Criteria for Establishment

The setting of specific guides and standards for indoor air is the product of proactive policies in this field on the part of the bodies responsible for their establishment and for maintaining the quality of indoor air at acceptable levels. In practice, the tasks are divided and shared among many entities responsible for controlling pollution, maintaining health, ensuring the safety of products, watching over occupational hygiene and regulating building and construction.

The establishment of a regulation is intended to limit or reduce the levels of pollution in indoor air. This goal can be achieved by controlling the existing sources of pollution, diluting indoor air with outside air and checking the quality of available air. This requires the establishment of specific maximum limits for the pollutants found in indoor air.

The concentration of any given pollutant in indoor air follows a model of balanced mass expressed in the following equation:

where:

C_i = the concentration of the pollutant in indoor air (mg/m³);

Q = the emission rate (mg/h);

V = the volume of indoor space (m³);

C_o = the concentration of the pollutant in outdoor air (mg/m³);

n = the ventilation rate per hour;

a = the pollutant decay rate per hour.

It is generally observed that—in static conditions—the concentration of pollutants present will depend in part on the amount of the compound released into the air from the source of contamination and its concentration in outdoor air, and on the different mechanisms by which the pollutant is removed. The elimination mechanisms include the dilution of the pollutant and its “disappearance” with time. All regulations, recommendations, guidelines and standards that may be set in order to reduce pollution must take stock of these possibilities.

Control of the Sources of Pollution

One of the most effective ways to reduce the levels of concentration of a pollutant in indoor air is to control the sources of contamination within the building. This includes the materials used for construction and decoration, the activities within the building and the occupants themselves.

If it is deemed necessary to regulate emissions that are due to the construction materials used, there are standards that limit directly the content in these materials of compounds for which harmful effects to health have been demonstrated. Some of these compounds are considered carcinogenic, like formaldehyde, benzene, some pesticides, asbestos, fibreglass and others. Another avenue is to regulate emissions by the establishment of emission standards.

This possibility presents many practical difficulties, chief among them being the lack of agreement on how to go about measuring these emissions, a lack of knowledge about their effects on the health and comfort of the occupants of the building, and the inherent difficulties of identifying and quantifying the hundreds of compounds emitted by the materials in question. One way to go about establishing emission standards is to start out from an acceptable level of concentration of the pollutant and to calculate a rate of emission that takes into account the environmental conditions—temperature, relative humidity, air exchange rate, loading factor and so forth—that are representative of the way in which the product is actually used. The main criticism levelled against this methodology is that more than one product may generate the same polluting compound. Emission standards are obtained from readings taken in controlled atmospheres where conditions are perfectly defined. There are published guides for Europe (COST 613 1989 and 1991) and for the United States (ASTM 1989). The criticisms usually directed against them are based on: (1) the fact that it is difficult to get comparative data and (2) the problems that surface when an indoor space has intermittent sources of pollution.

As for the activities that may take place in a building, the greatest focus is placed on building maintenance. In these activities the control can be established in the form of regulations about the performance of certain duties—like recommendations relating to the application of pesticides or the reduction of exposure to lead or asbestos when a building is being renovated or demolished.

Because tobacco smoke—attributable to the occupants of a building—is so often a cause of indoor air pollution, it deserves separate treatment. Many countries have laws, at the state level, that prohibit smoking in certain types of public space such as restaurants and theatres, but other arrangements are very common whereby smoking is permitted in certain specially designated parts of a given building.

When the use of certain products or materials is prohibited, these prohibitions are made based on their alleged detrimental health effects, which are more or less well documented for levels normally present in indoor air. Another difficulty that arises is that often there is not enough information or knowledge about the properties of the products that could be used in their stead.

Elimination of the Pollutant

There are times when it is not possible to avoid the emissions of certain sources of pollution, as is the case, for example, when the emissions are due to the occupants of the building. These emissions include carbon dioxide and bioeffluents, the presence of materials with properties that are not controlled in any way, or the carrying out of everyday tasks. In these cases one way to reduce the levels of contamination is with ventilation systems and other means used to clean indoor air.

Ventilation is one of the options most heavily relied on to reduce the concentration of pollutants in indoor spaces. However, the need to also save energy requires that the intake of outside air to renew indoor air be as sparing as possible. There are standards in this regard that specify minimum ventilation rates, based on the renewal of the volume of indoor air per hour with outdoor air, or that set a minimum contribution of air per occupant or unit of space, or that take into account the concentration of carbon dioxide considering the differences between spaces with smokers and without smokers. In the case of buildings with natural ventilation, minimum requirements have also been set for different parts of a building, such as windows.

Among the references most often cited by a majority of the existing standards, both national and international—even though it is not legally binding—are the norms published by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE). They were formulated to aid air-conditioning professionals in the design of their installations. In ASHRAE Standard 62-1989 (ASHRAE 1989), the minimum amounts of air needed to ventilate a building are specified, as well as the acceptable quality of indoor air required for its occupants in order to prevent adverse health effects. For carbon dioxide (a compound most authors do not consider a pollutant given its human origin, but that is used as an indicator of the quality of indoor air in order to establish the proper functioning of ventilation systems) this standard recommends a limit 1,000 ppm in order to satisfy criteria of comfort (odour). This standard also specifies the quality of outdoor air required for the renewal of indoor air.

In cases where the source of contamination—be it interior or exterior—is not easy to control and where equipment must be used to eliminate it from the environment, there are standards to guarantee their efficacy, such as those that state specific methods to check the performance of a certain type of filter.

Extrapolation from Standards of Occupational Hygiene to Standards of Indoor Air Quality

It is possible to establish different types of reference value that are applicable to indoor air as a function of the type of population that needs to be protected. These values can be based on quality standards for ambient air, on specific values for given pollutants (like carbon dioxide, carbon monoxide, formaldehyde, volatile organic compounds, radon and so on), or they can be based on standards usually employed in occupational hygiene. The latter are values formulated exclusively for applications in industrial environments. They are designed, first of all, to protect workers from the acute effects of pollutants—like irritation of mucous membranes or of the upper respiratory tract—or to prevent poisoning with systemic effects. Because of this possibility, many authors, when they are dealing with indoor environment, use as a reference the limit values of exposure for industrial environments established by the American Conference of Governmental Industrial Hygienists (ACGIH) of the United States. These limits are called threshold limit values (TLVs), and they include limit values for workdays of eight hours and work weeks of 40 hours.

Numerical ratios are applied in order to adapt TLVs to the conditions of the indoor environment of a building, and the values are commonly reduced by a factor of two, ten, or even one hundred, depending on the kind of health effects involved and the type of population affected. Reasons given for reducing the values of TLVs when they are applied to exposures of this kind include the fact that in non-industrial environments personnel are exposed simultaneously to low concentrations of several, normally unknown chemical substances which are capable of acting synergistically in a way that cannot be easily controlled. It is generally accepted, on the other hand, that in industrial environments the number of dangerous substances that need to be controlled is known, and is often limited, even though concentrations are usually much higher.

Moreover, in many countries, industrial situations are monitored in order to secure compliance with the established reference values, something that is not done in non-industrial environments. It is therefore possible that in non-industrial environments, the occasional use of some products can produce high concentrations of one or several compounds, without any environmental monitoring and with no way of revealing the levels of exposure that have occurred. On the other hand, the risks inherent in an industrial activity are known or should be known and, therefore, measures for their reduction or monitoring are in place. The affected workers are informed and have the means to reduce the risk and protect themselves. Moreover, workers in industry are usually adults in good health and in acceptable physical condition, while the population of indoor environments presents, in general, a wider range of health statuses. The normal work in an office, for example, may be done by people with physical limitations or people susceptible to allergic reactions who would be unable to work in certain industrial environments. An extreme case of this line of reasoning would apply to the use of a building as a family dwelling. Finally, as noted above, TLVs, just like other occupational standards, are based on exposures of eight hours a day, 40 hours a week. This represents less than one fourth of the time a person would be exposed if he or she remained continually in the same environment or were exposed to some substance for the entire 168 hours of a week. In addition, the reference values are based on studies that include weekly exposures and that take into account times of non-exposure (between exposures) of 16 hours a day and 64 hours on weekends, which makes it is very hard to make extrapolations on the strength of these data.

The conclusion most authors arrive at is that in order to use the standards for industrial hygiene for indoor air, the reference values must include a very ample margin of error. Therefore, the ASHRAE Standard 62-1989 suggests a concentration of one tenth of the TLV value recommended by the ACGIH for industrial environments for those chemical contaminants which do not have their own established reference values.

Regarding biological contaminants, technical criteria for their evaluation which could be applicable to industrial environments or indoor spaces do not exist, as is the case with the TLVs of the ACGIH for chemical contaminants. This could be due to the nature of biological contaminants, which exhibit a wide variability of characteristics that make it difficult to establish criteria for their evaluation that are generalized and validated for any given situation. These characteristics include the reproductive capacity of the organism in question, the fact that the same microbial species may have varying degrees of pathogenicity or the fact that alterations in environmental factors like temperature and humidity may have an effect upon their presence in any given environment. Nonetheless, in spite of these difficulties, the Bioaerosol Committee of the ACGIH has developed guidelines to evaluate these biological agents in indoor environments: Guidelines for the Assessment of Bioaerosols in the Indoor Environment (1989). The standard protocols that are recommended in these guidelines set sampling systems and strategies, analytical procedures, data interpretation and recommendations for corrective measures. They can be used when medical or clinical information points to the existence of illnesses like humidifier fever, hypersensitivity pneumonitis or allergies related to biological contaminants. These guidelines can be applied when sampling is needed in order to document the relative contribution of the sources of bioaerosols already identified or to validate a medical hypothesis. Sampling should be done in order to confirm potential sources, but routine sampling of air to detect bioaerosols is not recommended.

Existing Guidelines and Standards

Different international organizations such as the World Health Organization (WHO) and the International Council of Building Research (CIBC), private organizations such as ASHRAE and countries like the United States and Canada, among others, are establishing exposure guidelines and standards. For its part, the European Union (EU) through the European Parliament, has presented a resolution on the quality of air in indoor spaces. This resolution establishes the need for the European Commission to propose, as soon as possible, specific directives that include:

1. a list of substances to be proscribed or regulated, both in the construction and in the maintenance of buildings
2. quality standards that are applicable to the different types of indoor environments
3. prescriptions for the consideration, construction, management and maintenance of air-conditioning and ventilation installations
4. minimum standards for the maintenance of buildings that are open to the public.

Many chemical compounds have odours and irritating qualities at concentrations that, according to current knowledge, are not dangerous to the occupants of a building but that can be perceived by—and therefore annoy—a large number of people. The reference values in use today tend to cover this possibility.

Given the fact that the use of occupational hygiene standards is not recommended for the control of indoor air unless a correction is factored in, in many cases it is better to consult the reference values used as guidelines or standards for the quality of ambient air. The US Environmental Protection Agency (EPA) has set standards for ambient air intended to protect, with an adequate margin of safety, the health of the population in general (primary standards) and even its welfare (secondary standards) against any adverse effects that may be predicted due to a given pollutant. These reference values are, therefore, useful as a general guide to establish an acceptable standard of air quality for a given indoor space, and some standards like ASHRAE-92 use them as quality criteria for the renewal of air in a closed building. Table 1 shows the reference values for sulphur dioxide, carbon monoxide, nitrogen dioxide, ozone, lead and particulate matter.

Table 1. Standards of air quality established by the US Environmental Protection Agency

^a Primary standard. ^b Secondary standard. ^c Maximum value that should not be exceeded more than once a year. ^d Measured as particles of diameter ≤10 μm. Source: US Environmental Protection Agency. National Primary and Secondary Ambient Air Quality Standards. Code of Federal Regulations, Title 40, Part 50 (July 1990).

For its part, WHO has established guidelines intended to provide a baseline to protect public health from adverse effects due to air pollution and to eliminate or reduce to a minimum those air pollutants that are known or suspected of being dangerous for human health and welfare (WHO 1987). These guidelines do not make distinctions as to the type of exposure they are dealing with, and hence they cover exposures due to outdoor air as well as exposures that may occur in indoor spaces. Tables 2 and 3 show the values proposed by WHO (1987) for non-carcinogenic substances, as well as the differences between those that cause health effects and those that cause sensory discomfort.

Table 2. WHO guideline values for some substances in air based on known effects on human health other than cancer or odour annoyance.^a

^a Information in this table should be used in conjunction with the rationales provided in the original publication.
^b This value refers to indoor air only.
^c Exposure to this concentration should not exceed the time indicated and should not be repeated within 8 hours. Source: WHO 1987.

Table 3. WHO guideline values for some non-carcinogenic substances in air, based on sensory effects or annoyance reactions for an average of 30 minutes

^b In the manufacture of viscose it is accompanied by other odorous substances such as hydrogen sulphide and carbonyl sulphide. Source: WHO 1987.

For carcinogenic substances, the EPA has established the concept of units of risk. These units represent a factor used to calculate the increase in the probability that a human subject will contract cancer due to a lifetime’s exposure to a carcinogenic substance in air at a concentration of 1 μg/m³. This concept is applicable to substances that can be present in indoor air, such as metals like arsenic, chrome VI and nickel; organic compounds like benzene, acrylonitrile and polycyclic aromatic hydrocarbons; or particulate matter, including asbestos.

In the concrete case of radon, Table 20 shows the reference values and the recommendations of different organizations. Thus the EPA recommends a series of gradual interventions when the levels in indoor air rise above 4 pCi/l (150 Bq/m³), establishing the time frames for the reduction of those levels. The EU, based on a report submitted in 1987 by a task force of the International Commission on Radiological Protection (ICRP), recommends an average yearly concentration of radon gas, making a distinction between existing buildings and new construction. For its part, WHO makes its recommendations keeping in mind exposure to radon’s decay products, expressed as a concentration of equilibrium equivalent of radon (EER) and taking into account an increase in the risk of contracting cancer between 0.7 x 10^-4 and 2.1 x 10^-4 for a lifetime exposure of 1 Bq/m³ EER.

Table 4. Reference values for radon according to three organizations

^a Average annual concentration of radon gas.
^b Equivalent to a dose of 20 mSv/year.
^c Annual average.

Finally, it must be remembered that reference values are established, in general, based on the known effects that individual substances have on health. While this may often represent arduous work in the case of assaying indoor air, it does not take into account the possible synergistic effects of certain substances. These include, for example, volatile organic compounds (VOCs). Some authors have suggested the possibility of defining total levels of concentrations of volatile organic compounds (TVOCs) at which the occupants of a building may begin to react. One of the main difficulties is that, from the point of view of analysis, the definition of TVOCs has not yet been resolved to everyone’s satisfaction.

In practice, the future establishment of reference values in the relatively new field of indoor air quality will be influenced by the development of policies on the environment. This will depend on the advancements of knowledge of the effects of pollutants and on improvements in the analytical techniques that can help us to determine these values.

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