Dr. Klein-Szanto and colleagues at the Fox Chase Cancer Center employed a novel exposure system to explore the capacity of formaldehyde to cause cancerous changes in human epithelial cells. Formaldehyde is classified as a toxic air pollutant and is emitted in exhaust of motor vehicles, but whether or not formaldehyde is injurious to human health is controversial. The investigators obtained autopsy samples from human infant airways and from adult nasal tissue. They propagated the epithelial cells in culture and injected them into isolated segments of rat tracheas from which the epithelium had been removed. The tracheas then were placed under the skin of mice to allow the cells to attach and grow in the tracheas. For the exposures, the investigators inserted into the tracheas small silicon tubes containing different amounts of powdered formaldehyde. After 2, 4, 8, and 16 weeks, they evaluated the epithelial cells' appearance and determined the formaldehyde dose that produced the most cell changes. They also exposed the cells to formaldehyde alone and in combination with a potent carcinogen (a metabolite of benzo[a]pyrene) for 6 to 12 months and looked for cell changes suggesting cancer.