Passive smoking is the inhalation of tobacco smoke by individuals who are not actively smoking. It occurs when tobacco smoke diffuses into the surrounding atmosphere as an aerosol pollutant, which leads to its inhalation by nearby bystanders within the same environment. Such pollutant is known by various names, including passive smoke, second-hand smoke (SHS) or environmental tobacco smoke (ETS). Exposure causes many of the same health effects caused by active smoking, "There is sufficient evidence that involuntary smoking (exposure to secondhand or 'environmental' tobacco smoke) causes lung cancer in humans" although at a lower prevalence due to the reduced concentration of smoke that enters the airway.
According to a World Health Organization (WHO) report published in 2023, more than 1.3 million deaths are attributed to passive smoking worldwide every year. The health risks of passive smoke are a matter of scientific consensus, and have been a major motivation for smoking bans in workplaces and indoor venues, including restaurants, bars and night clubs, as well as some open public spaces.
Concerns around passive smoking have played a central role in the debate over the harms and regulation of tobacco products. Since the early 1970s, the tobacco industry has viewed public concern about the health effects of passive smoking as a serious threat to its business interests. Despite the industry's awareness of its harms as early as the 1980s, the tobacco industry coordinated a scientific controversy with the purpose of stopping regulation of their products.
Terminology Fritz Lickint created the term "passive smoking" ("Passivrauchen") in a publication in the German language during the 1930s. Terms used include "environmental tobacco smoke" to refer to the airborne matter, while "involuntary smoking" and "passive smoking" refer to exposure to second-hand smoke. The term "environmental tobacco smoke" can be traced back to a 1974 industry-sponsored meeting held in Bermuda, while the term "passive smoking" was first used in the title of a scientific paper in 1970. The Surgeon General of the United States prefers to use the phrase "second-hand smoke" rather than "environmental tobacco smoke", stating that "The descriptor 'second-hand' captures the involuntary nature of the exposure, while 'environmental' does not." Most researchers consider the term "passive smoking" to be synonymous with "second-hand smoke". In contrast, a 2011 commentary in Environmental Health Perspectives argued that research into "thirdhand smoke" renders it inappropriate to refer to passive smoking with the term "second-hand smoke", which the authors stated constitutes a pars pro toto.
The term "sidestream smoke" is sometimes used to refer to smoke that goes into the air directly from a burning cigarette, cigar, or pipe,: "During smoking of cigarettes, cigars, pipes and other tobacco productions, in addition to the mainstream smoke drawn and inhaled by the smokers, a stream of smoke is released between puffs into the air from the burning cone. Once released, this stream (also known as the sidestream smoke) is mixed with exhaled mainstream smoke as well as the air in an indoor environment to form the secondhand smoke to which ..." while "mainstream smoke" refers to smoke that a smoker exhales."Environmental tobacco smoke." Children's Environmental Health Centers, U.S. Environmental Protection Agency. .
Health effects
Passive smoke causes many of the diseases of direct smoking, including cardiovascular diseases, cancer, and respiratory diseases. It is associated with an overall increased risk of death in both adults and children. A study published in Lancet Public Health concluded that in 2023, exposure to second-hand smoke accounted for about 1.66 million deaths globally and about 44.8 million deaths and disability-adjusted life-years (DALYs). As of 2017, passive smoking causes about 900,000 deaths a year, which is about 1/8 of all deaths caused by smoking.
Passive smoking is a risk factor for lung cancer, and is estimated to cause over 7,000 deaths from lung cancer among non-smokers every year in the US, with three quarters of cases being people that have smoked previously. Similarly, passive smoking has been found to increase the risk of cervical cancer by a review from 2015, and by a 2016 review to significantly increase the risk of bladder cancer.
Second-hand smoke is a significant risk factor for breast cancer in females, as determined by a 2024 review. Historically, research has been less conclusive: The US Surgeon General in 2006 stated that there is "suggestive" evidence for passive smoking increasing the risk of breast cancer in females, but that it was not conclusive enough to assert causation, after the California Environmental Protection Agency found there to exist an increase in risk of 70% in younger, primarily premenopausal, females. Two years prior, in 2004, The IARC had concluded that there was no support for causation in never-smokers. In 2015, a meta-analysis found the evidence for moderate increase to breast cancer risk to have become "more substantial than a few years ago."
Exposure to second-hand smoke worsens existing asthma, allergies, and other respiratory conditions, increases the risks of developing asthma and chronic obstructive pulmonary disease, and has also been found associated with sinusitis. Among children and abdolescents, passive smoking is associated with a slightly higher risk of developing allergies, according to A 2014 systematic review, with evidence being weaker for adults.
Passive smoking may increase the risk of tuberculosis infection and accelerate the progression of the disease, as found by a 2015 meta-analysis, although noting that the evidence is weak. It may also increase the risk of carrying Neisseria meningitidis and Streptococcus pneumoniae, and in children increases the risk of middle ear infections and invasive meningococcal disease. A possible link to periodontitis has been found.
Passive smoking increases the likelihood of heart disease and reduced heart rate variability. Epidemiological studies have shown that passive cigarette smoking increases the risk of atherosclerosis, similarly to active smoking.
There is evidence for a correlation of passive smoke exposure with incidence of Type 2 diabetes, but it remains unclear whether the association is causal.
There is a strong association with an increased risk of stroke, disproportionately heavily at low levels of exposure. Exposure may also increase the risk of cognitive impairment and dementia in adults 50 and over, and causes children show reduced vocabulary and reasoning skills when compared with non-exposed children, as well as more general cognitive and intellectual deficits. Exposure is additionally associated with an increased risk of depressive symptoms.
A 2016 systematic review and meta-analysis found that passive smoking was associated with a higher rate of atopic dermatitis.
In children
Passive smoking in children carries many of the same adverse effects as in adults, such as those on the cardiovascular system, and increased risks and effects of asthma and allergies. Passive smoke exposure is associated with an almost doubled risk of hospitalization for asthma exacerbation among children with asthma.
Passive smoking increases the risk of lung infections, also including more severe illness with bronchiolitis and bronchitis, and of worse outcome, as well as increasing the risk of developing tuberculosis if exposed to a carrier. In the United States, it is estimated that passive smoke has been associated with between 150,000 and 300,000 lower respiratory tract infections in infants and children under 18 months of age, resulting in between 7,500 and 15,000 hospitalizations each year.
Exposure to second-hand smoke can lead to learning difficulties, developmental delays, executive function problems, and neurobehavioral effects. Animal models suggest a role for nicotine and carbon monoxide in neurocognitive problems.
As of 2015, most studies have found a significant association between passive smoking and sleep disordered breathing in children, but further studies are needed to determine whether this association is causal.
Anesthesia complications and increased chance of some negative surgical outcomes have also been noted.
During pregnancy
Pregnant females exposed to passive smoke exposure during pregnancy have higher risks of stillbirth and of delivering children of longer lengths, low weights, smaller head circumferences, or with congenital abnormalities such as neural tube defects. Maternal passive smoking increases the risk of non-syndromic orofacial clefts by 50% among their children.
There is evidence for a link between passive smoking and risk of giving premature birth, although the US Surgeon General in 2006 described the evidence of a causal link only as "suggestive". A 2014 meta-analysis found that laws limiting smoking decrease the rate of premature birth. A different 2014 meta-analysis found miscarriage to be 11% more common in exposed females, but did not conclude causation.
Presence of environmental smoke can cause sudden infant death syndrome (SIDS). In his 2006 report, the US Surgeon General concludes that there is evidence "sufficient to infer a causal relationship between exposure to passive smoke and sudden infant death syndrome." Passive smoking has been estimated to be associated with 430 SIDS deaths in the United States annually as of that year.
Evidence
Epidemiological studies show that non-smokers exposed to passive smoke are at risk for many of the health problems associated with direct smoking.
In 1992, a review estimated that passive smoke exposure was responsible for 35,000 to 40,000 deaths per year in the United States in the early 1980s. The absolute risk increase of heart disease due to ETS was 2.2%, while the attributable risk percent was 23%. A 1997 meta-analysis found that passive smoke exposure increased the risk of heart disease by a quarter, and two 1999 meta-analyses reached similar conclusions.
Evidence shows that inhaled sidestream smoke, the main component of passive smoke, is about four times more toxic than mainstream smoke. This fact has been known to the tobacco industry since the 1980s, though it kept its findings secret. Some scientists believe that the risk of passive smoking, in particular the risk of developing coronary heart diseases, may have been substantially underestimated.
In 1997, a meta-analysis on the relationship between passive smoke exposure and lung cancer concluded that such exposure caused lung cancer. The increase in risk was estimated to be 24% among non-smokers who lived with a smoker. In 2000, Copas and Shi reported that there was clear evidence of publication bias in the studies included in this meta-analysis. They further concluded that after correcting for publication bias, and assuming that 40% of all studies are unpublished, this increased risk decreased from 24% to 15%. This conclusion has been challenged on the basis that the assumption that 40% of all studies are unpublished was "extreme". In 2006, Takagi et al. reanalyzed the data from this meta-analysis to account for publication bias and estimated that the relative risk of lung cancer among those exposed to passive smoke was 1.19, slightly lower than the original estimate. A 2000 meta-analysis found a relative risk of 1.48 for lung cancer among men exposed to passive smoke, and a relative risk of 1.16 among those exposed to it at work. Another meta-analysis confirmed the finding of an increased risk of lung cancer among women with spousal exposure to passive smoke the following year. It found a relative risk of lung cancer of 1.29 for women exposed to passive smoke from their spouses. A 2014 meta-analysis noted that "the association between exposure to passive smoke and lung cancer risk is well established."