Can we trust cross-sectional studies when studying the risk of moisture-related problems indoor for asthma in children?

Most studies studying dampness as a risk factor for asthma are of a cross-sectional design. The aim of this study was to investigate if the association between moisture-related problems indoor and asthma found in cross-sectional questionnaire data can be confirmed in longitudinal analyses. The Dampness in Building and Health (DBH) study started in 2000 in Varmland, Sweden, with a baseline questionnaire to all children aged 1-5 y (n = 14,077) and five years later a follow-up questionnaire was distributed to children aged 6-8 y (n = 7,509). Moisture-related problems that were associated with asthma in cross-sectional analysis decreased or disappeared in the longitudinal analysis. However, the association between reports of moldy odor in the homes at baseline and incident asthma remained and became stronger. Our results suggest that cross-sectional data showing associations between moisture-related problems in homes and asthma in children partly can be explained by reporting bias.