Nasal tumours in rats after short-term exposure to a cytotoxic concentration of formaldehyde.

Male Wistar rats were exposed to 0, 10 or 20 ppm formaldehyde vapour for 4, 8 or 13 weeks (6 h/day; 5 days/week), and were then observed for periods up to 126 weeks. Transient growth retardation occurred in both test groups. Death rate was not noticeably affected by formaldehyde. Despite recovery periods of at most 126 weeks, the nasal respiratory and olfactory epithelium of many rats of the 20 ppm group exhibited non-neoplastic histopathological changes. Similar but much less severe changes of the respiratory epithelium were seen in a small number of rats of the 10 ppm group; the olfactory epithelium was not visibly affected in rats of this group. Nasal tumours considered to be induced by formaldehyde were seen only in the 20 ppm group and mainly in rats that had been exposed for 13 weeks, the incidence being 4.5% (6/132). These tumours comprised 3 squamous cell carcinomas, 1 carcinoma in situ and 2 polypoid adenomas, all originating from respiratory epithelium. It was concluded that rat nasal respiratory epithelium severely damaged by formaldehyde vapour often does not regenerate and in some cases develops tumours.