Bleaching of paper pulp represents a major industrial use of chlorine and could provide an environmentally significant source of atmospheric halocarbons. The related global production of chloroform is estimated at 3¿105 ton yr-1 and there could be additional production associated with atmospheric decomposition of perchloroethylene. Estimates are given for the production of methyl chloride, methyl bromide and methyl iodide, 5.2¿106, 7.7¿104, and 7.4¿105 ton yr-1 respectively. The relative yields of CH3Cl, CH3Br and CH3I are consistent with the hypothesis of a marine biological source for these compounds. Concentrations of other halocarbons observed in the atmosphere appear to indicate industrial sources.