Association between asthma and family size between 1977 and 1994

Study objectives-Several recent reports show a negative association between asthma and family size or birth order, but this association was not detect ed in data collected between 10 and 30 years ago. This study compared the a ssociation between sibship size and asthma in three surveys using the same methodology in 1977, 1985/86, and 1993/94. Design-Cross sectional comparison of the 1977, 1985/86, and 1993/94 surveys . Settings-Study areas in England and Scotland Participants-Parents of children between 5 to 11 years in England and Scotl and were asked about asthma and bronchitis attacks in the last 12 months, a nd wheeze in their child. Approximately 9000 children participated in each of the surveys. Results-The overall association between asthma, defined as asthma attacks o r wheeze, and total number of siblings was not significant (p=0.22), but an only child had a higher prevalence of asthma than children with siblings ( OR 0.87 95% CI 0.76 to 0.98). The interaction between year of survey and si bship size on asthma was not significant (p=0.36). There was no association between asthma and birth order. A significant interaction between social c lass and year of survey on asthma was detected (p=0.004). In the 1993/94 su rvey children whose fathers had a semi or unskilled manual occupation had a higher prevalence of asthma (16%) than children whose fathers belonged to other social classes (13%). Conclusions-This study provides only marginal support for a change over tim e of the association between sibship size and asthma. Based on recent repor ts the nature of the exposure agent that may explain the association remain s controversial. This study suggests a disproportionate increase of asthma in lower social classes.