The term third-hand smoke or "THS" 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 is contamination on the surfaces of objects that remains after the second-hand smoke has cleared.
Biomarkers of disease
It is important to acknowledge the biomarkers of third-hand smoke because they indicate the severity and presence of disease. In a recent study involving mice, biomarkers of THS were discovered after four weeks of initial exposure at equivalent levels to those found in homes of smokers. Researchers used a system that mimicked that of the exposure to humans and sought out biomarkers found in serum, liver, and brain tissues in the mice. The mice were then exposed to THS at varying months and at as early as one month, they began to show signs of increased circulating inflammatory cytokines, tumor necrosis factor, granulocyte macrophage colony stimulating factor, and an increase in the stress hormone epinephrine. Damage from THS exposure continued after two, four, and six months. Such damages included oxidative stress and molecular damage. Some of the mice also became hyperglycemic and hyperinsulinimic, which could mean that insulin resistance could be a consequence of long-term exposure. This study implies that increased exposure time to THS can have dramatic effects. Additional studies with human subjects are still needed to fully understand the implications of THS.
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 exposed 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 chemicals released in third-hand smoke reacts with indoor pollutants to create a toxic hazard to those exposed, especially children. Carcinogenic tobacco-specific nitrosamines (TSNAs) can be formed when the common indoor pollutant, nitrous acid, comes into contact with nicotine. One TSNA, called 4-(methylnitrosamino)-1-(3-pyridinyl)-1-butanone, is a potent lung-specific carcinogen ubiquitous in tobacco smoke and smokers' homes. The carcinogens found in THS are known as tobacco-specific nitrosamines.
A study was done in 2016 to look at how long third-hand smoke stayed in three different fabrics over a timespan of 72 hours and post washing. The three different fiber types included wool, cotton, and polyester. Levels of THS were measured using a self-designed surface acoustic wave gas sensor (SAW) which measures a frequency change when a compound is laid down on the surface of the sensor. The results of this study found that third-hand smoke does tend to stay in wool the most right after smoking and polyester the least. Wool had the slowest desorption while polyester had the fastest. Also, the study concluded that even though doing laundry and washing these fibers with detergent was an effective way to get rid of some of the smoke, there was still about an average of 300 Hz of THS residue left on all the fibers.
A 2010 study published in the Proceedings of the National Academy of Sciences found that nicotine residue which coats 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.
A study in 2004 measured the levels of nicotine in dust in the homes of three different groups of families. Homes where parents smoked with children present in the home had the highest levels of nicotine found in dust in all rooms of the house, including the rooms of infants and children. Homes where parents attempted to limit exposure of cigarette smoke to their children had lower levels of nicotine found in dust. Homes that had not been smoked in did not contain any traces of nicotine.
Exposure and health effects
Humans can be exposed to THS through inhalation, skin contact, or ingestion. There are also many surfaces that can accumulate THS compounds. Common surfaces that humans come into contact with daily include couches, furniture, curtains, and car seats. THS 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 or 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 still developing systems.
According to a study conducted by Northrup, 22% of infants and children are exposed to SHS/THS in their homes each year, comprising a major proportion of the 126 million nonsmokers exposed to harmful tobacco products annually.
Though research is limited, there are many harmful health effects that have been linked to THS exposure. THS has the potential to impair wound healing by altering the body’s natural anti-inflammatory response, remodel respiratory structure due to increased collagen deposits in airways, and cause permanent damage to DNA. Other health effects include asthma, increased cough morbidity, and other respiratory conditions. If pregnant mothers are exposed to THS, it can slow fetal lung development in the third trimester. In children, THS exposure has also been linked to sudden infant death syndrome (SIDS) as a potential cause factor, and has also been linked to cognitive and memory deficits in growing children. Whether or not these largely theoretical relationships are actually causal in humans at realistic exposure levels remains to be seen.
Public awareness and solutions
Third-hand smoke is a relatively newly postulated concept, and public awareness of it is lower than that of second-hand smoke. A 2013 study with six focus groups in metro and rural Georgia (USA) asked participants whether they had heard of THS, most of the participants had not heard about it and did not know what third-hand smoke was. Research on THS is growing, but does not compare to the 40 years of research on the effects of second-hand smoke. Yet, it is estimated that 5%-60% of second-hand smoke-related harm may actually be attributable to third-hand smoke exposure. THS poses such a risk because its exposure can linger much longer than second-hand smoke. Third-hand smoke-contaminated surfaces like carpet, walls, and car interiors are also especially hard to clean whereas second-hand smoke can be removed with ventilation.
Smoking rates in the United States have fallen considerably from a high of 42.4% in 1965 to around 17.8% in 2016. But the downward trend is slowing and it is estimated that 23% to 42% of adults in America with low education or living in poverty still smoke. This exposes millions of nonsmokers, many of which are children.
Awareness campaigns for THS are steadily growing in number and primarily focus on the health risks for young children. A 2014 study published in 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 are 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 by smoking outside and showering and changing clothes before coming into 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.
Policy in the United States
Smoking restrictions have been in place in the US as early as 1975 starting in Minnesota and carrying on with various local and state governments legislating smoke-free or clean indoor air laws. By 1999, all 50 US states and the District of Columbia had some public smoking restrictions. Currently, 26 states have implemented comprehensive “smoke-free” laws that ban smoking in worksites, restaurants and bars and other public areas.
However, while many states have laws in place to ensure smoke-free public areas, children especially receive the majority of their exposure while at home, in private spaces.
An exposure assessment study performed by the Lawrence Berkeley National lab found that 80% to 90% of combusted cigarette nicotine adsorbs into surfaces. It has been shown that deposited nicotine may desorb or resuspend from non-cleaned surfaces and adsorb or redeposit elsewhere, including previously cleaned surfaces.
According to the Americans for NonSmokers’ Rights organization website, “Parents, landlords, business owners and others need to be aware of the health risks of exposure to thirdhand smoke and recognize that eliminating smoking is the only way to protect against tobacco's smoke contamination.”  This concept is referred to as the "cessation imperative" in a 2017 paper; the only way to fully protect people from exposure to thirdhand smoke is for smokers to quit smoking because even smoking in places when others are not present can expose people to tobacco smoke contaminants.
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