Background: We explored methods to develop uncorrelated variables for epidemiological analysis models. They were used to examine associations between respiratory health outcomes and multiple household risk factors.
Methods: We analysed data collected in the Four Chinese Cities Study (FCCS) to examine health effects on prevalence rates of respiratory symptoms and illnesses in 7058 school children living in the four Chinese cities: Lanzhou, Chongqing, Wuhan, and Guangzhou. We used factor analysis approaches to reduce the number of the children's lifestyle/household variables and to develop new uncorrelated 'factor' variables. We used unconditional logistic regression models to examine associations between the factor variables and the respiratory health outcomes, while controlling for other covariates.
Results: Five factor variables were derived from 21 original variables: heating coal smoke, cooking coal smoke, socioeconomic status, ventilation, and environmental tobacco smoke (ETS) and parental asthma. We found that higher exposure to heating coal smoke was associated with higher reporting of cough with phlegm, wheeze, and asthma. Cooking coal smoke was not associated with any of the outcomes. Lower socioeconomic status was associated with lower reporting of persistent cough and bronchitis. Higher household ventilation was associated with lower reporting of persistent cough, persistent phlegm, cough with phlegm, bronchitis, and wheeze. Higher exposure to ETS and the presence of parental asthma were associated with higher reporting of persistent cough, persistent phlegm, cough with phlegm, bronchitis, wheeze, and asthma.
Conclusions: Our study suggests that independent respiratory effects of exposure to indoor air pollution, heating coal smoke, and ETS may exist for the studied children.