Estimates have been obtained for all half-lives of several chlorinated aliphatic pollutants in the Canadian troposphere. The rates of reaction are strongly dependent on the magnitude of the second-order rate constant for reaction of the pollutant with OH, and on the intensity of solar radiation , which is determined both by season and geographical location. Of the pollutants studied, trichloroethylene has the shortest half-life of 2–5 days in summer, depending on location. Dichloromethane, 1,2-dichloroethane and tetrachloroethylene are each about one order of magnitude longer lived than trichloroethylene; 1,1,1-trichloroethane and 1,1,2,2-tetrachloroethane are about two orders of magnitude longer lived than trichloroethylene. Under urban conditions, high rates of oxidation, and hence short half-lives, are favoured by high tropospheric concentrations of ozone and low concentrations of NO2. With the exception of 1,1,1-trichloroethane, none of these substances is predicted to pose a significants threat to stratospheric ozone.