A concise assessment of the risks to human health and the environment posed by exposure to 2 2-dichloro-1 1 1-trifluoroethane (HCFC-123) a volatile liquid used as a refrigerant in commercial and industrial air-conditioning installations, in gaseous fire extinguishers as a foam-blowing agent, and in metal and electronics cleaning. Although HCFC-123 is known to contribute to ozone depletion, the significance of its role in global warming is far smaller than that of chlorofluorocarbons and bromofluorocarbons, which are being phased out in compliance with the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. Releases of HCFC-123 to the environment are primarily to ambient air. In the atmosphere the chemical has an estimated lifetime of less than two years. The risk to aquatic organisms is considered low. While exposure of the general public is expected to be minimal, occupational exposures during manufacture and use of products containing the chemical are of concern. In laboratory animals, exposure via inhalation has been shown to cause liver lesions, central nervous system depression, and adrenaline-induced cardiac arrhythmia. In studies involving repeated exposure, the principal targets of toxicity were the liver, the hypothalamic-pituitary-gonadal endocrine system, and the central nervous system. Although an increased incidence of benign tumors of the liver and other organs has been observed in one species, the relevance of these findings to carcinogenicity in humans could not be clarified. Adverse effects on the liver have been detected in some studies of exposed workers. Central nervous system, depression, and an increased likelihood of adrenaline-induced cardiac arrhythmia are the principle effects in humans of brief exposure.