Parental smoking was categorized into three groupsaccording to the reported smoking history (neversmoked, ex-smoker or current smoker). Parental smoking was also coded by the number of cigarettes currentlyIS ET AL.smoked per day by each parent. A positive history ofparental respiratory illness was defined as a history ofbronchitis, emphysema or asthma of either parent. Tocontrol for the association between socioeconomic status(SES) and respiratory health, analyses were adjustedfor a three-level index of SES based on the mean number of years of schooling of the parents (12 yr). In single-parent households the years of education of that parent were used.