Exposure Guidelines for Residential Indoor Air Quality

2.0 Purpose and Scope

2.1 Objectives

Generally, people can be at home for as much as 70% of their time and, for some segments of the population (the very young, the old and the infirm), this percentage can be much higher. In developing recommendations for the domestic environment, the Working Group considered it prudent to assume that certain segments of the population are at home on a continuous basis. Furthermore, some individuals may be at special risk from indoor air pollution. Such individuals comprise those whose physiological processes either are not fully developed or are deteriorating, or for whom pathological or physiological changes impair the ability to surmount the adverse effects of exposure to a pollutant. Therefore, a primary objective was:

to develop guidelines for the concentrations of selected contaminants of residential indoor air, taking into account such factors as the sensitivity of groups at special risk and the sources and mechanisms of action of contaminants.

These guidelines may not provide complete protection for the hypersensitive portion of the population which requires extraordinary measures to achieve such protection.

The need to specify ventilation rates for domestic premises that reflect the indoor air quality guidelines was also considered but was deemed to be beyond the expertise of members of the Working Group. Moreover, a prescription for ventilation requirements is only one of several strategies that might be adopted for controlling the presence of airborne contaminants in the home. Other possibilities include specifications for building design and materials and consumer products. In many instances, measures to minimize exposure to chemical contaminants can be taken by the occupants. Public education is an important strategy for attaining an acceptable quality of indoor air, and the Working Group therefore established as its second objective:

to develop, where practicable, other guidelines or recommendations for measures that will preserve or improve air quality in domestic premises.

2.2 Definitions of Indoor Air Quality

As a guiding principle, air within domestic premises should be sufficiently free from biological, physical and chemical contaminants to ensure that there is a negligible risk to the health and safety of the occupants.

Following an examination of the available information, 17 chemical substances or groups of substances, and one radio-active, were selected for detailed review because of their potential to cause adverse health effects and their possible presence indoors. Guidelines expressed in terms of concentration ranges were developed for nine of these. For the others it was not possible to express the guidelines in quantitative terms, either because the data base was inadequate or because human exposure limits were deemed inappropriate; where possible, however, recom-mendations are made in this document on measures that can be taken to control indoor exposure to contaminants.

The guidelines in this document therefore encompass two categories of contaminants:

• those for which recommendations are expressed in terms of ranges of concentrations;
• those for which recommendations are specified as practical measures that can be taken to reduce exposure.

Section 4, Part A of this document deals with those contaminants for which exposure limits could be derived exclusively on the basis of health considerations (see Section 3.2.1). These are specified for the following substances:

• aldehydes (total);
• carbon dioxide;
• carbon monoxide;
• nitrogen dioxide;
• ozone;
• particulate matter;
• sulphur dioxide;
• water vapour.

The potential for adverse effects from long-term exposures as well as from shorter-term higher-level exposures has been considered. Two kinds of exposure limits are therefore specified.

The Acceptable Long-Term Exposure Range (ALTER) is that concentration range to which it is believed from existing information that a person may be exposed over a lifetime without undue risk to health.

The Acceptable Short-Term Exposure Range (ASTER) is that concentration range to which it is believed from existing information that a person may be exposed over the specified time period without undue risk to health.

An important consideration in deriving acceptable exposure ranges is the possibility of interactive effects, since many contaminants are likely to be simultaneously present in the home environment. Where possible, due account was taken of the potential for synergistic and additive effects. However, in most cases there were insufficient or inadequate data to completely resolve this problem.

Section 4, Part B contains recommendations for long-term exposure to formaldehyde and radon, on the basis of carcinogenic or possible carcinogenic effects. For formaldehyde, the guideline was not established exclusively on the basis of health considerations; rather it was set as low as possible taking into account the cost and technical feasibility of attainment, as well as the associated health risks. The risk associated with exposure to radon in homes is still the subject of research. This report has taken a conservative position and has established a guideline for radon that recommends action above a level at which the risk to health is considered to be significant. This guideline also notes that, as any level of exposure to radon constitutes a risk, homeowners may wish to reduce levels as low as practicable (see Section 3.2.2).

Section 4, Part C of these guidelines deals with those contaminants for which the formulation of exposure ranges was deemed inappropriate or was not feasible. This group comprises:

• biological agents;
• chlorinated hydrocarbons;
• fibrous materials;
• lead;
• pest control products;
• polycyclic aromatic hydrocarbons;
• product aerosols;
• tobacco smoke.

Recommendations for procedures that would reduce or eliminate exposure in the home are provided for these substances.

In developing the recommendations for these 18 substances or groups, the available literature was critically reviewed. The criteria and rationale on which these guidelines are based are briefly outlined in this document. A more detailed description of the scientific criteria and reference material is available on request from the Environmental Health Directorate, Health and Welfare Canada.

It is recognized that the above lists do not fully represent the range of compounds found in the home environment. As new data become available, the need for developing exposure guidelines for additional contaminants or groups and revising the current guidelines should also be considered.

2.3 General "Indicators" of Indoor Air Quality

2.3.1 Carbon Dioxide

Increasingly, complaints made by occupants of some large buildings are being linked with poor indoor air quality. The occurrence of some of these complaints (headache, fatigue, unpleasant odours, stuffiness and undue warmth) has been associated with elevated concentrations of carbon dioxide. Whether a concentration range could be prescribed for carbon dioxide that would be indicative of acceptable air quality from the viewpoint of comfort or aesthetic considerations was therefore considered.

Carbon dioxide is produced by human respiration and can be reduced significantly only by ventilation of the building. The concentration of carbon dioxide in indoor air is therefore closely related to the ventilation rate. The degree of ventilation needed to maintain carbon dioxide at a low level within a building also helps to reduce the levels of other indoor pollutants and to improve overall indoor air quality. Carbon dioxide is useful as an indicator of general air quality only in buildings where there are significant metabolic or combustion sources of carbon dioxide. Otherwise, carbon dioxide levels will be low over a wide range of ventilation rates (for example, in large houses with only one or two occupants and no unvented combustion appliances).

The trend in recent years toward minimizing ventilation within houses in order to reduce energy consumption for heating and cooling can lead to increases in carbon dioxide levels within residences and to a general deterioration in indoor air quality.

In several studies, comfort factors have been correlated with carbon dioxide concentrations. Collectively, these studies suggest that carbon dioxide concentrations above 1800 mg/m³ (1000 ppm) are indicative that there is an inadequate supply of fresh air, although complaints have been documented at concentrations as low as 1100 mg/m³ (600 ppm). However, from a review of the direct physiological effects of exposure to carbon dioxide, as opposed to subjective symptoms, a higher maximum exposure concentration is recommended (see Section 4.A.2).

It must be noted that these studies were conducted in buildings with mechanical ventilation systems and with occupancy rates quite different from those of residences. Moreover, the effects observed are probably not attributable to the presence of elevated concentrations of carbon dioxide, but rather to undesirable concentrations of other substances that result from inadequate ventilation, and for which carbon dioxide provides a suitable surrogate parameter. Therefore, caution must be used in interpreting carbon dioxide concentrations as a general indication of residential indoor air quality.

2.3.2 Water Vapour

Relative humidity has also been considered as an indicator of indoor air quality. Air-to-air heat exchangers, which are designed to bring in outside air and to turn on and off at preset relative humidity levels, have been developed. The control of indoor air pollutants other than excessive water vapour by heat exchangers operating in this way may not be satisfactory for the following reasons:

• in a large house with a low occupancy, relative humidity may not rise sufficiently to trigger air exchange, although other pollutants may be present at unacceptable concentrations;
• changes in occupancy throughout the day affect the rate at which water vapour is generated, whereas other pollutants may be emitted continuously;
• the variation in indoor relative humidity with geographical area and season necessitates different control settings for each location and season.

Therefore, relative humidity is not suitable as a general indicator of residential indoor air quality.