Nitrosamines are used in the manufacturing cosmetics, cured meats and other consumables, metals, and rubber products, including latex gloves and condoms. 
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, as the compounds inhibit formation of nitrosamines.
Tobacco-specific 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.)
Nitrosamines can also be found in e-cigarette vapour, though this is reported to be trace amounts, possibly from the propylene glycol and/or vegetable glycerine used in manufacture.
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.
The risk for the induction of tumors from nitrosamines in condoms is considered very low, but not impossible.  Researchers at the Chemical and Veterinary Investigation Institute in Stuttgart reported that harmful levels of carcinogenic nitrosamines were released by 29 of 32 condom brands tested when they came into contact with bodily fluids. The researchers have urged condom manufacturers to seek alternative, non-carcinogenic chemicals for use in place of nitrosamines. 
Processed meat consumption has been associated with an increased risk of stomach cancer in some epidemiological studies. Nitrosamines may be responsible for this association. 
Examples of nitrosamines
|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|
- Nitroamine (without the 's'), compounds of the formula R2N-NO2.
- Nitroso, compounds of the formula R-NO
- Tobacco-specific nitrosamines
- Vogel A I, Practical Organic Chemistry, 3rd Ed. Impression 1962 p. 649
- Vogel A I, Practical Organic Chemistry, 3rd Ed. Impression 1962 p. 1074
- Honikel, Karl-Otto (January–February 2008). "The use and control of nitrate and nitrite for the processing of meat products". Meat Science 78: 68–76. doi:10.1016/j.meatsci.2007.05.030.
- Gregory N. Connolly, and Howard Saxner (August 21, 2001). "Informational Update Research on Tobacco Specific Nitrosamines (TSNAs) in Oral Snuff and a Request to Tobacco Manufacturers to Voluntarily Set Tolerance Limits For TSNAs in Oral Snuff".
- "E-cigarettes: harmless inhaled or exhaled, No second hand smoke". Health New Zealand. Retrieved 27 June 2014.
- "Nitrosamines and Cancer".
- Jakszyn, P; Gonzalez, CA (2006). "Nitrosamine and related food intake and gastric and oesophageal cancer risk: A systematic review of the epidemiological evidence". World journal of gastroenterology : WJG 12 (27): 4296–303. PMC 4087738. PMID 16865769.
- Hecht, Steven S.; Borukhova, Anna; Carmella, Steven G. "Tobacco specific nitrosamines" Chapter 7; of "Nicotine safety and toxicity" Society for Research on Nicotine and Tobacco; 1998 - 203 pages
- [ NIH Substance Profile]
- [ Spectrum; Chemical Fact Sheet]
- [ Safety data for N-nitrosodiethylamine]