ranking to evaluate the significance of the other factors.

Ease of use

For any control to be effective the worker must be able to perform his or her job tasks with the control in place. For example, if the control method selected is substitution, then the worker must know the hazards of the new chemical, be trained in safe handling procedures, understand proper disposal procedures, and so on. If the control is isolation—placing an enclosure around the substance or the worker—the enclosure must allow the worker to do his or her job. If the control measures interfere with the tasks of the job, the worker will be reluctant to use them and may find ways to accomplish the tasks that could result in increased, not decreased, exposures.

Cost

Every organization has limits on resources. The challenge is to maximize the use of those resources. When hazardous exposures are identified and an intervention strategy is being developed, cost must be a factor. The “best buy” many times will not be the lowest- or highest-cost solutions. Cost becomes a factor only after several viable methods of control have been identified. Cost of the controls can then be used to select the controls that will work best in that particular situation. If cost is the determining factor at the outset, poor or ineffective controls may be selected, or controls that interfere with the process in which the employee is working. It would be unwise to select an inexpensive set of controls that interfere with and slow down a manufacturing process. The process then would have a lower throughput and higher cost. In very short time the “real” costs of these “low cost” controls would become enormous. Industrial engineers understand the layout and overall process; production engineers understand the manufacturing steps and processes; the financial analysts understand the resource allocation problems. Occupational hygienists can provide a unique insight into these discussions due to their understanding of the specific employee’s job tasks, the employee’s interaction with the manufacturing equipment as well as how the controls will work in a particular setting. This team approach increases the likelihood of selecting the most appropriate (from a variety of perspectives) control.

Adequacy of warning properties

When protecting a worker against an occupational health hazard, the warning properties of the material, such as odour or irritation, must be considered. For example, if a semiconductor worker is working in an area where arsine gas is used, the extreme toxicity of the gas poses a significant potential hazard. The situation is compounded by arsine’s very poor warning properties—the workers cannot detect the arsine gas by sight or smell until it is well above acceptable levels. In this case, controls that are marginally effective at keeping exposures below acceptable levels should not be considered because excursions above acceptable levels cannot be detected by the workers. In this case, engineering controls should be installed to isolate the worker from the material. In addition, a continuous arsine gas monitor should be installed to warn workers of the failure of the engineering controls. In situations involving high toxicity and poor warning properties, preventive occupational hygiene is practised. The occupational hygienist must be flexible and thoughtful when approaching an exposure problem.

Acceptable level of exposure

If controls are being considered to protect a worker from a substance such as acetone, where the acceptable level of exposure may be in the range of 800 ppm, controlling to a level of 400 ppm or less may be achieved relatively easily. Contrast the example of acetone control to control of 2-ethoxyethanol, where the acceptable level of exposure may be in the range of 0.5 ppm. To obtain the same per cent reduction (0.5 ppm to 0.25 ppm) would probably require different controls. In fact, at these low levels of exposure, isolation of the material may become the primary means of control. At high levels of exposure, ventilation may provide the necessary reduction. Therefore, the acceptable level determined (by the government, company, etc.) for a substance can limit the selection of controls.

Frequency of exposure

When assessing toxicity the classic model uses the following relationship:

TIME x CONCENTRATION = DOSE 

Dose, in this case, is the amount of material being made available for absorption. The previous discussion focused on minimizing (lowering) the concentration portion of this relationship. One might also reduce the time spent being exposed (the underlying reason for administrative controls). This would similarly reduce the dose. The issue here is not the employee spending time in a room, but how often an operation (task) is performed. The distinction is important. In the first example, the exposure is controlled by removing the workers when they are exposed to a selected amount of toxicant; the intervention effort is not directed at controlling the amount of toxicant (in many situations there may be a combination approach). In the second case, the frequency of the operation is being used to provide the appropriate controls, not to determine a work schedule. For example, if an operation such as degreasing is performed routinely by an employee, the controls may include ventilation, substitution of a less toxic solvent or even automation of the process. If the operation is performed rarely (e.g., once per quarter) personal protective equipment may be an option (depending on many of the factors described in this section). As these two examples illustrate, the frequency with which an operation is performed can directly affect the selection of controls. Whatever the exposure situation, the frequency with which a worker performs the tasks must be considered and factored into the control selection.

Route of exposure obviously is going to affect the method of control. If a respiratory irritant is present, ventilation, respirators, and so on, would be considered. The challenge for the occupational hygienist is identifying all routes of exposure. For example, glycol ethers are used as a carrier solvent in printing operations. Breathing-zone air concentrations can be measured and controls implemented. Glycol ethers, however, are rapidly absorbed through intact skin. The skin represents a significant route of exposure and must be considered. In fact, if the wrong gloves are chosen, the skin exposure may continue long after the air exposures have decreased (due to the employee continuing to use gloves that have experienced breakthrough). The hygienist must evaluate the substance—its physical properties, chemical and toxicological properties, and so on—to determine what routes of exposure are possible and plausible (based on the tasks performed by the employee).

In any discussion of controls, one of the factors that must be considered is the regulatory requirements for controls. There may well be codes of practice, regulations, and so on, that require a specific set of controls. The occupational hygienist has flexibility above and beyond the regulatory requirements, but the minimum mandated controls must be installed. Another aspect of the regulatory requirements is that the mandated controls may not work as well or may conflict with the best judgement of the occupational hygienist. The hygienist must be creative in these situations and find solutions that satisfy the regulatory as well as best practice goals of the organization.

Training and Labelling

Regardless of what form of intervention is eventually selected, training and other forms of notification must be provided to ensure that the workers understand the interventions, why they were selected, what reductions in exposure are expected, and the role of the workers in achieving those reductions. Without the participation and understanding of the workforce, the interventions will likely fail or at least operate at reduced efficiency. Training builds hazard awareness in the workforce. This new awareness can be invaluable to the occupational hygienist in identifying and reducing previously unrecognized exposures or new exposures.

Training, labelling and related activities may be part of a regulatory compliance scheme. It would be prudent to check the local regulations to ensure that whatever type of training or labelling is undertaken satisfies the regulatory as well as operational requirements.

Conclusion

In this short discussion on interventions, some general considerations have been presented to stimulate thought. In practice, these rules become very complex and often have significant ramifications for employee and company health. The occupational hygienist’s professional judgement is essential in selecting the best controls. Best is a term with many different meanings. The occupational hygienist must become adept at working in teams and soliciting input from the workers, management and technical staff.

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Workplace exposure assessment is concerned with identifying and evaluating agents with which a worker may come in contact, and exposure indices can be constructed to reflect the amount of an agent present in the general environment or in inhaled air, as well as to reflect the amount of agent that is actually inhaled, swallowed or otherwise absorbed (the intake). Other indices include the amount of agent that is resorbed (the uptake) and the exposure at the target organ. Dose is a pharmacological or toxicological term used to indicate the amount of a substance administered to a subject. Dose rate is the amount administered per unit of time. The dose of a workplace exposure is difficult to determine in a practical situation, since physical and biological processes, like inhalation, uptake and distribution of an agent in the human body cause exposure and dose to have complex, non-linear relationships. The uncertainty about the actual level of exposure to agents also makes it difficult to quantify relationships between exposure and health effects.

For many occupational exposures there exists a time window during which the exposure or dose is most relevant to the development of a particular health-related problem or symptom. Hence, the biologically relevant exposure, or dose, would be that exposure which occurs during the relevant time window. Some exposures to occupational carcinogens are believed to have such a relevant time window of exposure. Cancer is a disease with a long latency period, and hence it could be that the exposure which is related to the ultimate development of the disease took place many years before the cancer actually manifested itself. This phenomenon is counter-intuitive, since one would have expected that cumulative exposure over a working lifetime would have been the relevant parameter. The exposure at the time of manifestation of disease may not be of particular importance.

The pattern of exposure—continuous exposure, intermittent exposure and exposure with or without sharp peaks—may be relevant as well. Taking exposure patterns into account is important for both epidemiological studies and for environmental measurements which may be used to monitor compliance with health standards or for environmental control as part of control and prevention programmes. For example, if a health effect is caused by peak exposures, such peak levels must be monitorable in order to be controlled. Monitoring which provides data only about long-term average exposures is not useful since the peak excursion values may well be masked by averaging, and certainly cannot be controlled as they occur.

The biologically relevant exposure or dose for a certain endpoint is often not known because the patterns of intake, uptake, distribution and elimination, or the mechanisms of biotransformation, are not understood in sufficient detail. Both the rate at which an agent enters and leaves the body (the kinetics) and the biochemical processes for handling the substance (biotransformation) will help determine the relationships between exposure, dose and effect.

Environmental monitoring is the measurement and assessment of agents at the workplace to evaluate ambient exposure and related health risks. Biological monitoring is the measurement and assessment of workplace agents or their metabolites in tissue, secreta or excreta to evaluate exposure and assess health risks. Sometimes biomarkers, such as DNA-adducts, are used as measures of exposure. Biomarkers may also be indicative of the mechanisms of the disease process itself, but this is a complex subject, which is covered more fully in the chapter Biological Monitoring and later in the discussion here.

A simplification of the basic model in exposure-response modelling is as follows:

exposure uptake distribution,

elimination, transformationtarget dosephysiopathologyeffect

Depending on the agent, exposure-uptake and exposure-intake relationships can be complex. For many gases, simple approximations can be made, based on the concentration of the agent in the air during the course of a working day and on the amount of air that is inhaled. For dust sampling, deposition patterns are also related to particle size. Size considerations may also lead to a more complex relationship. The chapter Respiratory System provides more detail on the aspect of respiratory toxicity.

Exposure and dose assessment are elements of quantitative risk assessment. Health risk assessment methods often form the basis upon which exposure limits are established for emission levels of toxic agents in the air for environmental as well as for occupational standards. Health risk analysis provides an estimate of the probability (risk) of occurrence of specific health effects or an estimate of the number of cases with these health effects. By means of health risk analysis an acceptable concentration of a toxicant in air, water or food can be provided, given an a priori chosen acceptable magnitude of risk. Quantitative risk analysis has found an application in cancer epidemiology, which explains the strong emphasis on retrospective exposure assessment. But applications of more elaborate exposure assessment strategies can be found in both retrospective as well as prospective exposure assessment, and exposure assessment principles have found applications in studies focused on other endpoints as well, such as benign respiratory disease (Wegman et al. 1992; Post et al. 1994). Two directions in research predominate at this moment. One uses dose estimates obtained from exposure monitoring information, and the other relies on biomarkers as measures of exposure.

Exposure Monitoring and Prediction of Dose

Unfortunately, for many exposures few quantitative data are available for predicting the risk for developing a certain endpoint. As early as 1924, Haber postulated that the severity of the health effect (H) is proportional to the product of exposure concentration (X) and time of exposure (T):

H=X x T

Haber’s law, as it is called, formed the basis for development of the concept that time-weighted average (TWA) exposure measurements—that is, measurements taken and averaged over a certain period of time—would be a useful measure for the exposure. This assumption about the adequacy of the time-weighted average has been questioned for many years. In 1952, Adams and co-workers stated that “there is no scientific basis for the use of the time-weighted average to integrate varying exposures …” (in Atherly 1985). The problem is that many relations are more complex than the relationship that Haber’s law represents. There are many examples of agents where the effect is more strongly determined by concentration than by length of time. For example, interesting evidence from laboratory studies has shown that in rats exposed to carbon tetrachloride, the pattern of exposure (continuous versus intermittent and with or without peaks) as well as the dose can modify the observed risk of the rats developing liver enzyme level changes (Bogers et al. 1987). Another example is bio-aerosols, such as α-amylase enzyme, a dough improver, which can cause allergic diseases in people who work in the bakery industry (Houba et al. 1996). It is unknown whether the risk of developing such a disease is mainly determined by peak exposures, average exposure, or cumulative level of exposure. (Wong 1987; Checkoway and Rice 1992). Information on temporal patterns is not available for most agents, especially not for agents that have chronic effects.

The first attempts to model exposure patterns and estimate dose were published in the 1960s and 1970s by Roach (1966; 1977). He showed that the concentration of an agent reaches an equilibrium value at the receptor after an exposure of infinite duration because elimination counterbalances the uptake of the agent. In an eight-hour exposure, a value of 90% of this equilibrium level can be reached if the half-life of the agent at the target organ is smaller than approximately two-and-a-half hours. This illustrates that for agents with a short half-life, the dose at the target organ is determined by an exposure shorter than an eight-hour period. Dose at the target organ is a function of the product of exposure time and concentration for agents with a long half-life. A similar but more elaborate approach has been applied by Rappaport (1985). He showed that intra-day variability in exposure has a limited influence when dealing with agents with long half-lives. He introduced the term dampening at the receptor.

The information presented above has mainly been used to draw conclusions on appropriate averaging times for exposure measurements for compliance purposes. Since Roach’s papers it is common knowledge that for irritants, grab samples with short averaging times have to be taken, while for agents with long half-lives, such as asbestos, long-term average of cumulative exposure has to be approximated. One should however realize that the dichotomization into grab sample strategies and eight-hour time average exposure strategies as adopted in many countries for compliance purposes is an extremely crude translation of the biological principles discussed above.

An example of improving an exposure assessment strategy based on pharmocokinetic principles in epidemiology can be found in a paper of Wegman et al. (1992). They applied an interesting exposure assessment strategy by using continuous monitoring devices to measure personal dust exposure peak levels and relating these to acute reversible respiratory symptoms occurring every 15 minutes.A conceptual problem in this kind of study, extensively discussed in their paper, is the definition of a health-relevant peak exposure. The definition of a peak will, again, depend on biological considerations. Rappaport (1991) gives two requirements for peak exposures to be of aetiological relevance in the disease process: (1) the agent is eliminated rapidly from the body and (2) there is a non-linear rate of biological damage during a peak exposure. Non-linear rates of biological damage may be related to changes in uptake, which in turn are related to exposure levels, host susceptibility, synergy with other exposures, involvement of other disease mechanisms at higher exposures or threshold levels for disease processes.

These examples also show that pharmacokinetic approaches can lead elsewhere than to dose estimates. The results of pharmacokinetic modelling can also be used to explore the biological relevance of existing indices of exposure and to design new health-relevant exposure assessment strategies.

Pharmacokinetic modelling of the exposure may also generate estimates of the actual dose at the target organ. For instance in the case of ozone, an acute irritant gas, models have been developed which predict the tissue concentration in the airways as a function of the average ozone concentration in the airspace of the lung at a certain distance from the trachea, the radius of the airways, the average air velocity, the effective dispersion, and the ozone flux from air to lung surface (Menzel 1987; Miller and Overton 1989). Such models can be used to predict ozone dose in a particular region of the airways, dependent on environmental ozone concentrations and breathing patterns.

In most cases estimates of target dose are based on information on the exposure pattern over time, job history and pharmacokinetic information on uptake, distribution, elimination and transformation of the agent. The whole process can be described by a set of equations which can be mathematically solved. Often information on pharmacokinetic parameters is not available for humans, and parameter estimates based on animal experiments have to be used. There are several examples by now of the use of pharmacokinetic modelling of exposure in order to generate dose estimates. The first references to modelling of exposure data into dose estimates in the literature go back to the paper of Jahr (1974).

Although dose estimates have generally not been validated and have found limited application in epidemiological studies, the new generation of exposure or dose indices is expected to result in optimal exposure-response analyses in epidemiological studies (Smith 1985, 1987). A problem not yet tackled in pharmacokinetic modelling is that large interspecies differences exist in kinetics of toxic agents, and therefore effects of intra-individual variation in pharmacokinetic parameters are of interest (Droz 1992).

Biomonitoring and Biomarkers of Exposure

Biological monitoring offers an estimate of dose and therefore is often considered superior to environmental monitoring. However, the intra-individual variability of biomonitoring indices can be considerable. In order to derive an acceptable estimate of a worker’s dose, repeated measurements have to be taken, and sometimes the measurement effort can become larger than for environmental monitoring.

This is illustrated by an interesting study on workers producing boats made of plastic reinforced with glass fibre (Rappaport et al. 1995). The variability of styrene exposure was assessed by measuring styrene in air repeatedly. Styrene in exhaled air of exposed workers was monitored, as well as sister chromatid exchanges (SCEs). They showed that an epidemiological study using styrene in the air as a measure of exposure would be more efficient, in terms of numbers of measurements required, than a study using the other indices of exposure. For styrene in air three repeats were required to estimate the long-term average exposure with a given precision. For styrene in exhaled air, four repeats per worker were necessary, while for the SCEs 20 repeats were necessary. The explanation for this observation is the signal-to-noise ratio, determined by the day-to-day and between-worker variability in exposure, which was more favourable for styrene in air than for the two biomarkers of exposure. Thus, although the biological relevance of a certain exposure surrogate might be optimal, the performance in an exposure-response analysis can still be poor because of a limited signal-to-noise ratio, leading to misclassification error.

Droz (1991) applied pharmacokinetic modelling to study advantages of exposure assessment strategies based on air sampling compared to biomonitoring strategies dependent on the half-life of the agent. He showed that biological monitoring is also greatly affected by biological variability, which is not related to variability of the toxicological test. He suggested that no statistical advantage exists in using biological indicators when the half-life of the agent considered is smaller than about ten hours.

Although one might tend to decide to measure the environmental exposure instead of a biological indicator of an effect because of variability in the variable measured, additional arguments can be found for choosing a biomarker, even when this would lead to a greater measurement effort, such as when a considerable dermal exposure is present. For agents like pesticides and some organic solvents, dermal exposure can be of greater relevance that the exposure through the air. A biomarker of exposure would include this route of exposure, while measuring of dermal exposure is complex and results are not easily interpretable (Boleij et al. 1995). Early studies among agricultural workers using “pads” to assess dermal exposure showed remarkable distributions of pesticides over the body surface, depending on the tasks of the worker. However, because little information is available on skin uptake, exposure profiles cannot yet be used to estimate an internal dose.

Biomarkers can also have considerable advantages in cancer epidemiology. When a biomarker is an early marker of the effect, its use could result in reduction of the follow-up period. Although validation studies are required, biomarkers of exposure or individual susceptibility could result in more powerful epidemiological studies and more precise risk estimates.

Time Window Analysis

Parallel to the development of pharmacokinetic modelling, epidemiologists have explored new approaches in the data analysis phase such as “time frame analysis” to relate relevant exposure periods to endpoints, and to implement effects of temporal patterns in the exposure or peak exposures in occupational cancer epidemiology (Checkoway and Rice 1992). Conceptually this technique is related to pharmacokinetic modelling since the relationship between exposure and outcome is optimized by putting weights on different exposure periods, exposure patterns and exposure levels. In pharmacokinetic modelling these weights are believed to have a physiological meaning and are estimated beforehand. In time frame analysis the weights are estimated from the data on the basis of statistical criteria. Examples of this approach are given by Hodgson and Jones (1990), who analysed the relationship between radon gas exposure and lung cancer in a cohort of UK tin miners, and by Seixas, Robins and Becker (1993), who analysed the relationship between dust exposure and respiratory health in a cohort of US coal miners. A very interesting study underlining the relevance of time window analysis is the one by Peto et al. (1982).

They showed that mesothelioma death rates appeared to be proportional to some function of time since first exposure and cumulative exposure in a cohort of insulation workers. Time since first exposure was of particular relevance because this variable was an approximation of the time required for a fibre to migrate from its place of deposition in the lungs to the pleura. This example shows how kinetics of deposition and migration determine the risk function to a large extent. A potential problem with time frame analysis is that it requires detailed information on exposure periods and exposure levels, which hampers its application in many studies of chronic disease outcomes.

Concluding Remarks

In conclusion, the underlying principles of pharmacokinetic modelling and time frame or time window analysis are widely recognized. Knowledge in this area has mainly been used to develop exposure assessment strategies. More elaborate use of these approaches, however, requires a considerable research effort and has to be developed. The number of applications is therefore still limited. Relatively simple applications, such as the development of more optimal exposure assessment strategies dependent on the endpoint, have found wider use. An important issue in the development of biomarkers of exposure or effect is validation of these indices. It is often assumed that a measurable biomarker can predict health risk better than traditional methods. However, unfortunately, very few validation studies substantiate this assumption.

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The History of Occupational Exposure Limits

Over the past 40 years, many organizations in numerous countries have proposed occupational exposure limits (OELs) for airborne contaminants. The limits or guidelines that have gradually become the most widely accepted both in the United States and in most other countries are those issued annually by the American Conference of Governmental Industrial Hygienists (ACGIH), which are termed threshold limit values (TLVs) (LaNier 1984; Cook 1986; ACGIH 1994).

The usefulness of establishing OELs for potentially harmful agents in the working environment has been demonstrated repeatedly since their inception (Stokinger 1970; Cook 1986; Doull 1994). The contribution of OELs to the prevention or minimization of disease is now widely accepted, but for many years such limits did not exist, and even when they did, they were often not observed (Cook 1945; Smyth 1956; Stokinger 1981; LaNier 1984; Cook 1986).

It was well understood as long ago as the fifteenth century, that airborne dusts and chemicals could bring about illness and injury, but the concentrations and lengths of exposure at which this might be expected to occur were unclear (Ramazinni 1700).

As reported by Baetjer (1980), “early in this century when Dr. Alice Hamilton began her distinguished career in occupational disease, no air samples and no standards were available to her, nor indeed were they necessary. Simple observation of the working conditions and the illness and deaths of the workers readily proved that harmful exposures existed. Soon however, the need for determining standards for safe exposure became obvious.”

The earliest efforts to set an OEL were directed to carbon monoxide, the toxic gas to which more persons are occupationally exposed than to any other (for a chronology of the development of OELs, see figure 1. The work of Max Gruber at the Hygienic Institute at Munich was published in 1883. The paper described exposing two hens and twelve rabbits to known concentrations of carbon monoxide for up to 47 hours over three days; he stated that “the boundary of injurious action of carbon monoxide lies at a concentration in all probability of 500 parts per million, but certainly (not less than) 200 parts per million”. In arriving at this conclusion, Gruber had also inhaled carbon monoxide himself. He reported no symptoms or uncomfortable sensations after three hours on each of two consecutive days at concentrations of 210 parts per million and 240 parts per million (Cook 1986).

Figure 1. Chronology of occupational exposure levels (OELS).

The earliest and most extensive series of animal experiments on exposure limits were those conducted by K.B. Lehmann and others under his direction. In a series of publications spanning 50 years they reported on studies on ammonia and hydrogen chloride gas, chlorinated hydrocarbons and a large number of other chemical substances (Lehmann 1886; Lehmann and Schmidt-Kehl 1936).

Kobert (1912) published one of the earlier tables of acute exposure limits. Concentrations for 20 substances were listed under the headings: (1) rapidly fatal to man and animals, (2) dangerous in 0.5 to one hour, (3) 0.5 to one hour without serious disturbances and (4) only minimal symptoms observed. In his paper “Interpretations of permissible limits”, Schrenk (1947) notes that the “values for hydrochloric acid, hydrogen cyanide, ammonia, chlorine and bromine as given under the heading ‘only minimal symptoms after several hours’ in the foregoing Kobert paper agree with values as usually accepted in present-day tables of MACs for reported exposures”. However, values for some of the more toxic organic solvents, such as benzene, carbon tetrachloride and carbon disulphide, far exceeded those currently in use (Cook 1986).

One of the first tables of exposure limits to originate in the United States was that published by the US Bureau of Mines (Fieldner, Katz and Kenney 1921). Although its title does not so indicate, the 33 substances listed are those encountered in workplaces. Cook (1986) also noted that most of the exposure limits through the 1930s, except for dusts, were based on rather short animal experiments. A notable exception was the study of chronic benzene exposure by Leonard Greenburg of the US Public Health Service, conducted under the direction of a committee of the National Safety Council (NSC 1926). An acceptable exposure for human beings based on long-term animal experiments was derived from this work.

According to Cook (1986), for dust exposures, permissible limits established before 1920 were based on exposures of workers in the South African gold mines, where the dust from drilling operations was high in crystalline free silica. In 1916, an exposure limit of 8.5 million particles per cubic foot of air (mppcf) for the dust with an 80 to 90% quartz content was set (Phthisis Prevention Committee 1916). Later, the level was lowered to 5 mppcf. Cook also reported that, in the United States, standards for dust, also based on exposure of workers, were recommended by Higgins and co-workers following a study at the south-western Missouri zinc and lead mines in 1917. The initial level established for high quartz dusts was ten mppcf, appreciably higher than was established by later dust studies conducted by the US Public Health Service. In 1930, the USSR Ministry of Labour issued a decree that included maximum allowable concentrations for 12 industrial toxic substances.

The most comprehensive list of occupational exposure limits up to 1926 was for 27 substances (Sayers 1927). In 1935 Sayers and Dalle Valle published physiological responses to five concentrations of 37 substances, the fifth being the maximum allowable concentration for prolonged exposure. Lehmann and Flury (1938) and Bowditch et al. (1940) published papers that presented tables with a single value for repeated exposures to each substance.

Many of the exposure limits developed by Lehmann were included in a monograph initially published in 1927 by Henderson and Haggard (1943), and a little later in Flury and Zernik’s Schadliche Gase (1931). According to Cook (1986), this book was considered the authoritative reference on effects of injurious gases, vapours and dusts in the workplace until Volume II of Patty’s Industrial Hygiene and Toxicology (1949) was published.

The first lists of standards for chemical exposures in industry, called maximum allowable concentrations (MACs), were prepared in 1939 and 1940 (Baetjer 1980). They represented a consensus of opinion of the American Standard Association and a number of industrial hygienists who had formed the ACGIH in 1938. These “suggested standards” were published in 1943 by James Sterner. A committee of the ACGIH met in early 1940 to begin the task of identifying safe levels of exposure to workplace chemicals, by assembling all the data which would relate the degree of exposure to a toxicant to the likelihood of producing an adverse effect (Stokinger 1981; LaNier 1984). The first set of values were released in 1941 by this committee, which was composed of Warren Cook, Manfred Boditch (reportedly the first hygienist employed by industry in the United States), William Fredrick, Philip Drinker, Lawrence Fairhall and Alan Dooley (Stokinger 1981).

In 1941, a committee (designated as Z-37) of the American Standards Association, which later became the American National Standards Institute, developed its first standard of 100 ppm for carbon monoxide. By 1974 the committee had issued separate bulletins for 33 exposure standards for toxic dusts and gases.

At the annual meeting of the ACGIH in 1942, the newly appointed Subcommittee on Threshold Limits presented in its report a table of 63 toxic substances with the “maximum allowable concentrations of atmospheric contaminants” from lists furnished by the various state industrial hygiene units. The report contains the statement, “The table is not to be construed as recommended safe concentrations. The material is presented without comment” (Cook 1986).

In 1945 a list of 132 industrial atmospheric contaminants with maximum allowable concentrations was published by Cook, including the then current values for six states, as well as values presented as a guide for occupational disease control by federal agencies and maximum allowable concentrations that appeared best supported by the references on original investigations (Cook 1986).

At the 1946 annual meeting of ACGIH, the Subcommittee on Threshold Limits presented their second report with the values of 131 gases, vapours, dusts, fumes and mists, and 13 mineral dusts. The values were compiled from the list reported by the subcommittee in 1942, from the list published by Warren Cook in Industrial Medicine (1945) and from published values of the Z-37 Committee of the American Standards Association. The committee emphasized that the “list of M.A.C. values is presented … with the definite understanding that it be subject to annual revision.”

Intended use of OELs

The ACGIH TLVs and most other OELs used in the United States and some other countries are limits which refer to airborne concentrations of substances and represent conditions under which “it is believed that nearly all workers may be repeatedly exposed day after day without adverse health effects” (ACGIH 1994). (See table 1).  In some countries the OEL is set at a concentration which will protect virtually everyone. It is important to recognize that unlike some exposure limits for ambient air pollutants, contaminated water, or food additives set by other professional groups or regulatory agencies, exposure to the TLV will not necessarily prevent discomfort or injury for everyone who is exposed (Adkins et al. 1990). The ACGIH recognized long ago that because of the wide range in individual susceptibility, a small percentage of workers may experience discomfort from some substances at concentrations at or below the threshold limit and that a smaller percentage may be affected more seriously by aggravation of a pre-existing condition or by development of an occupational illness (Cooper 1973; ACGIH 1994). This is clearly stated in the introduction to the ACGIH’s annual booklet Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices (ACGIH 1994).

Table 1. Occupational exposure limits (OELs) in various countries (as of 1986)

Source: Cook 1986.

This limitation, although perhaps less than ideal, has been considered a practical one since airborne concentrations so low as to protect hypersusceptibles have traditionally been judged infeasible due to either engineering or economic limitations. Until about 1990, this shortcoming in the TLVs was not considered a serious one. In light of the dramatic improvements since the mid-1980s in our analytical capabilities, personal monitoring/sampling devices, biological monitoring techniques and the use of robots as a plausible engineering control, we are now technologically able to consider more stringent occupational exposure limits.

The background information and rationale for each TLV are published periodically in the Documentation of the Threshold Limit Values (ACGIH 1995). Some type of documentation is occasionally available for OELs set in other countries. The rationale or documentation for a particular OEL should always be consulted before interpreting or adjusting an exposure limit, as well as the specific data that were considered in establishing it (ACGIH 1994).

TLVs are based on the best available information from industrial experience and human and animal experimental studies—when possible, from a combination of these sources (Smith and Olishifski 1988; ACGIH 1994). The rationale for choosing limiting values differs from substance to substance. For example, protection against impairment of health may be a guiding factor for some, whereas reasonable freedom from irritation, narcosis, nuisance or other forms of stress may form the basis for others. The age and completeness of the information available for establishing occupational exposure limits also varies from substance to substance; consequently, the precision of each TLV is different. The most recent TLV and its documentation (or its equivalent) should always be consulted in order to evaluate the quality of the data upon which that value was set.

Even though all of the publications which contain OELs emphasize that they were intended for use only in establishing safe levels of exposure for persons in the workplace, they have been used at times in other situations. It is for this reason that all exposure limits should be interpreted and applied only by someone knowledgeable of industrial hygiene and toxicology. The TLV Committee (ACGIH 1994) did not intend that they be used, or modified for use:

• as a relative index of hazard or toxicity
• in the evaluation of community air pollution
• for estimating the hazards of continuous, uninterrupted exposures or other extended work periods
• as proof or disproof of an existing disease or physical condition
• for adoption by countries whose working conditions differ from those of the United States.

The TLV Committee and other groups which set OELs warn that these values should not be “directly used” or extrapolated to predict safe levels of exposure for other exposure settings. However, if one understands the scientific rationale for the guideline and the appropriate approaches for extrapolating data, they can be used to predict acceptable levels of exposure for many different kinds of exposure scenarios and work schedules (ACGIH 1994; Hickey and Reist 1979).

Philosophy and approaches in setting exposure limits

TLVs were originally prepared to serve only for the use of industrial hygienists, who could exercise their own judgement in applying these values. They were not to be used for legal purposes (Baetjer 1980). However, in 1968 the United States Walsh-Healey Public Contract Act incorporated the 1968 TLV list, which covered about 400 chemicals. In the United States, when the Occupational Safety and Health Act (OSHA) was passed it required all standards to be national consensus standards or established federal standards.

Exposure limits for workplace air contaminants are based on the premise that, although all chemical substances are toxic at some concentration when experienced for a period of time, a concentration (e.g., dose) does exist for all substances at which no injurious effect should result no matter how often the exposure is repeated. A similar premise applies to substances whose effects are limited to irritation, narcosis, nuisance or other forms of stress (Stokinger 1981; ACGIH 1994).

This philosophy thus differs from that applied to physical agents such as ionizing radiation, and for some chemical carcinogens, since it is possible that there may be no threshold or no dose at which zero risk would be expected (Stokinger 1981). The issue of threshold effects is controversial, with reputable scientists arguing both for and against threshold theories (Seiler 1977; Watanabe et al. 1980, Stott et al. 1981; Butterworth and Slaga 1987; Bailer et al. 1988; Wilkinson 1988; Bus and Gibson 1994). With this in mind, some occupational exposure limits proposed by regulatory agencies in the early 1980s were set at levels which, although not completely without risk, posed risks that were no greater than classic occupational hazards such as electrocution, falls, and so on. Even in those settings which do not use industrial chemicals, the overall workplace risks of fatal injury are about one in one thousand. This is the rationale that has been used to justify selecting this theoretical cancer risk criterion for setting TLVs for chemical carcinogens (Rodricks, Brett and Wrenn 1987; Travis et al. 1987).

Occupational exposure limits established both in the United States and elsewhere are derived from a wide variety of sources. The 1968 TLVs (those adopted by OSHA in 1970 as federal regulations) were based largely on human experience. This may come as a surprise to many hygienists who have recently entered the profession, since it indicates that, in most cases, the setting of an exposure limit has come after a substance has been found to have toxic, irritational or otherwise undesirable effects on humans. As might be anticipated, many of the more recent exposure limits for systemic toxins, especially those internal limits set by manufacturers, have been based primarily on toxicology tests conducted on animals, in contrast to waiting for observations of adverse effects in exposed workers (Paustenbach and Langner 1986). However, even as far back as 1945, animal tests were acknowledged by the TLV Committee to be very valuable and they do, in fact, constitute the second most common source of information upon which these guidelines have been based (Stokinger 1970).

Several approaches for deriving OELs from animal data have been proposed and put into use over the past 40 years. The approach used by the TLV Committee and others is not markedly different from that which has been used by the US Food and Drug Administration (FDA) in establishing acceptable daily intakes (ADI) for food additives. An understanding of the FDA approach to setting exposure limits for food additives and contaminants can provide good insight to industrial hygienists who are involved in interpreting OELs (Dourson and Stara 1983).

Discussions of methodological approaches which can be used to establish workplace exposure limits based exclusively on animal data have also been presented (Weil 1972; WHO 1977; Zielhuis and van der Kreek 1979a, 1979b; Calabrese 1983; Dourson and Stara 1983; Leung and Paustenbach 1988a; Finley et al. 1992; Paustenbach 1995). Although these approaches have some degree of uncertainty, they seem to be much better than a qualitative extrapolation of animal test results to humans.

Approximately 50% of the 1968 TLVs were derived from human data, and approximately 30% were derived from animal data. By 1992, almost 50% were derived primarily from animal data. The criteria used to develop the TLVs may be classified into four groups: morphological, functional, biochemical and miscellaneous (nuisance, cosmetic). Of those TLVs based on human data, most are derived from effects observed in workers who were exposed to the substance for many years. Consequently, most of the existing TLVs have been based on the results of workplace monitoring, compiled with qualitative and quantitative observations of the human response (Stokinger 1970; Park and Snee 1983). In recent times, TLVs for new chemicals have been based primarily on the results of animal studies rather than human experience (Leung and Paustenbach 1988b; Leung et al. 1988).

It is noteworthy that in 1968 only about 50% of the TLVs were intended primarily to prevent systemic toxic effects. Roughly 40% were based on irritation and about two per cent were intended to prevent cancer. By 1993, about 50% were meant to prevent systemic effects, 35% to prevent irritation, and five per cent to prevent cancer. Figure 2 provides a summary of the data often used in developing OELs. 

Figure 2. Data often used in developing an occupational exposure.

Limits for irritants

Prior to 1975, OELs designed to prevent irritation were largely based on human experiments. Since then, several experimental animal models have been developed (Kane and Alarie 1977; Alarie 1981; Abraham et al. 1990; Nielsen 1991). Another model based on chemical properties has been used to set preliminary OELs for organic acids and bases (Leung and Paustenbach 1988).

Limits for carcinogens

In 1972, the ACGIH Committee began to distinguish between human and animal carcinogens in its TLV list. According to Stokinger (1977), one reason for this distinction was to assist the stakeholders in discussions (union representatives, workers and the public) in focusing on those chemicals with more probable workplace exposures.

Do the TLVs Protect Enough Workers?

Beginning in 1988, concerns were raised by numerous persons regarding the adequacy or health protectiveness of TLVs. The key question raised was, what percentage of the working population is truly protected from adverse health effects when exposed to the TLV?

Castleman and Ziem (1988) and Ziem and Castleman (1989) argued both that the scientific basis of the standards was inadequate and that they were formulated by hygienists with vested interests in the industries being regulated.

These papers engendered an enormous amount of discussion, both supportive of and opposed to the work of the ACGIH (Finklea 1988; Paustenbach 1990a, 1990b, 1990c; Tarlau 1990).

A follow-up study by Roach and Rappaport (1990) attempted to quantify the safety margin and scientific validity of the TLVs. They concluded that there were serious inconsistencies between the scientific data available and the interpretation given in the 1976 Documentation by the TLV Committee. They also note that the TLVs were probably reflective of what the Committee perceived to be realistic and attainable at the time. Both the Roach and Rappaport and the Castleman and Ziem analyses have been responded to by the ACGIH, who have insisted on the inaccuracy of the criticisms.

Although the merit of the Roach and Rappaport analysis, or for that matter, that of Ziem and Castleman, will be debated for a number of years, it is clear that the process by which TLVs and other OELs will be set will probably never be as it was between 1945 and 1990. It is likely that in coming years, the rationale, as well as the degree of risk inherent in a TLV, will be more explicitly described in the documentation for each TLV. Also, it is certain that the definition of “virtually safe” or “insignificant risk” with respect to workplace exposure will change as the values of society change (Paustenbach 1995, 1997).

The degree of reduction in TLVs or other OELs that will undoubtedly occur in the coming years will vary depending on the type of adverse health effect to be prevented (central nervous system depression, acute toxicity, odour, irritation, developmental effects, or others). It is unclear to what degree the TLV committee will rely on various predictive toxicity models, or what risk criteria they will adopt, as we enter the next century.

Standards and Nontraditional Work Schedules

The degree to which shift work affects a worker’s capabilities, longevity, mortality, and overall well-being is still not well understood. So-called nontraditional work shifts and work schedules have been implemented in a number of industries in an attempt to eliminate, or at least reduce, some of the problems caused by normal shift work, which consists of three eight-hour work shifts per day. One kind of work schedule which is classified as nontraditional is the type involving work periods longer than eight hours and varying (compressing) the number of days worked per week (e.g., a 12-hours-per-day, three-day workweek). Another type of nontraditional work schedule involves a series of brief exposures to a chemical or physical agent during a given work schedule (e.g., a schedule where a person is exposed to a chemical for 30 minutes, five times per day with one hour between exposures). The last category of nontraditional schedule is that involving the “critical case” wherein persons are continuously exposed to an air contaminant (e.g., spacecraft, submarine).

Compressed workweeks are a type of nontraditional work schedule that has been used primarily in non-manufacturing settings. It refers to full-time employment (virtually 40 hours per week) which is accomplished in less than five days per week. Many compressed schedules are currently in use, but the most common are: (a) four-day workweeks with ten-hour days; (b) three-day workweeks with 12-hour days; (c) 4-1/2–day workweeks with four nine-hour days and one four-hour day (usually Friday); and (d) the five/four, nine plan of alternating five-day and four-day workweeks of nine-hour days (Nollen and Martin 1978; Nollen 1981).

Of all workers, those on nontraditional schedules represent only about 5% of the working population. Of this number, only about 50,000 to 200,000 Americans who work nontraditional schedules are employed in industries where there is routine exposure to significant levels of airborne chemicals. In Canada, the percentage of chemical workers on nontraditional schedules is thought to be greater (Paustenbach 1994).

One Approach to Setting International OELs

As noted by Lundberg (1994), a challenge facing all national committees is to identify a common scientific approach to setting OELs. Joint international ventures are advantageous to the parties involved since writing criteria documents is both a time- and cost-consuming process (Paustenbach 1995).

This was the idea when the Nordic Council of Ministers in 1977 decided to establish the Nordic Expert Group (NEG). The task of the NEG was to develop scientifically-based criteria documents to be used as a common scientific basis of OELs by the regulatory authorities in the five Nordic countries (Denmark, Finland, Iceland, Norway and Sweden). The criteria documents from the NEG lead to the definition of a critical effect and dose-response/dose-effect relationships. The critical effect is the adverse effect that occurs at the lowest exposure. There is no discussion of safety factors and a numerical OEL is not proposed. Since 1987, criteria documents are published by the NEG concurrently in English on a yearly basis.

Lundberg (1994) has suggested a standardized approach that each county would use. He suggested building a document with the following characteristics:

• A standardized criteria document should reflect the up-to-date knowledge as presented in the scientific literature.
• The literature used should preferably be peer-reviewed scientific papers but at least be available publicly. Personal communications should be avoided. An openness toward the general public, particularly workers, decreases the kind of suspiciousness that recently has been addressed toward documentation from the ACGIH.
• The scientific committee should consist of independent scientists from academia and government. If the committee should include scientific representatives from the labour market, both employers and employees should be represented.
• All relevant epidemiological and experimental studies should be thoroughly scrutinized by the scientific committee, especially “key studies” that present data on the critical effect. All observed effects should be described.
• Environmental and biological monitoring possibilities should be pointed out. It is also necessary to thoroughly scrutinize these data, including toxicokinetic data.
• Data permitting, the establishment of dose-response and dose-effect relationships should be stated. A no observable effect level (NOEL) or lowest observable effect level (LOEL) for each observed effect should be stated in the conclusion. If necessary, reasons should be given as to why a certain effect is the critical one. The toxicological significance of an effect is thereby considered.
• Specifically, mutagenic, carcinogenic and teratogenic properties should be pointed out as well as allergic and immunological effects.
• A reference list for all studies described should be given. If it is stated in the document that only relevant studies have been used, there is no need to give a list of references not used or why. On the other hand, it could be of interest to list those databases that have been used in the literature search.

There are in practice only minor differences in the way OELs are set in the various countries that develop them. It should, therefore, be relatively easy to agree upon the format of a standardized criteria document containing the key information. From this point, the decision as to the size of the margin of safety that is incorporated in the limit would then be a matter of national policy.

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