English: Tobacco-specific N-nitrosamines is formed by N-nitrosation of alkaloids in tobacco. NNN (N′-nitrosonornicotine) and NNK (4-(metylnitrosamino)-1-(3-pyridyl)-1-butanon) are among the most important and most potent carcinogens in tobacco and tobacco smoke. NNAL (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol) is a metabolite of NNK and determination of total NNAL (NNAL and its glucuronides) in urine is often used to assess the possible role of TSNA in tumor development. A significant relationship between total NNAL has been found in serum samples from smokers and lung cancer risk. NNN and NNK are classified by International Agency for Cancer Research (IARC) as human carcinogens.