Repeated 4-week inhalation exposure of rats: effect of low-, intermediate, and high-humidity chamber atmospheres.

A subacute nose-only inhalation study with low (approximately 3%), medium (approximately 40%), and high humidity (approximately 80%) has been performed on young adult Wistar rats. Exposure was 6-hr/day on 5 days/week for 4 consecutive weeks. Rats housed individually in the animal holding room, deprived of feed and water during exposure of the remaining groups, served as concurrent controls (sham controls). This study served the purpose to assess whether toxicologically significant effects occur when rats are repeatedly exposed to lower or higher humidity chamber atmospheres than proposed by current testing guidelines. For analysis, conventional end-points as required by common testing guidelines were considered, i.e., clinical observations before and after exposure, rectal temperatures, body weights, feed and water consumption. At the end of the 4-week exposure period, ophthalmological and gross pathological examinations were made and major organ weights determined. The histopathological examinations comprised the nasal cavities, larynx, trachea, and lungs. There was no apparent evidence of humidity-related effects on nose-only exposed rats. When compared with non-exposed sham controls, however, body weights, water and feed consumption were markedly reduced in all nose-only exposure groups. In summary, it can be concluded that rats tolerated either humidity atmosphere without any specific effects. As far as there were differences to sham controls they appear to be more controlled by the differences in the exposure patterns (nose-only versus normal housing) than differences in the humidity of chamber atmospheres. Thus, deviations of current testing guidelines for repeated exposure inhalation studies with regard to humidity, do not appear to have any appreciable impact on the study outcome.