Evaluates the risks to human health and the environment posed by two partially halogenated chlorofluorocarbons: dichlorofluoromethane (HCFC 21) and chlorodifluoromethane (HCFC 22). These two methane derivatives were selected for evaluation because of their potential use as substitutes for those fully halogenated chlorofluorocarbons that are being phased out as a result of the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. The evaluation is intended to assist industry in its urgent search for acceptable substitute chemicals, most notably for use as refrigerants, as propellants in aerosols, and as blowing agents for the production of polystyrene. While data on human toxicity are thoroughly reviewed, the greatest challenge is to find the most accurate models for predicting levels of release to the environment and estimating the potential of these chemicals to deplete the ozone layer.