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