Exposure Guidelines for Residential Indoor Air Quality

4.0 Guidelines and Recommendations (continued)

Part B. Substances with Exposure Guidelines - Carcinogenic Effects

For substances designated as human or potential human carcinogens, a continuing effort should be made to reduce exposure to the lowest possible level. It is recognized that attainment of this goal must be considered in light of the cost and feasibility of remedial measures and technological changes. It was, therefore, considered desirable to specify exposure guidelines in terms of both what can be attained now (action level) and what should be striven for as a longer-term objective (target level).

4.B.1 Formaldehyde* (see note)

The exposure guidelines for formaldehyde in residential indoor air are:

• ACTION LEVEL: 120 µg/m³ (0.10 ppm);
• TARGET LEVEL: 60 µg/m³ (0.05 ppm).

*Also see Section 4.A.1, "Aldehydes".

Formaldehyde is a colourless gas with a pungent odour. It combines readily with water to form a non-volatile compound and has a tendency to be absorbed onto surfaces and textiles, such as carpets and curtains. An equilibrium is established between formaldehyde in air and that adsorbed on surfaces and within wood products such as particle board.

Formaldehyde is formed naturally in the environment; outdoor concentrations in remote regions are about 7 µg/m³ (0.006 ppm). Outdoor levels in Canadian locations are about 10 µg/m³ (0.008 ppm). Man-made sources of formaldehyde include combustion and the decomposition of formaldehyde resins used in wood, paper, textiles or urea-formaldehyde foam insulation (UFFI). In two large Canadian surveys, average formaldehyde concentrations of 14 and 42 µg/m³ (0.011 and 0.034 ppm) were found in homes that did not have UFFI; higher average levels were detected in homes with UFFI (66 µg/m³ ; 0.054 ppm). Levels in mobile homes which generally contain a high proportion of urea-formaldehyde resin pressed wood products tend to be even higher than the concentrations in homes with UFFI.

Formaldehyde is a vitally important intermediate in the normal metabolism of cells. It serves as a building block for the synthesis of purines, pyrimidines and many amino acids and lipids and is a key molecule in one-carbon metabolism. It is present at low levels in body fluids, and exposure to exogenous sources does not lead to any appreciable increase in these levels.

Formaldehyde gas is a sensory irritant, primarily affecting the nasal passages, respiration and the eyes (see Section 4.A.1). In addition, in two well-conducted bioassays and a more limited study, formaldehyde has been found to be carcinogenic in two strains of rats, producing a high incidence of nasal squamous cell carcinomas (38% to 50%) following administration of approximately 18 mg/m³ (15 ppm). Formaldehyde is also genotoxic in a number of assays and is weakly mutagenic in cultured human cells as well as in other mammalian cells, Drosophila, fungi and bacteria. Although the epidemiological studies conducted to date provide little convincing evidence that formaldehyde is carcinogenic in human populations, the possibility cannot be excluded owing to limitations of the available data.

Because of the possible carcinogenicity of formaldehyde, it would be prudent to reduce indoor levels as much as possible. The action level of 120 µg/m³ is the lowest concentration considered to be feasible at the present time. However, it is recommended that in future, and where remedial measures are taken, every effort be made to reduce concentrations to below the target value (60 mg/m³).

4.B.2 Radon

The exposure guideline for radon in residential indoor air is:

ACTION LEVEL: 800 Bq/m³ as an annual average concentration in the normal living area.

Radon, a naturally occurring noble gas and a member of the uranium-238 decay chain, is a chemically inert gas which diffuses from its local site of production. Radon decays with a half-life of 3.82 days via an alpha particle emission into a series of radionuclides with short half-lives called radon progeny (radon daughter products) and eventually to the radionuclide lead-210, which has a relatively long half-life. The radon progeny are chemically reactive particulates which attach almost immediately to aerosol particles in the atmosphere.

The average worldwide concentration of radium in soil is 25 Bq/kg; this medium constitutes the main source of radon in the global atmosphere. The outdoor concentration range in continental North America is 0.7 to 35 Bq/m³, with an average concentration of 7.0 Bq/m³. Sources of radon in indoor air include transport of soil gas into basements, domestic use of radon-bearing well water, building materials and natural gas supplies.

Levels of radon in indoor air are normally higher than outdoor levels and depend mainly on the source strengths and ventilation rate. There have been several surveys of radon in Canadian homes, the results of which indicate indicate significant differences across the country. In a survey of 14 000 homes in Canada conducted between 1977 and 1980, 0.1% of homes had radon levels in excess of 800 Bq/m³. The geometric means for the communities monitored in the cross-Canada survey ranged from a low of 5.2 Bq/m³ to a high of 57 Bq/m³.

Inhalation of radon and radon progeny leads to radiation exposure of the bronchial tissue of the lungs with a resultant risk of cancer. More than 95% of the radiation dose results from the deposition and decay of the radon progeny. The increased risk of lung cancer among uranium miners exposed to radon and radon progeny has been well documented in a number of epidemiological studies. A study of the correlation of lung cancer mortality with concentrations of radon progeny in indoor air in 18 Canadian cities has been conducted. Despite the relatively wide variation of indoor concentrations in different cities, no association between radon and lung cancer mortality rates was apparent. It is conceivable that domestic radon exposure in Canada may result in such a small increase of lung cancer risk that it would be detectable only in a more definitive health effects approach such as a large case-control study.

Because radon is a potential human carcinogen, it would be prudent to reduce indoor levels as much as possible. In addition, in view of the seasonal variability in the composition of indoor air, an annual averaging time is recommended for the application of the exposure guideline.

Therefore, it is recommended that remedial measures be taken where the annual average concentration level of radon in the normal living area of a home is found to exceed 800 Bq/m³. Because there is some risk at any level of radon exposure, homeowners may wish to reduce levels of radon to as low a level as practicable.