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

A subacute nose-only inhalation study with low (approximate to 3 %), medium (approximate to 40 %), and high humidity (approximate to 80 %) has been pe rformed on young adult Wistar rats. Exposure was 6-hr/day on 5 days/week fo r 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 a ssess whether toxicologically significant effects occur when rats are repea tedly exposed to lower or higher humidity chamber atmospheres than proposed by current testing guidelines. For analysis, conventional end-points as re quired by common testing guidelines were considered, i.e., clinical observa tions before and after exposure, rectal temperatures, body weights, feed an d water consumption. At the end of the 4-week exposure period, ophthalmolog ical 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-re lated effects on nose-only exposed rats. When compared with non-exposed sha m 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 no r mal housing) than differences in the humidity of chamber atmospheres. Thu s, deviations of current testing guidelines for repeated exposure inhalatio n studies with regard to humidity, do not appear to have any appreciable im pact on the study outcome.