The term third-hand smoke is a neologism coined by a research team from the Dana–Farber/Harvard Cancer Center. The 'third-hand' component of the term is a reference to the remnants on surfaces after "second-hand smoke" has cleared out. The term first-hand smoke refers to what is inhaled into the smoker's own lungs, while second-hand smoke is a mixture of exhaled smoke and other substances leaving the smoldering end of the cigarette that enters the atmosphere and can be inhaled by others; third-hand smoke, by that token, is contamination on the surfaces of objects that remains after the second-hand smoke has cleared.
Although "second-hand smoke" dissipates from a room or confined space after a short period of time, nicotine and other components of the smoke tend to coat a space's surfaces including skin, hair and clothing and continue to emit toxins. Third-hand smoke is environmental tobacco smoke that has oxidized with environmental nitrous acid to create carcinogens not seen in cigarette ingredients or tobacco smoke. The carcinogens found in third hand smoke are known as tobacco-specific nitrosamines. A 2010 study published in the Proceedings of the National Academy of Sciences journal found that the residue of the nicotine coating smokers as well as, interior car or room surfaces can react with nitrous acid present in the air to create tobacco-specific nitrosamines, carcinogens found in tobacco products. This was demonstrated by spraying what was termed a "high but reasonable" level of nitrous acid (about 4–12 times the levels typically found in homes) onto cellulose substrates used to wipe the interior of a vehicle that had been heavily smoked in over time. Similar results were found when cellulose substrates were kept (without wiping) in the same vehicle for three days when smoking occurred. Ensuring ventilation while a cigarette is smoked does not eliminate the deposition of third-hand smoke in an enclosed space, according to the study's authors. The study found that eleven carcinogenic compounds could be found in third-hand smoke, including the radioactive element polonium-210. Third-hand smoke is thought to potentially cause the greatest harm to infants and young children, because younger children are more likely to put their hands in their mouths, be cuddled up to a smoker with toxins on their skin and clothes. Infants also crawl on the floor and eat from their hands without washing them first, ingesting the toxins into their systems.
Public awareness and implications
Third-hand smoke is a relatively newly postulated concept, and public awareness of it is lower than that of second-hand smoke. A 2014 study published in the journal Pediatrics demonstrated that parents are more likely to attempt to quit smoking if they become convinced that third-hand smoke is harmful to children and more likely to have smoke-free home and car policies if they are aware of the dangers of third-hand smoke. It was recommended for parents to safeguard their children by ensuring they have a smoke free zone. One such way recommended for smokers to protect family, friends and others is washing skin and hair and changing clothes before coming in to contact with others. However, research has shown that parents who are heavy smokers (> 10 cigarettes per day) are less likely to believe that third-hand smoke is harmful to children. Third-hand smoke is one of the issues promoting indoor smoking bans, especially supported by performers such as musicians, who are forced to bring third-hand smoke contamination into their homes through contaminated instruments and cases. 
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