Experimental Carcinogenesis of the Lung. Inhalation of Gaseous Formaldehyde or an Aerosol of Coal Tar by C3H Mice

Abstract

Inhalation of gaseous formaldehyde at atmospheric levels ≥0.10 mg per liter increased the incidence of the following changes in the trachea and the major bronchi of C3H mice: basal-cell hyperplasia, stratification of the epithelium, and squamous-cell metaplasia. The incidence of the first 2 changes was increased above normal in mice inhaling 0.05 mg of formaldehyde per liter. Atypical metaplastic changes in the trachea that did not progress to an invasive carcinoma were observed in animals in contact with 0.2 mg of formaldehyde per liter. Animals repeatedly subjected to inhalation of 0.05 mg of formaldehyde per liter of air for 35 weeks, followed by 29 weeks of repeated exposure to 0.15 mg of formaldehyde per liter, showed no evidence of development of pulmonary neoplasms when killed after 64 weeks. In contrast, 5 of 33 mice subjected to repeated inhalation of an aerosol of coal tar at 0.30 mg per liter of air developed squamous-cell tumors, one of which was a carcinoma. One mouse of a parallel group that inhaled the same concentration of coal tar, after a preliminary 35-week period of repeated inhalation of formaldehyde (0.1 mg per liter), developed a squamous-cell tumor. If one considers the small number of animals surviving into the period of development of tumors, the difference in incidence of neoplasms is not significant. In addition, the familiar peripheral adenoma was seen in 1 mouse of each group. Microscopic examination of sections of the lungs of a group of 30 unexposed C3H mice, 82 weeks old, disclosed no adenomas or squamous-cell tumors.