In 415 nonsmoking asthmatic children who were seen consecutively, asthma symptoms were more severe if the mother was a smoker than if she was a nonsmoker. This applied to both sexes but was more marked in boys than in girls. There were also other indications that sons were the more severely affected: the forced expiratory volume at 1 second, the forced expiratory flow rate during the middle half of the forced vital capacity, and the provocation concentration of histamine needed to result in a 20% decrease in the forced expiratory volume at 1 second were significantly decreased only in the sons, and lung function test results were significantly less in sons than in daughters of mothers who smoked. When the 415 children were stratified according to age, lung function improved significantly with increasing age in the children of nonsmokers; in children of smokers, by contrast, symptoms and lung function test results became progressively worse. As well, there was a correlation between these indications of asthma severity and the number of years the child had been exposed to the mother's smoke. It appeared that, compared with girls, boys were more sensitive to passive smoking, and that its adverse effect increased with age and with duration of exposure.

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