Passive smoking is the involuntary inhalation of smoke, called secondhand smoke (SHS) or environmental tobacco smoke (ETS), from tobacco products. It occurs when tobacco smoke permeates any environment, causing its inhalation by people within that environment. Scientific evidence shows that continuous exposure to secondhand tobacco smoke causes disease, disability, and death.

Passive smoking has played a central role in the debate over the harms and regulation of tobacco products. Since the early 1970s, the tobacco industry has been concerned about passive smoking as a serious threat to its business interests; harm to "innocent bystanders" was perceived as a motivator for stricter regulation of tobacco products. Despite an early awareness of the likely harms of secondhand smoke, the tobacco industry coordinated to engineer a scientific controversy with the aim of forestalling regulation of their products. Currently, the health risks of secondhand smoke are a matter of scientific consensus, and these risks have been one of the major motivations for smoking bans in workplaces and indoor public places, including restaurants, bars and night clubs.

Long-term effects

Research has generated scientific evidence that secondhand smoke (that is, in the case of cigarettes, a mixture of smoke released from the smoldering end of the cigarette and smoke exhaled by the smoker) causes the same problems as direct smoking, including cardiovascular disease, lung cancer, and lung ailments such bronchitis and asthma attacks.

A wide array of negative effects are attributed, in whole or in part, to frequent, long term exposure to second hand smoke. Some of these effects include:

• Cancer:
• General: overall increased risk;^{, pp. 30–46} 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: the effect of passive smoking on lung cancer has been extensively studied. A series of studies from the USA from 1986–2003,US Environmental Protection Agency. PDF: 1=Respiratory health effects of passive smoking: Lung cancer and other disorders the UK in 1998, Australia in 1997 and internationally in 2004 have consistently shown a significant increase in relative risk among those exposed to passive smoke.
• Breast cancer: The California Environmental Protection Agency concluded in 2005 that passive smoking increases the risk of breast cancer in younger, primarily premenopausal women by 70% and the US Surgeon General has concluded that the evidence is "suggestive," but still insufficient to assert such a causal relationship. In contrast, the International Agency for Research on Cancer concluded in 2004 that there was "no support for a causal relation between involuntary exposure to tobacco smoke and breast cancer in never-smokers."
• Renal cell carcinoma (RCC): A recent study shows an increased RCC risk among never smokers with combined home/work exposure to passive smoking.
• Passive smoking does not appear to be associated with pancreatic cancer.
• Ear, nose, and throat: risk of ear infections.
• Circulatory system: risk of heart disease,^{, ch. 8} reduced heart rate variability, higher heart rate.
• Epidemiological studies have shown that both active and passive cigarette smoking increase the risk of atherosclerosis.
• Lung problems:
• Risk of asthma.^{, pp. 555–558}
• Cognitive impairment and dementia: Exposure to second-hand smoke may increase the risk of cognitive impairment and dementia in adults 50 and over.
• Pregnancy:
• Low birth weight^{, part B, ch. 3}.^{, pp. 198–205}
• Premature birth^{, part B, ch. 3} (Note that evidence of the causal link is only described as "suggestive" by the US Surgeon General in his 2006 report.^{, pp. 194–197})
• General:
• Worsening of asthma, allergies, and other conditions.
• Risk to children:
• 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."^{, p. 194}
• Asthma^{, pp. 311–319}
• Lung infections
• More severe illness with bronchiolitis, and worse outcome
• Increased risk of developing tuberculosis if exposed to a carrier
• Allergies
• Crohn's disease.
• Learning difficulties, developmental delays, and neurobehavioral effects. Animal models suggest a role for nicotine and carbon monoxide in neurocognitive problems.
• An increase in tooth decay (as well as related salivary biomarkers) has been associated with passive smoking in children.
• Overall increased risk of death in both adults, where it is estimated to kill 53,000 nonsmokers per year, making it the 3^rd leading cause of preventable death in the U.S. and in children.^{, pp. 376–380}