Enzymes in the body detoxify or render inactive the aromatic rings. Studies with PAH s have shown structural similarities between intermediates and led to the bay region theory of PAH carcinogenesis. For a compound to be highly mutagenic and carcinogenic, diol epoxides must be generated in the bay region formed by the polycyclic structures of the molecules. Some carcinogenic hydrocarbons do not possess a bay region and have different structural requirements to be active carcinogens. PAH s from automobile exhaust, industrial pollution, pesticides, and chlorinated organic compounds in drinking water are known carcinogens.
Smoking is a clear cause of cancer.
However, there is a significant number of cancers in people with no clear evidence of exposure to defined carcinogens. Some of these mutations could be in genes that are involved in regulation of cell proliferation and differentiation. At the molecular level, mutations can involve substitutions, insertions, or deletions of one or more nucleotides in a DNA molecule. The spontaneous mutation rate in human somatic and germ-line cells has been estimated to be one mistake in 10 billion base pairs per each cell division. This is highly unlikely.