Changes in cancer incidence patterns among immigrants is revealing - the patterns gradually change from those prevalent in the country of origin to approximately those of the host country (as happened with Japanese immigrants to California and Hawaii). The recent increase in malignant melanoma is the direct result of changes in clothing habits and the fashion of being tanned. Other increases result from medical practices - the increase in thyroid cancer as a consequence of childhood exposure of the head and neck to X-rays, and the increase in endometrial cancer as a consequence of the use of menopausal oestrogens. The most important environmental factor in carcinogenesis is tobacco smoking, which alone accounts for 25-35% of all cancer deaths in men and 5-10% in women.
In 1998, the California Air Resources Board identified diesel exhaust as a "Toxic Air Contaminant" based on a review of animal and epidemiological studies, which strongly suggest a causal relationship between occupational diesel exhaust exposure and lung cancer. Already in 1989, the International Agency for Research on Cancer (IARC) had concluded that diesel engine exhaust is "probably carcinogenic to humans" (Group 2A), while gasoline engine exhaust was classified as "possibly carcinogenic to humans" (Group 2B).