Environmental tobacco smoke is an important contaminant of indoor air. For a non-smoker living with a smoker the exposure is equivalent to about 1% of that from actively smoking 20 cigarettes a day (based on plasma cotinine). There is strong and consistent evidence that passive smoking increases the risk of lung cancer. It is estimated that there is an increase in risk of 24% (95% confidence interval 11–38%) compared to unexposed non-smokers, and several hundred lung cancer deaths per year in Britain are attributable to environmental tobacco smoke exposure. Passive smoking is associated with an increase in risk of chronic respiratory disease in adults of 25% (10–43%), and increases the risk of acute respiratory illness in children, by 50–100%. It is likely that passive smoking increases the risk of ischaemic heart disease, and that exposure in pregnancy lowers birthweight, but there is inconsistency between different estimates of the magnitude of risk. The overall hazard is sufficient to justify measures to restrict smoking in public places and workplaces, and to discourage people from smoking in their homes.