AbstractGuinea‐pigs were exposed to 400 ppm or 5500 ppm of isoprophyl alcohol (IPA) for 24 successive hours, and they were killed immediately after the exposure period. The ciliary activities of the nose and trachea were examined. In addition, the epithelium of the nose and trachea was observed by scanning and transmission electron microscopy.The present study showed that exposure to 400 or 5500 ppm of IPA vapor caused deterioration of the ciliary activity and also some pathological changes. Although our study revealed that exposure to 400 ppm of IPA vapor can affect the mucosa of the nose and trachea, the ciliary activity of the 400‐ppm exposure group was not too poor and morphological changes were rather mild. Recovery from such degeneration might be rapid. Therefore, our study supports the justification of the allowable IPA level recommended by AC