In a cross-sectional study on 433 schoolchildren (aged 6-9 years) from 9 schools in Austria we observed associations between housing factors like passive smoking and lung function as well as improved lung function in children who had been breast-fed. The latter findings urged the question whether protective effects of breast feeding act on environmental stressors or if they act independently. Therefore the effect of passive smoking on lung function was stratified by breast-feeding. Detrimental effects of passive smoking were significant but restricted to the group of 53 children without breast-feeding. Breast feeding counteracts the effect of environmental stressors on the growing respiratory organs.