Exposure Guidelines for Residential Indoor Air Quality

1.0 Introduction

1.1 Background

Criteria for acceptable air quality have existed for many years for the industrial workplace and outdoor environments. Only recently, however, has widespread attention been drawn to potential hazards posed by the presence of airborne contaminants in non-industrial indoor environments. Several factors have contributed to this awareness:

• a trend toward tighter building construction, as incentives are given to conserve fuels;
• the presence of an increasingly complex array of synthetic chemicals used in building and insulating products, furnishings, consumer products, and hobby and craft materials;
• the use of alternative heating systems which, if not properly installed or designed, can release combustion by-products;
• a recognition that prolonged exposure even to very low concentrations of chemical contaminants may result in delayed toxic effects;
• the realization that a very high proportion of the average individual's time is spent indoors.

1.2 Sources of Indoor Air Contaminants

The quality of indoor air is influenced both by the quality of outdoor air and by the emission characteristics of indoor sources.

1.2.1 Outdoor Sources

In almost all inhabited enclosed spaces, there is a continuous exchange of air with the outside. Therefore, all contaminants of outdoor air are likely to be present indoors. Important pollutants in this category include carbon monoxide, oxides of nitrogen, oxides of sulphur, particulate matter, ozone (and other photo-chemical oxidants) and lead.

These pollutants originate, to a large extent, from automobile and factory emissions and other combustion processes. Generally, in the absence of indoor sources of these contaminants, concentrations indoors will be close to or lower than those outdoors.

Radon-222 is a naturally occurring radioactive gas that decays to non-gaseous radioactive species which can be adsorbed onto suspended particulate matter and hence be deposited in the lung. Underlying soil and domestic well water are the most likely potential sources of radon in buildings.

1.2.2 Indoor Sources

Internally generated airborne pollutants fall into one of three categories:

• those formed in combustion processes for heating and cooking;
• those derived from construction materials and furnishings;
• those related to human activity or presence.

Concentrations of contaminants in the first and last categories tend to vary with time; those in the second are likely to be more constant, as long as air exchange rates remain constant.

(a) Combustion Processes. Furnaces and other combustion appliances can be sources of indoor pollutants, notably carbon monoxide, especially if they are not properly vented or routinely serviced. Since combustion in wood-burning stoves is much less complete than in oil and gas furnaces, pollutant emissions from them can be greater. Though by-products should be vented to the outside, leaks and improper operation of these appliances can cause emissions to the indoors. Contaminants associated with wood-burning stoves and fireplaces include carbon monoxide, oxides of nitrogen and sulphur, aldehydes and polycyclic aromatic hydrocarbons.

A major potential source of combustion by-products is gas-fired stoves. Emissions from the oven and pilot light are not always vented and can contribute to indoor levels of carbon monoxide, nitrogen oxides and formaldehyde.

Kerosene heaters are becoming increasingly popular for space heating. Since these systems are often unvented, the potential for high contaminant levels again exists. In particular, the improper use of kerosenes with high sulphur contents or of poorly designed units could result in emission of oxides of sulphur as well as some of the other combustion by-products previously mentioned.

(b) Building Products and Furnishings. Synthetic polymers used in furnishings and decorative materials can slowly degrade, releasing small quantities of the original constituents or reaction by-products. Draperies, rugs and fabrics, the great majority of which contain man-made fibres, are sources of a variety of organic, and potentially microbiological, contaminants.

Formaldehyde is released from wood laminates and particle board in which formaldehyde-containing resins have been used. Urea-formaldehyde foam insulation is a significant source of formaldehyde and possibly other gaseous products.

Fibrous materials such as asbestos and fibrous glass are present in some building materials and may be released to the indoor environment especially when such products are disturbed during building alterations.

(c) Human Activity. The variety of contaminants that result from human activity is extremely broad.

Tobacco smoking is a major source of indoor air pollution. While smokers subject themselves to mainstream smoke, bystanders can be involuntarily exposed to significant amounts of respirable particles, carbon monoxide and oxides of nitrogen, as well as numerous harmful organic contaminants, in sidestream smoke. Over 50 components of cigarette smoke are known to cause adverse health effects, and 12 of these are known or suspected carcinogens.

Human metabolic activity itself influences air quality by reducing the concentration of oxygen and increasing the level of carbon dioxide. Respiration, perspiration and food preparation add water vapour as well as odour-producing substances to the indoor atmosphere. A large variety of biological agents may be present in the home, for example, microorganisms from occupants, pets and insects; microbial growths may also occur on moist surfaces or in stagnant water. Pollens, spores, cell debris and insects are present in dust originating both indoors and outdoors.

Air fresheners, furniture waxes, polishes, cleansers, paints, pesticidal formulations, fabric protectors, deodorants and other products frequently used in the home are sources of various inorganic and organic chemicals. Many substances found in the workplace may also occur in the home as a result of hobby or craft activities. Moreover, workers exposed to chemicals in the workplace may bring these contaminants into the home on their clothing. In some circumstances this may be a means by which significant amounts of potentially harmful substances are introduced into indoor air.