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Male Sprague Dawley rats were exposed by inhalation for 6 h per day on one or three successive days to 0, 2, 6 or 20 ppm formaldehyde or 0, 0.2 or 0.6 ppm acrolein. Nasal and tracheal epithelial and free lung cells were then analyzed for proliferative responses. The proliferative response was expressed as the proportion of DNA synthesizing cells determined by the 5-bromodeoxyuridine (BrdU) labeling technique. Single exposure to both irritants increased the DNA synthesizing (proliferating) cells by a factor of 3. After three exposures the increase was distinctly lower. All sites analyzed showed approximately the same concentration/response pattern. Significant changes in cell proliferation could be detected at 2 ppm formaldehyde and 0.2 ppm acrolein.

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