Many non-smokers feel immediate effects on breathing in tobacco smoke. They can suffer from coughing, headache, eye irritation, sore throat, sneezing and runny nose, nausea, breathing problems, and irregular heartbeat (particularly for people with heart disease).
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1. Cancers

Extended exposure to tobacco smoke causes lung cancer in non-smokers. IARC recently determined tobacco smoke pollution to be carcinogenic to humans10:“There is sufficient evidence that involuntary smoking causes lung cancer in humans.”

IARC found that non-smokers living with a smoker have a 20-30% increase in risk of lung cancer compared with those who live in non-smoking households, controlling for some potential sources of bias and confounding. As with active smoking, there is a dose-response relationship between a non-smoker’s risk of lung cancer and the number of years of exposure to the tobacco smoke.

For non-smokers exposed in the workplace, IARC reviewed other published meta-analyses, which give an increased risk of lung cancer of 16-19%.

Finland, Germany and the US National Toxicology Programme have listed tobacco smoke as a workplace carcinogen and, as long ago as 1992, the US Environmental Protection Agency classified tobacco smoke as a Class A (known human) carcinogen.
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2. Respiratory diseases

IARC reported that adverse effects of tobacco smoke on respiratory symptoms have been observed, with the strongest evidence being for a causal relationship with chronic respiratory symptoms10. In 1998, a review found a small but significant association between passive smoking and adult-onset asthma and COPD. This review estimated that adults exposed at home or in the workplace had a 40-60% increased risk of asthma compared with adults who were not exposed in these places.

In people with asthma, exposure to tobacco smoke is associated not only with more severe symptoms, but also with lower quality of life, reduced lung function and increased use of health services for asthma, including hospital admissions.

In 2001, the European Community Respiratory Health Survey80 (involving nearly 8,000 adults aged 20-48 years from 46 centres in 16 countries) found that passive smoking was significantly associated with nocturnal chest tightness, nocturnal breathlessness and increased bronchial responsiveness. Further analyses from this study have found that both intra-uterine (see below) and environmental exposure to parental tobacco smoking was related to children having more respiratory symptoms and poorer lung function in adulthood.
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3. Cardiovascular diseases

People who live with smokers have a 25% increased risk of coronary heart disease. Exposure to tobacco smoke has consistently been found to be associated with an increased relative risk of CHD in cohort studies and case-controlled studies, in males and females, and for exposure at home and in the workplace.

It may seem odd that the relative risks for active and passive smoking on lung cancer are very different, namely, ~20 for active smoking and ~1.2 for passive smoking, while the relative risks for active and passive smoking on CHD are relatively close, namely 1.8 for active smoking and 1.25 for passive smoking. However, the dose-response relationship for CHD is non-linear, and we now understand that the mechanisms concerning smoking and cancer and smoking and cardiovascular disease are very different. Even small amounts of smoke may have immediate influences on clotting and thrombus formation, and long-term effects on the development of arteriosclerosis, all important factors in CHD and cardiovascular disease. Such a non-linear response helps make the epidemiological estimates of the cardiovascular effects of passive smoking biologically plausible.

There is some evidence that second-hand smoke increases the risk of stroke. Bonita et al. found that regular exposure to tobacco smoke pollution increased the risk of stroke in non-smokers by 82%. A recent cohort study found a 50% increased risk of first ischaemic stroke among females exposed to environmental tobacco smoke at home.
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4. Effects of tobacco smoke on the health of children

Exposure to tobacco smoke is especially dangerous to young children and infants. It increases the risk of lower respiratory tract infections, such as pneumonia and bronchitis, causes coughing and wheezing, and is associated with reduced lung growth and with middle-ear disease, including recurrent ear infections in children. It is also a risk factor for new cases of asthma and increases the severity of symptoms in children with asthma.

A Dutch review of the evidence recently estimated that the increase in risk of respiratory infections in children with or without asthma varied from 20-50%75. In the UK, it has been estimated that, each year, more than 17,000 children aged under 5 years are admitted to hospitals because of respiratory illness caused by exposure to other peoples’ cigarette smoke.
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5. Exposure to tobacco smoke during pregnancy and infancy

As with maternal smoking in pregnancy, passive smoking (i.e. when non-smoking females are exposed to other peoples’ smoke during pregnancy) also reduces birth weight in the offspring of non-smoking mothers. There is also evidence that exposure to tobacco smoke pollution after birth is a risk factor for SIDS in babies of non-smoking mothers. Exposure to tobacco smoke in pregnancy and infancy adversely affects certain cognitive abilities and behavioural characteristics of children.
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6. Other effects

Second-hand tobacco smoke is especially dangerous to people with pre-existing respiratory or  cardiovascular conditions. Such individuals make up a substantial proportion of the population.
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from: Tobacco or Health in The European Union, 2004