Thirdhand smoke refers to residual tobacco smoke constituents that remain on surfaces and in dust after tobacco has been smoked. These substances are then re-emitted as gases or react with other compounds in the environment to create other substances. 1, 2
Research shows that smoking in the home is associated with persistent high levels of tobacco toxins long after smoking has ended. 2, 3 Vapour phase constituents adsorb onto walls, furniture, clothes, toys and other objects within 10 minutes to hours after a cigarette has been smoked inside the house. 4 These substances are then re-emitted into the air over the course of hours, weeks and months. 2, 4 Particulate matter from secondhand smoke settles and coats household surfaces and contaminates dust, where it may react with vapour phase constituents or, to a small extent, become re-suspended in the air. 4, 5 Some of the toxic constituents found in thirdhand smoke are not present in secondhand smoke or mainstream smoke and are highly toxic. 1, 6 The main constituents of thirdhand smoke are nicotine, 3-ethenylpyridine, phenol, cresols, naphthalene (potentially fatal to those with G-6-PD deficiency), formaldehyde and tobacco-specific nitrosamines. 7
Nicotine adsorbed onto indoor surfaces reacts with nitrous acid in the air to form tobacco-specific nitrosamines, which are potent carcinogens. 8 It is not known whether these nitrosamines react to form other compounds or whether they build up with time. However, they are quite stable in indoor conditions and are likely to accumulate. 9, 10 Ultrafine particles are also produced through the oxidative ageing of secondhand smoke. 11
Thirdhand smoke accumulates in smokers’ homes, with dust and surfaces being the two major reservoirs of smoke constituents. Nicotine has been found in smokers’ homes at levels 12 to 21 times those in non-smokers’ homes. 3 Once present it persists for months to years even if tobacco is no longer smoked. Nicotine can permeate all parts of the indoor environment and strongly absorb onto a variety of materials. Cleaning may not adequately remove nicotine and other thirdhand smoke constituents and instead be re-suspended and re-deposited on cleaned surfaces.
Thirdhand smoke persists in homes, cars and hotel rooms months after the last tobacco was smoked. Home surface nicotine levels have been demonstrated to decrease after a change in occupancy from smoker to non-smoker, however, nicotine is still present in the air, on surfaces and on the fingers of new non-smoking residents’ two months after moving into homes of former smokers. 3, 12 Finger nicotine levels for non-smokers who have moved into former homes of smokers are seven to eight times higher than those in non-smoker homes. 3
Even with indoor smoking bans, levels of contamination in homes and exposure among children remain higher than in smoke-free households. 4, 13, 14 This may be partly due to smoke drift from outdoor areas through open windows and doors and residual tobacco smoke coming from the lungs of smokers after they have finished a cigarette. However, indoor contamination is also likely to be due to past indoor smoking as well as introducing smoke constituents through contaminated clothing, skin and dust carried into the home. 4 The introduction of nicotine to environments where smoking has never occurred has been demonstrated in a study of nicotine contamination in a neonatal intensive care unit. 15 In this study nicotine was tested for on hospital furniture and in the urine of newborn babies who had a parent who was a smoker. Nicotine contamination was present on the cots and other hospital furniture as well as in the urine of the newborn babies participating in the study.
In contrast to secondhand smoke that is inhaled, thirdhand smoke can be inhaled, ingested or absorbed through skin. Young children are more likely to be at risk of thirdhand smoke exposure because they typically spend more time indoors and tend to be active near the floor, close to contaminated dust, carpets, blankets and other objects. Young children frequently put objects in their mouths, and tend to ingest more dust than adults. They also tend to be in close physical contact with their smoking parents, whose clothes, hair and skin tend to be coated with thirdhand smoke. 4
Thirdhand smoke is an emerging area of research and it is not yet possible to fully understand the health hazards from thirdhand smoke. However, in vitro and animal studies have demonstrated a range of health effects from thirdhand smoke that are likely to be relevant for human health, including toxicity to the liver and lung, reduced wound healing and hyperactive behaviour. 16-18
While the proportion of harm that is attributable to thirdhand smoke (compared with secondhand smoke) is unknown, there is increasing evidence and consensus that thirdhand smoke contributes significant harm.
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References
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