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What is secondhand smoke?

Secondhand smoke (SHS) is also known as environmental tobacco smoke (ETS). SHS is a mixture of 2 forms of smoke that come from burning tobacco:

  • Sidestream smoke ? the smoke that comes from the end of a lighted cigarette, pipe, or cigar and
  • Mainstream smoke ? the smoke that is exhaled by a smoker
Even though we think of these as the same, they aren't. The sidestream smoke has higher concentrations of cancer-causing agents (carcinogens) than the mainstream smoke. And, it contains smaller particles than mainstream smoke, which make their way into the body's cells more easily.

When non-smokers are exposed to SHS it is called involuntary smoking or passive smoking. Non-smokers who breathe in SHS take in nicotine and other toxic chemicals just like smokers do. The more SHS you are exposed to, the higher the level of these harmful chemicals in your body.

Why is secondhand smoke a problem?

Secondhand smoke causes cancer.
Secondhand smoke is classified as a "known human carcinogen" (cancer-causing agent) by the US Environmental Protection Agency (EPA), the US National Toxicology Program, and the International Agency for Research on Cancer (IARC), a branch of the World Health Organization.

Tobacco smoke contains more than 7,000 chemical compounds. More than 250 of these chemicals are known to be harmful, and more than 60 are known to cause cancer.

SHS has been linked to lung cancer. There is also some evidence suggesting it may be linked with childhood leukemia and cancers of the larynx (voice box), pharynx (throat), brain, bladder, rectum, stomach, and breast.

Secondhand smoke and breast cancer.
Whether SHS increases the risk of breast cancer is an issue that is still being studied. Both mainstream and SHS contain about 20 chemicals that, in high concentrations, cause breast cancer in rodents. And we know that in humans, chemicals from tobacco smoke reach breast tissue and are found in breast milk.

But a link between SHS and breast cancer risk in human studies is still being debated. This is partly because breast cancer risk has not been shown to be increased in active smokers. One possible explanation for this is that tobacco smoke may have different effects on breast cancer risk in smokers and in those who are exposed to SHS.

A report from the California Environmental Protection Agency in 2005 concluded that the evidence regarding SHS and breast cancer is "consistent with a causal association" in younger women. This means the SHS acts as if it could be a cause of breast cancer in these women. The 2006 US Surgeon General's report, The Health Consequences of Involuntary Exposure to Tobacco Smoke, found that there is "suggestive but not sufficient" evidence of a link at this point.

Research is still being done, but women should be told that this possible link to breast cancer is yet another reason to avoid being around SHS.

Secondhand smoke causes other kinds of diseases and deaths.
Secondhand smoke can cause harm in many ways. Each year in the United States alone, it is responsible for:

  • An estimated 46,000 deaths from heart disease in people who are current non-smokers
  • About 3,400 lung cancer deaths as a result of breathing SHS
  • Worse asthma and asthma-related problems in up to 1 million asthmatic children
  • Between 150,000 and 300,000 lower respiratory tract infections in children under 18 months of age, and lung infections resulting in 7,500 to 15,000 hospitalizations each year