 this study compared susceptibility to respiratory morbidity in a cohort of 9-year - old children exposed congenitally and postnatally to environmental tobacco smoke ( ets ) to susceptibility in a cohort of unexposed children . 
 the epidemiologic study included 1129 children : 594 boys and 535 girls attending the second grade of grammar schools in krakw , poland . 
 we found strong evidence that children exposed to ets in their homes were more susceptible to acute respiratory tract illnesses than unexposed children . a dose - response relationship between degree of exposure [ for lower ets exposure , odds ratio ( or ) 
 = 1.32 ; for higher ets exposure , or = 1.74 ] supports a causal explanation for the association observed . 
 the significant trend of increased risk of respiratory infections due to ets level in nonatopic children whose mothers did not smoke cigarettes during pregnancy suggests a direct effect of ets exposure on the child 's respiratory health . 
 ets combined with allergy nearly tripled the risk of acute respiratory tract illness ( or = 3.39 ; 95% ci , 1.93 - 5.93 ) , and maternal smoking during pregnancy had a modifying effect on the risk of respiratory illnesses due to ets after accounting for atopy . 
 the stronger effect of ets in atopic children and in those whose mothers smoked during pregnancy may be result of biologic interaction of endogenous and environmental factors . 
 the results of this study are of relevance to public health policy , as children with higher risk of respiratory infections may be more susceptible to environmental hazards later in adolescence or in adulthood . 
 respiratory infections also increase demands for medical interventions in terms of outpatient services and hospital administrations . 
 in addition , respiratory illnesses cause missed school days , and caring for a sick child may lead to absenteeism from work.imagesfigure 1.figure 2.figure 3.figure 4 . 
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