In general terms, the phrase "third-hand tobacco smoke" ("THS") refers to the tobacco particulates and gas that get on clothes, in hair, etc. You have smelled it when you approach a smoker, even if they are smoking at that time. Technically, it consists of residual tobacco smoke pollutants that 1) remain on surfaces and in dust after tobacco has been smoked, 2) are re-emitted back into the gas phase, or 3) react with oxidants and other compounds in the environment to yield secondary pollutants. Although obnoxious, is it dangerous? At this stage of toxicological research, it depends.
In 1991 nicotine was first reported as being detected in the dust of homes of smokers. See http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=Search&doptcmdl=Citation&defaultField=Title%20Word&term=Hein%5Bauthor%5D%20AND%20Indoor%20dust%20exposure%3A%20an%20unnoticed%20aspect%20of%20involuntary%20smoking.%20AND%20Arch%20Environ%20Health%5BJournal%5D%20AND%201991%5BPublication%20Date%5D%20AND%2098%5BPagination%5D.
In 2004, the amount present in house dust was quantified. See http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=Search&doptcmdl=Citation&defaultField=Title%20Word&term=Matt%5Bauthor%5D%20AND%20Households%20contaminated%20by%20environmental%20tobacco%20smoke%3A%20sources%20of%20infant%20exposures.%20AND%20Tob%20Control%5BJournal%5D%20AND%202004%5BPublication%20Date%5D%20AND%2029%5BPagination%5D.
In 2008, the amount present was quantified for the interior of cars of smokers. See http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=Search&doptcmdl=Citation&defaultField=Title%20Word&term=Matt%5Bauthor%5D%20AND%20Residual%20tobacco%20smoke%20pollution%20in%20used%20cars%20for%20sale%3A%20air%2C%20dust%2C%20and%20surfaces.%20AND%20Nicotine%20Tob%20Res%5BJournal%5D%20AND%202008%5BPublication%20Date%5D%20AND%201467%5BPagination%5D.
In 2010, research showed that nicotine was present in homes two months after the smokers had left. See http://tobaccocontrol.bmj.com/content/20/1/e1.abstract.
Studies have also shown that some tobacco smoke compounds adsorb onto interior walls of homes, and over time desorb. See http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=Search&doptcmdl=Citation&defaultField=Title%20Word&term=Singer%5Bauthor%5D%20AND%20Gas-phase%20organics%20in%20environmental%20tobacco%20smoke.%201.%20Effects%20of%20smoking%20rate%2C%20ventilation%2C%20and%20furnishing%20level%20on%20emission%20factors.%20AND%20Environ%20Sci%20Technol%5BJournal%5D%20AND%202002%5BPublication%20Date%5D%20AND%20846%5BPagination%5D and http://cat.inist.fr/?aModele=afficheN&cpsidt=15352925.
In terms of the potential for harm, recent research has raised some concerns. Research has shown that nicotine adsorbed onto surfaces reacts with nitrous acid, a common air pollutant from vehicle exhaust and improperly vented stoves, as well as from burning tobacco, to form tobacco-specific nitrosamines. See http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=Search&doptcmdl=Citation&defaultField=Title%20Word&term=Sleiman%5Bauthor%5D%20AND%20Formation%20of%20carcinogens%20indoors%20by%20surface-mediated%20reactions%20of%20nicotine%20with%20nitrous%20acid%2C%20leading%20to%20potential%20thirdhand%20smoke%20hazards.%20AND%20Proc%20Natl%20Acad%20Sci%20USA%5BJournal%5D%20AND%202010%5BPublication%20Date%5D%20AND%206576%5BPagination%5D.
Some of these nitrosamines are classified as carcinogenic, likely to be carcinogenic, or potential mutagens. See http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=Search&doptcmdl=Citation&defaultField=Title%20Word&term=Crespi%5Bauthor%5D%20AND%20A%20tobacco%20smoke-derived%20nitrosamine%2C%204-%28methylnitrosamino%29-1-%283-pyridyl%29-1-butanone%2C%20is%20activated%20by%20multiple%20human%20cytochrome%20P450s%20including%20the%20polymorphic%20human%20cytochrome%20P4502D6.%20AND%20Carcinogenesis%5BJournal%5D%20AND%201991%5BPublication%20Date%5D%20AND%201197%5BPagination%5D; http://monographs.iarc.fr/ENG/Monographs/vol89/mono89.pdf; and, http://ntp.niehs.nih.gov/index.cfm?objectid=32BA9724-F1F6-975E-7FCE50709CB4C932.
Evidence has also developed that THS reacts to form ultra-fine particulates smaller than 100 nm. See, for example, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2974714/.
Yet, despite all this suggestive evidence, proof of harm has not been formally demonstrated; that being said, there is reason for serious concern. However, such research about harm from THS is about to begin. The California Tobacco-Related Disease Research Program, which is funded by the California tobacco tax, recently made $3.75 million of funding available for studying THS and cigarette butt waste. See http://www.trdrp.org/fundingopps/callrfp.asp.
This is clearly a subject about which further information will be developed over the next several years.
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