Tobacco smoke is a major component of indoor air pollution. Exposure to environmental tobacco smoke (ETS) is prevalent worldwide despite growing awareness of its adverse health effects on non-smokers. ETS contains the same toxic substances as identified in mainstream tobacco smoke. Cotinine (a metabolite of nicotine) can be measured in urine and serum of non-smokers exposed to ETS and reflects the degree of exposure. In children, exposure to ETS leads to reduced lung function, increased risk of lower respiratory tract illnesses, acute exacerbation of asthma resulting in hospitalization, increased prevalence of non-allergic bronchial hyperresponsiveness, increased risk for sudden infant death syndrome (SIDS) and possibly increased risk for asthma. Exposure to ETS is responsible for excess cost to the family's financial resources and demands on health services. In adults, exposure to ETS is associated with increased risk of lung cancer, particularly in those with high exposure and acute and chronic respiratory symptoms that improve after the cessation of exposure. Healthcare providers should advocate for non-smokers' rights in the community and support legislation to limit tobacco exposure.