24, 28, 33, 40), it was an independent risk factor in this study. Unfortunately, sample size became too small to determine whether the delivery of heat through forced air, radiant heat, or water radiators modified that effect.

It is not clear why the present study did not find that boys were at increased risk of asthma because many previous studies did (12, 19, 22, 25, 50), although not all (20). Finally, contrary to most studies (14, 19, 25, 40, 50, 51), the present study showed an association between asthma and breast feeding. Only one other study reported that breast feeding was a protective factor (19). The younger age of our study subjects is a likely explanation for these discrepancies. Indeed, the protective effect may not last beyond early infancy.

Misclassification of outcome is a potential concern in most studies of childhood asthma, including this one. However, had many cases not been asthmatics and many controls underdiagnosed, it is unlikely that the study would have shown increased risks for markers of atopy, family history, and previous infections. Potential selection bias needs to be addressed. Controls living in the same census tract were considered a reasonable choice for the study base. However, only families who still resided at the address given in the files were sampled as controls. If the studied factors were associated with mobility, then the proportion of controls exposed to these factors would have been underestimated in this study. There are some indications that this was not the case. For instance, a recent national survey (52) showed that among Quebec women aged 20–44 years in 1986, 37.5 percent were regular smokers, which is identical to the proportion found among control mothers in this study. In addition, in 1983, the prevalence of asthma in 3- and 7-year-old children in Montreal was estimated to be 6.4 percent (53), which is close to the 5.4 percent found among controls in the present study. We also note that socioeconomic factors, which may be associated with mobility, were controlled for in the analysis.

In conclusion, this incident density case-control study showed that even after accounting for personal susceptibility, family history, past infections, and factors related to the indoor environment contribute significantly to the incidence of asthma. For future studies to have a greater impact on public health, it will be necessary to assess exposure-response relations and to relate findings to suggested protective standards. Obtaining reliable quantitative measurements will be the challenge to future studies.

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