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Nitrosamines are used in the manufacture of some cosmetics, pesticides, and in most rubber products.
In foods, nitrosamines are produced from nitrites and secondary amines, which often occur in the form of proteins. Their formation can occur only under certain conditions, including strongly acidic conditions such as that of the human stomach. High temperatures, as in frying, can also enhance the formation of nitrosamines. The presence of nitrosamines may be identified by the Liebermann nitroso reaction  (not to be confused with the Liebermann's reaction).
Under acidic conditions the nitrite forms nitrous acid (HNO2), which is protonated and splits into the nitrosonium cation N≡O+ and water: H2NO2+ = H2O + NO+. The nitrosonium cation then reacts with an amine to produce nitrosamine.
These processes lead to significant levels of nitrosamines in many foodstuffs, especially beer, fish, and fish byproducts, and also in meat and cheese products preserved with nitrite pickling salt. The U.S. government established limits on the amount of nitrites used in meat products in order to decrease cancer risk in the population. There are also rules about adding ascorbic acid or related compounds to meat, in order to inhibit nitrosamine formation. 
Nitrosamines can also be found in tobacco smoke, American dip snuff, chewing tobacco, and to a much lesser degree, snus. (127.9 PPM for American dip snuff compared to 2.8 PPM in Swedish snuff or snus.)
In 1956, two British scientists, John Barnes and Peter Magee, reported that dimethylnitrosamine produced liver tumours in rats. Research was undertaken and approximately 90% of nitrosamine compounds were deemed to be carcinogenic.
In the 1970s, there was an increased frequency of liver cancer found in Norwegian farm animals. The farm animals had been fed on herring meal, which was preserved using sodium nitrite. The sodium nitrite had reacted with dimethylamine in the fish and produced dimethylnitrosamine.
Nitrosamines can cause cancers in a wide variety of animal species, a feature that suggests that they may also be carcinogenic in humans. At present, available epidemiological evidence from case-control studies on nitrite and nitrosamine intake supports a positive association with gastric cancer risk. Regarding oesophageal cancer, available evidence supports a positive association between nitrite and nitrosamine intake and GC, between meat and processed meat intake and GC and OC, and between preserved fish, vegetable and smoked food intake and GC, but is not conclusive.
|Substance Name||CAS #||Synonyms||Molecular Formula||Physical Appearance||Carcinogenicity Category|
|N-Nitrosonornicotine||16543-55-8||NNN||C9H11N3O||Light Yellow Low Melting Solid|
|4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone||64091-91-4||NNK, 4'-(nitrosomethylamino)-1-(3-pyridyl)-1-butanone||C10H15N3O2||Light Yellow Oil|
|N-Nitrosodimethylamine||62-75-9||Dimethylnitrosamine, N,N-Dimethylnitrosamine, NDMA, DMN||C2H6N2O||Yellow liquid||EPA-B2; IARC-2A; OSHA Carcinogen; TLV-A3|
|N-Nitrosodiethylamine||55-18-5||diethylnitrosamide, diethylnitrosamine, N,N-diethylnitrosamine, N-ethyl-N-nitrosoethanamine, diethylnitrosamine, DANA, DENA, DEN, NDEA||C4H10N2O||Yellow liquid||EPA-B2; IARC-2A|
|N-Nitrosoanatabine||71267-22-6||NAT||C10H11N3O||Clear Yellow to Orange Oil||IARC-3|