Household environmental tobacco smoke and risks of asthma

The reported prevalence of childhood asthma/wheeze is increasing around the world. The changing pattern of these diseases has not been fully explained, in part because of an incomplete understanding of its pathogenesis. The change has been too rapid to be accounted for by changes in gene frequencies. It is also unlikely that it can be totally accounted for by changes in either clinical diagnostic patterns or increased recognition of respiratory symptoms by the general population.
This shift does, however, suggest a role for environmental exposures in the etiology of this evolving epidemic. It is estimated that around 1.3 billion people worldwide smoke, and the number is predicted to increase in the coming years as smoking rates continue to increase among youth, primarily among young girls.
Exposure to environmental tobacco smoke (ETS) is common in children and causes substantial morbidity. Estimates of population attributable risk for household ETS exposure in children range from 9% for asthma prevalence to 25% for hospital admissions due to respiratory symptoms. The World Health Organization estimates that approximately half of the children in the world are exposed to ETS, mostly in their homes. In Taiwan, schoolchildren are not typically exposed in public due to the legislative ban on public smoking and regular health promotion campaigns; home exposure is likely the dominant source of ETS. It was reported that approximately 60% Taiwanese children under the age of 17 were exposed to household ETS, and therefore an estimated 3.3 million children were at risk for adverse health effects from this exposure.
Evidence of the effects of ETS exposure on childhood respiratory outcomes is inconsistent. Although an effect for paternal smoking has been reported, exposure to maternal smoking has consistently had the strongest association with adverse respiratory health effects. The strength of the association between maternal smoking and asthma/wheeze is in part due to larger ETS doses from maternal smoking than from other sources. Although there is evidence that maternal smoking during pregnancy increases the risk of asthma/wheeze in childhood, the effects of in utero exposure on the occurrence of respiratory symptoms have not been extensively studied or compared with other household sources of ETS.
The Taiwan Children Health Study (TCHS) offers an opportunity to investigate the effects of household ETS exposure on the occurrence of respiratory symptoms in Taiwanese children. At cohort entry, we used lifetime tobacco smoke exposure histories to investigate the relationships of multiple sources of ETS exposure with parental- reported respiratory outcomes. The individual and joint effects of parental smoking habits were also examined for associations with early-onset and late-onset childhood asthma.