- 34 Comments: It is obviously true that "the relative significance of chemical agents, viruses and radiation in the incidence of human cancer is not known." It is true that the “details of the mechanisms of cancer induction by chemical agents and viruses also are poorly understood." However, Z~< would point out that many experts in carcinogenesis would regard these mechanisms as better understood than the mechanisms of cancer induction by radiation. The 3rd sentence of this paragraph states flatly and without elaboration that "the proposed chemical carcinogens in cigarette smoke and in polluted urban environmenta have not been demonstrated to be carcinogenic at the low concentrations involved." Even if this were true, it may be only a question of more time and investigation, as it has been with understanding the effects of radiation. Furthermore, the same might be said for the low alpha radiation activity by itself in regard to cigarette smoke, as compared with the effects of the rest of the smoke (see previous comments on the author's page 2). The fourth sentence of this paragraph states: "For all of these reasons it is deemed Likely that radiation, and alpha radiation in particular, may be the principal agent of human cancer."' The "reasons" referred to in this paragraph, even if they were true, would hardly be reasons for anything but further research, and they certainly do not provide an acceptable scientific basis for this enormously sweeping generalization and summary dismissal of all other known and suspected agents and conditions which may cause or help to cause human cancer. The fifth sentence implies that NCI has greater interest in interest radiation but ascribes this etiological factors other than