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Smoking causes cancer. That may be enough for some people, but for those interested in what happens in between, what follows is one explanation. Upon inhalation of tobacco smoke, polycyclic aromatic hydrocarbons and certain nitrosamines are deposited in body tissue. These are metabolized by cytochrome P450 and other enzymes, forming reactive metabolic intermediates that form aberrant bonds with DNA strands. Bonds that go unaltered by repair mechanisms can induce permanent mutations. After a thousand or so inhalations, an arbitrarily large…