Cigarette smoke contains about 4,000 chemicals. Carbon dioxide, carbon monoxide, methane, and nicotine are some of the major components, with lesser amounts of acetone, acetylene, formaldehyde, propane, hydrogen cyanide, toluene, and many others.

Why does the risk of cancer diminish rapidly after one quits the cigarette smoking habit?

Exposure of a premalignant cell to a promoter converts the cell to an irreversibly malignant state. Promotion is a slow process, and exposure to the promoter must be sustained for a certain period of time. This requirement explains why the risk of cancer diminishes rapidly after one quits smoking; both cancer initiators and promoters appear to be contained in tobacco smoke.