Proposed residential indoor air quality guidelines for formaldehyde

6. Summary of Critical Effects and Derivation of Guidelines

6.1 Short-term Effects

Based on human clinical studies and on animal experiments, the primary effects of acute exposure to formaldehyde are the irritation of the mucosa of the upper respiratory tract and the eyes.

Several studies assessed effects of short-term exposure to formaldehyde in healthy and asthmatic adults but only one of them (Kulle et al. 1993) included a range of exposure levels enabling the assessment of exposure-response relationship. In this study, the most sensitive effect was eye irritation: the LOAEL for this outcome was 1,230 μg/m³ and the NOAEL was 615 μg/m³ . Another study (Pazdrak et al. 1993) found a subclinical inflammatory response at 615 μg/m³ , the only exposure level tested.

6.2 Chronic Effects Other Than Cancer

Epidemiological studies on the effects of chronic formaldehyde exposure consistently found respiratory and allergic effects at levels below 123 μg/m³ (Krzyzanowski et al. 1990; Smedje et al. 1997; Garrett et al. 1999; Franklin et al. 2000; Smedje and Norback 2001; Rumchev et al. 2002).

In one of these studies (Garrett et al. 1999), formaldehyde levels in homes (96-hour samples collected four times over 1 year) were associated with increased risk of atopy, after ruling out confounding from other indoor air pollutants. In another study (Rumchev et al. 2002), formaldehyde levels (8-hour samples taken in summer and winter) were significantly associated with hospitalization for asthma in children aged 6 months to 3 years, again after ruling out confounding from other indoor air pollutants. No effects were found in children exposed to 10 to 29 μg/m³ and 30 to 49 μg/m³ formaldehyde, a non-significant increase of risk was observed at 50 to 59 μg/m³ (OR 1.2) and a significantly increased risk was observed at 60 μg/m³ (OR 1.39, p<0.05). An association between low-level exposure to formaldehyde and the development of allergic sensitization and/or asthma is biologically plausible as it is consistent with observations in animals: formaldehyde-enhanced allergic sensitization to ovalbumin in mice and guinea pigs (Tarkowski and Gorski 1995; Riedel et al. 1996).The observed dose-response relationship, the strong design of this study (especially the control of confounding variables) and the strong biological plausibility of the association observed make this study appear to be the most suitable for risk assessment because of extensive controlling for potential confounders. Based on its findings, long-term exposure to formaldehyde levels below 50 μg/m³ appear not to be associated with adverse effects.

6.3 Cancer

There is evidence from toxicological and epidemiological studies that inhaled formaldehyde is carcinogenic; this effect appears to be limited to the nasal cavity (Environment Canada, Health Canada 2001). The IARC has classified formaldehyde as carcinogenic to humans (Group 1), based on sufficient evidence both in humans and in animals.

However, formaldehyde-induced carcinogenicity appears to be a consequence of proliferative regeneration following cytotoxicity (CIIT 1999; Environment Canada, Health Canada 2001). Based on a dose-response model developed by CIIT, the additional risk of respiratory cancer associated with a lifelong formaldehyde exposure ranging from 1.23 and 123 μg/m³ in non-smokers ranged from 2.3 × 10^-10 to 2.7 × 10^-8 (Environment Canada, Health Canada 2001). The risk of cancer associated with formaldehyde levels sufficiently low to prevent irritation and inflammatory responses appears therefore to be negligible.

6.4 Proposed Guidelines

It is recommended that a guideline be established for short-term (1-hour averaged) exposures to formaldehyde at 123 μg/m³ (100 ppb) (i.e. one tenth of the lowest concentration at which eye irritation was reported in the 1993 Kulle et al. controlled exposure study).

It is recommended that the guideline for long-term (8-hour averaged) exposure to formaldehyde be based on the NOAEL derived from the Rumchev (2002) case-control study of childhood asthma. Based on this study, the guideline would be 50 μg/m³ (40 ppb). Although formaldehyde is probably carcinogenic to humans, the cancer risk associated with a lifelong exposure to that concentration of formaldehyde is estimated to be negligible.