Biologic factors in cancer etiology refer largely to the role of four classes of external agents in carcinogenesis: physical, chemical and biological agents, and diet.

Physical Agents:
* Ionizing radiation can be background from cosmic rays and earth sources of radioactivity. More important are cumulative exposures from medical diagnostic and treatment procedures, and from commercial and occupational sources. Exposures have also occurred with warfare, as in the atomic bombs dropped on Hiroshima and Nagasaki in Japan in world war II. Leukemias and cancers of the breast, lung and thyroid are typical but cancers of the stomach, colon, bladder, and potentially any human tumor may be seen.
* Nonionizing radiation of solar origin, especially the ultraviolet (UV)B wavelengths, are associated with basal and squamous skin cancers and with malignant melanoma. Certain inherited skin types (Celtic skin) are at greater risk. Commercial sources such as tanning parlors also provide risk.
* Particles can also be important. The cancer risk with asbestos relates to fiber length and toughness. The risk from particles in air pollution is related to their size and propensity to settle in the lung.
 

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