Tobacco-specific nitrosamines are relevant to both smoking and smokeless tobacco use but are of particular prominence in the use of smokeless tobacco because polycyclic aromatic hydrocarbons are generated by combustion and are therefore not implicated in smokeless tobacco carcinogenesis. These compounds are formed by the nitrosation of nicotine and its metabolites. The nature and amount of these compounds depends on the tobacco species, the growing conditions, the amount of fermentation and ageing, and how finely the tobacco is cut or powdered. Smokeless tobacco products, as with many cigarette brands, usually have an ingredient that increases the pH, to promote the availability of nicotine for absorption. Such agents include ammonia, ammonium
