The evidence from a large number of studies indicates that exposures to current outdoor air pollution increase respiratory morbidity in children. Children with asthma, and those with asthma-like symptoms but without a diagnosis of asthma, are considered to be at highest risk of experiencing short-term and/or longer-term adverse health effects. Many outdoor air pollutants readily penetrate indoors. Indoor air quality can deteriorate quickly when persistent and uncontrolled emissions occur and the ventilation/air exchange rate is reduced. It has been estimated that children spend 90% of their time indoors, including in school buildings, vehicles and public indoor environments. Environmental tobacco smoke is a well-recognized persistent indoor air contaminant with adverse health effects in children of all ages. Uncontrolled moisture in the indoor environment is increasingly recognized to significantly increase the risk of respiratory morbidity in children. The evidence that air pollutants singly and in combination with other environmental factors elicit adverse health responses in asthmatic and non-asthmatic children and adolescents, appears irrefutable.