Formaldehyde is ubiquitous in the environment. As an aqueous solution, it causes burns on contact with the skin and eyes, and the vapour is irritating to the eyes and respiratory system. It is a sensitizing agent and may affect susceptible individuals at levels below the current British TLV of 2 ppm. It has been tested for teratogenicity in several animal species by a variety of routes, but was not positive in any of the studies. Inhalation of the vapour has been shown to cause nasal cancer in rats and mice. However, epidemiological studies have failed to confirm whether or not the compound is carcinogenic in humans.