Few epidemics have been as devastating and preventable as that caused by tobacco consumption. Cigarette smoking became highly prevalent in most developed countries through the 20thcentury; with a lag of several decades, the rise of smoking was followed by epidemic increases in smoking-related diseases, including lung and other cancers, heart disease, and chronic lung disease. The issue of exposure to secondhand smoke (SHS) and health has a much briefer history, although the irritating nature of tobacco smoke to the nonsmoker has long been chronicled. Initial investigations focused upon parental smoking and lower respiratory illnesses in infants; studies of lung function and respiratory symptoms in children soon followed. WHAT IS SECONDHAND SMOKE? — Secondhand smoke (SHS) is one of several terms used for the involuntary exposure of nonsmokers to tobacco smoke from the smoking of others. The smoke inhaled by nonsmokers has also been referred to as environmental tobacco smoke (ETS), but this term has fallen out of use. More than one billion adults worldwide are smokers, implying that some SHS exposure is almost unavoidable for children and for the two-thirds of adults who do not smoke.The term “thirdhand smoke” (THS) has been used to refer to smoke components deposited on surfaces, along with metabolites of these components generated through oxidation. These toxins may be absorbed through the skin, by ingestion, or inhalation of resuspended dust, but the potential health effects are not yet well studied. Evidence of adverse respiratory effects of SHS exposure for children was included in the 1986 reports of the United States Surgeon General and the National Research Council. Subsequent public health reports from the United States and elsewhere have identified additional health risks associated with SHS exposure in children. These reports describe the following specific risks for children, each of which is discussed in detail below:Quality of life and costsPrematurity and perinatal mortalityFetal growth and developmentSudden infant death syndrome (SIDS)Respiratory symptoms and illnessLower respiratory illnessesChronic respiratory symptomsAsthmaReduced lung functionAtherogenesisMiddle ear diseaseIn addition, there is suggestive but not definitive evidence for an association between SHS exposure and dental caries, renal function, and childhood cancers.Prematurity and perinatal mortality — The level of tobacco exposure of the fetus of a mother who smokes is the same as the level for an active smoker. Maternal smoking is associated with increased perinatal mortality (stillbirths and neonatal deaths). Maternal smoking during pregnancy – Active smoking by pregnant women, resulting in passive smoking for the developing fetus, increases risk for a variety of adverse health effects in children; these effects are hypothesized to result primarily from transplacental exposure of the fetus to tobacco smoke components. In particular, maternal smoking during pregnancy reduces birth weight by about 200 g on average. Sudden infant death syndrome — Sudden infant death syndrome (SIDS) refers to the unexpected death of a seemingly healthy infant while asleep. Maternal smoking during pregnancy has been causally associated with SIDS. An estimated 25 to 40 percent of SIDS cases are related to smoking during pregnancy.