“All scientific work is incomplete—whether it be observational or experimental. All scientific work is liable to be upset or modified by advancing knowledge. That does not confer upon us a freedom to ignore the knowledge we already have, or to postpone the action it appears to demand at a given time.” (Bradford Hill, 1965)

scientific knowledge is never complete.¹ Scientists are sometimes surprised by new findings which question well established causal hypotheses. One hypothesis which has been accepted for several years and is now being questioned is that for formaldehyde carcinogenicity, particularly at low levels of exposure and in relation to cancer sites other than the upper respiratory tract.

Formaldehyde is currently classified by IARC as a probable human carcinogen based on animal studies with neoplastic lesions at the point of contact, the nasal cavity, but limited evidence of human respiratory tract carcinogenicity.² This IARC classification is based on several long term chronic bioassays, mechanistic information developed over 20 years, and 39 epidemiology studies. The studies on formaldehyde carcinogenicity represent one of the more extensive databases that IARC has reviewed.

Formaldehyde causes nasal cavity tumours in rats and at higher exposure levels in …