thumb|right|Tobacco smoke in an [[Irish pub before a smoking ban came into effect on 29 March 2004]]

Passive smoking is the inhalation of tobacco smoke, called passive smoke, secondhand smoke (SHS) or environmental tobacco smoke (ETS), by individuals other than the active smoker. 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. Exposure to secondhand tobacco smoke causes many of the same health effects caused by active smoking, 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 secondhand smoke are a matter of scientific consensus,

Concerns around secondhand smoke 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 over secondhand smoke as a serious threat to its business interests. Despite the industry's awareness of the harms of secondhand smoke 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 secondhand 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 term "sidestream smoke" is sometimes used to refer to smoke that goes into the air directly from a burning cigarette, cigar, or pipe, while "mainstream smoke" refers to smoke that a smoker exhales.

Health effects

Secondhand smoke causes many of the same diseases as direct smoking, including cardiovascular diseases, lung cancer, and respiratory diseases. These include:

  • Cancer:
  • General: overall increased risk; reviewing the evidence accumulated on a worldwide basis, the International Agency for Research on Cancer concluded in 2004 that "Involuntary smoking (exposure to secondhand or 'environmental' tobacco smoke) is carcinogenic to humans."
  • Lung cancer: Passive smoking is a risk factor for lung cancer. In the United States, secondhand smoke is estimated to cause more than 7,000 deaths from lung cancer a year among non-smokers. A quarter of all cases occur in people who have never smoked.
  • Breast cancer: The California Environmental Protection Agency concluded in 2005 that passive smoking increases the risk of breast cancer in younger, primarily premenopausal females by 70%
  • Cervical cancer: A 2015 overview of systematic reviews found that exposure to secondhand smoke increased the risk of cervical cancer.
  • Circulatory system: risk of heart disease and reduced heart rate variability.
  • Epidemiological studies have shown that both active and passive cigarette smoking increase the risk of atherosclerosis.
  • Passive smoking is strongly associated with an increased risk of stroke, and this increased risk is disproportionately high at low levels of exposure.
  • Lung problems:
  • Risk of asthma
  • Risk of chronic obstructive pulmonary disease (COPD)
  • According to a 2015 review, passive smoking may increase the risk of tuberculosis infection and accelerate the progression of the disease, but the evidence remains weak.
  • The majority of studies on the association between secondhand smoke exposure and sinusitis have found a significant association between the two.
  • Cognitive impairment and dementia: Exposure to secondhand smoke may increase the risk of cognitive impairment and dementia in adults 50 and over. Children exposed to secondhand smoke show reduced vocabulary and reasoning skills when compared with non-exposed children as well as more general cognitive and intellectual deficits.
  • Mental health: Exposure to secondhand smoke is associated with an increased risk of depressive symptoms.
  • During pregnancy:
  • Miscarriage: a 2014 meta-analysis found that maternal secondhand smoke exposure increased the risk of miscarriage by 11%.
  • Low birth weight
  • Premature birth (Evidence of the causal link is described only as "suggestive" by the US Surgeon General in his 2006 report.) Laws limiting smoking decrease premature births.
  • Stillbirth and congenital malformations in children
  • Recent studies comparing females exposed to secondhand smoke and non-exposed females, demonstrate that females exposed while pregnant have higher risks of delivering a child with congenital abnormalities, longer lengths, smaller head circumferences, and neural tube defects.
  • General:
  • Worsening of asthma, allergies, and other conditions. A 2014 systematic review and meta-analysis found that passive smoking was associated with a slightly increased risk of allergic diseases among children and adolescents; the evidence for an association was weaker for adults.
  • Type 2 diabetes. It remains unclear whether the association between passive smoking and diabetes is causal.
  • Risk of carrying Neisseria meningitidis or Streptococcus pneumoniae.
  • Overall increased risk of death in both adults, where it was estimated to kill 53,000 nonsmokers per year in the U.S in 1991, and in children. The World Health Organization states that passive smoking causes about 600,000 deaths a year, and about 1% of the global burden of disease. As of 2017, passive smoking causes about 900,000 deaths a year, which is about 1/8 of all deaths caused by smoking.
  • Skin conditions: A 2016 systematic review and meta-analysis found that passive smoking was associated with a higher rate of atopic dermatitis.

Risk to children

thumb|Old prevention poster from [[New Zealand. "When a child breathes air filled with cigarette smoke it can be as bad as if he actually smoked the cigarette himself."]]

thumb|Children playing while their mothers smoke, 1945. Scenes such as this were common before the dangers of passive smoke were widely recognized.

  • Sudden infant death syndrome (SIDS). In his 2006 report, the US Surgeon General concludes: "The evidence is sufficient to infer a causal relationship between exposure to secondhand smoke and sudden infant death syndrome." Secondhand smoking has been estimated to be associated with 430 SIDS deaths in the United States annually.
  • Asthma. Secondhand smoke exposure is also associated with an almost doubled risk of hospitalization for asthma exacerbation among children with asthma.
  • Lung infections, also including more severe illness with bronchiolitis and worse outcome, as well as increased risk of developing tuberculosis if exposed to a carrier. In the United States, it is estimated that secondhand 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.
  • Maternal passive smoking increases the risk of non-syndromic orofacial clefts by 50% among their children.
  • Learning difficulties, developmental delays, executive function problems, and neurobehavioral effects. Animal models suggest a role for nicotine and carbon monoxide in neurocognitive problems.
  • Invasive meningococcal disease.
  • Anesthesia complications and some negative surgical outcomes.
  • Sleep disordered breathing: 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.
  • Adverse effects on the cardiovascular system of children.

Evidence

thumb|Exposure to secondhand smoke by age, race, and poverty level in the US in 2010

Epidemiological studies show that non-smokers exposed to secondhand smoke are at risk for many of the health problems associated with direct smoking.

In 1992, a review estimated that secondhand 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 secondhand 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 secondhand 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 secondhand 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". A 2000 meta-analysis found a relative risk of 1.48 for lung cancer among men exposed to secondhand 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 secondhand smoke the following year. It found a relative risk of lung cancer of 1.29 for women exposed to secondhand smoke from their spouses. A 2014 meta-analysis noted that "the association between exposure to secondhand smoke and lung cancer risk is well established."

A minority of epidemiologists have found it hard to understand how secondhand smoke, which is more diluted than actively inhaled smoke, could have an effect that is such a large fraction of the added risk of coronary heart disease among active smokers. One proposed explanation is that secondhand smoke is not simply a diluted version of "mainstream" smoke, but has a different composition with more toxic substances per gram of total particulate matter.

In 2004, the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) reviewed all significant published evidence related to tobacco smoking and cancer. It concluded:

With the release of formerly classified tobacco industry documents through the Tobacco Master Settlement Agreement, it was found (by Elisa Ong and Stanton Glantz) that the controversy over the WHO's alleged suppression of data had been engineered by Philip Morris, British American Tobacco, and other tobacco companies in an effort to discredit scientific findings which would harm their business interests. A WHO inquiry, conducted after the release of the tobacco-industry documents, found that this controversy was generated by the tobacco industry as part of its larger campaign to cut the WHO's budget, distort the results of scientific studies on passive smoking, and discredit the WHO as an institution. This campaign was carried out using a network of ostensibly independent front organizations and international and scientific experts with hidden financial ties to the industry.

EPA lawsuit

In 1993, the United States Environmental Protection Agency (EPA) issued a report estimating that 3,000 lung cancer related deaths in the United States were caused by passive smoking annually.

Philip Morris, R.J. Reynolds Tobacco Company, and groups representing growers, distributors and marketers of tobacco took legal action, claiming that the EPA had manipulated this study and ignored accepted scientific and statistical practices.

The United States District Court for the Middle District of North Carolina ruled in favor of the tobacco industry in 1998, finding that the EPA had failed to follow proper scientific and epidemiologic practices and had "cherry picked" evidence to support conclusions which they had committed to in advance. The court stated in part, "EPA publicly committed to a conclusion before research had begun...adjusted established procedure and scientific norms to validate the Agency's public conclusion... In conducting the ETS Risk Assessment, disregarded information and made findings on selective information; did not disseminate significant epidemiologic information; deviated from its Risk Assessment Guidelines; failed to disclose important findings and reasoning..."

In 2002, the EPA successfully appealed this decision to the United States Court of Appeals for the Fourth Circuit. The EPA's appeal was upheld on the preliminary grounds that their report had no regulatory weight, and the earlier finding was vacated.

In 1998, the U.S. Department of Health and Human Services, through the publication by its National Toxicology Program of the 9th Report on Carcinogens, listed environmental tobacco smoke among the known carcinogens, observing of the EPA assessment that "The individual studies were carefully summarized and evaluated."

Tobacco-industry funding of research

The tobacco industry's role in funding scientific research on secondhand smoke has been controversial. A review of published studies found that tobacco-industry affiliation was strongly correlated with findings exonerating secondhand smoke; researchers affiliated with the tobacco industry were 88 times more likely than independent researchers to conclude that secondhand smoke was not harmful. In a specific example which came to light with the release of tobacco-industry documents, Philip Morris executives successfully encouraged an author to revise his industry-funded review article to downplay the role of secondhand smoke in sudden infant death syndrome. The 2006 U.S. Surgeon General's report criticized the tobacco industry's role in the scientific debate:

This strategy was outlined at an international meeting of tobacco companies in 1988, at which Philip Morris proposed to set up a team of scientists, organized by company lawyers, to "carry out work on ETS to keep the controversy alive." All scientific research was subject to oversight and "filtering" by tobacco-industry lawyers: