Trihalomethanes: II. Reversibility of toxicological changes produced by chloroform, bromodichloromethane, chlorodibromomethane and bromoform in rats.

Groups of 20 male and 20 female rats were fed 0, 5, 50, 500 or 2500 ppm trihalomethanes (chloroform, bromodichloromethane, chlorodibromomethane and bromoform) in their drinking water for 90 days. Ten rats from each group were killed at this time and the remaining animals were fed tap water for a further 90 days before they were sacrificed. Suppression of growth rate was observed in male and female rats fed 2500 ppm chloroform and bromodichloromethane (BDCM) for 90 days. After the subsequent 90-day recovery period, no effects on body weight gain in either sex were apparent. Food consumption was depressed in all groups of males and females receiving 2500 ppm chloroform, chlorodibromomethane (CDBM) and BDCM for 90 days and in these same groups during the recovery period. Bromoform at 2500 ppm suppressed food consumption of male rats but this effect was not evident 90 days after cessation of exposure. Decreased lymphocyte counts were only observed in the recovery groups fed chloroform (500 ppm, male), CDBM (2500 ppm, female) and bromoform (2500 ppm, females). Histological changes in the livers and thyroids of male and female rats were qualitatively and quantitatively similar for the four trihalomethanes. These changes were mild in nature, and were not evident 90 days after cessation of exposure. These data indicate that trihalomethanes can produce biochemical hematological and histological changes but most of these were reversible when exposure had terminated.