Social-class gradient in childhood asthma : the nature of the link and issues of measurement

Abstract

The objectives of this thesis were: (i) to examine the relationship and the nature of the link of socioeconomic status (SES) to an objective indicator of childhood asthma; (ii) to quantify the agreement between area-based SES measures and SES assessed at the level of the individual; and (iii) to compare the test characteristics of asthma as evidenced by questionnaire, exercise and methacholine challenges without assuming a gold standard. We carried out a cross-sectional study of 989 Montreal elementary school children in an area where there is universal access to medical care through state-funded health insurance. A questionnaire was completed by parents, and the children performed spirometry before and after a free-running exercise challenge. Street address information and parental occupation were used to establish neighborhood and individual-level SES indicators respectively. A sub-sample of 332 children underwent allergy skin and methacholine bronchoprovocation tests, and indoor home measurements of NO$sb2$, PM$sb{10}$, dust-mites and various types of moulds were performed.Low SES was associated with an increased likelihood of exercise-induced bronchospasm (EIB), lower FEV$sb1$ and FVC, but not with FEV$sb1$/FVC, a measure of airway calibre. With lower SES the likelihood of positive allergy skin tests decreased for cat, trees and grasses but increased for cockroach and moulds.Of the host factors examined, birthweight was positively associated with FVC and FEV$sb1$ while airway responsiveness to methacholine increased as birthweight decreased. Pre-term birth independent of birthweight was associated with a lower FEV$sb1$/FVC. Of the environmental factors, increasing exposure to household tobacco smoke and smoking during pregnancy were found to be associated with larger FVC and lower FEV$sb1$/FVC. The current presence of a cat and lower respiratory infection prior to the age of 2 years (LRI) were associated with EIB. Increasing usual dietary salt intake was also found to be associated with higher methacholine responsiveness. Part of the observed relationship between SES and indicators of asthma was mediated by the indoor environmental pollutants.There was poor agreement between the area-based and the individual-level SES measures even when used to classify children into broad SES categories. Using a Bayesian approach, comparison of the three tests for measuring asthma revealed exercise challenge to perform least well compared to a questionnaire or methacholine provocation in terms of the test characteristics when the purpose was to chose a single test for identifying asthma in community based studies.The results provide evidence suggesting that SES is an important risk factor for childhood asthma. This further suggests that excess morbidity and mortality from asthma in inner city areas of large American cities is not only due to differential access to health care but that under-utilization of health care as well as host, dietary and environmental factors associated with social disadvantage are also important