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Abstract It has become rather generally accepted among rubber technologists that ozone stands alone as the cause of exposure check-cracking of rubber. Newton, Crabtree and Kemp, and Fielding are apparently supporters of this view, and the present authors have concurred. The viewpoint, however, need not be considered as fixed. The experiment with nitrogen dioxide, cited by Haagen-Smit, raises interesting speculation of a possible exception, unless indeed the explanation lies in an intermediate formation of ozone in the system nitrogen dioxide-oxygen-sunlight. (This likelihood was suggested by Haagen-Smit in a private communication to one of the authors.) Interesting as some of these variant possibilities might be, however, the present authors record their belief in the simple rather than the complex explanation of rubber checking in Southern California, namely, that of ozone. The general similarity of pattern between the city and noncity stations argues against any dominant effect due to special components of the city atmosphere. The general prevalence of checking in the whole area, relative to that of the country at large, is generally consistent with the high level of ozone indicated by the measurements given here. Under present information, it seems unlikely to us that effects of alternately proposed chemical agents will bulk importantly in the total effect. The further development of information on these questions may be awaited with much interest.