Higher asthma occurrence in an urban than a suburban area: role of house dust mite skin allergy

Understanding of geographical differences in asthma prevalence may be helpful in explaining recent increases in the occurrence of asthma. We wondered whether differences in allergic sensitization or other factors could explain differences in reported occurrence of asthma between an urban centre and a neighbouring suburban area. From the European Community Respiratory Health Survey (ECRHS) questionnaire, responses on asthma symptoms and risk factors and results of 11 skin allergy tests were available from 656 young adults living in urban or south suburban Antwerp, Belgium. Answers to five asthma questions were selected as dependent variables, and eight personal or environmental risk factors, as well as house dust mite (HDM) allergy, as independent variables. The effect of each independent variable on the association of asthma variables with area was assessed. Prior asthma diagnosis, present asthma symptoms, the selected risk factors and HDM allergy were all more frequently recorded in urban Antwerp. Difference in HDM allergy accounted for most of the difference in prior (mostly childhood) asthma diagnosis, since correction for it decreased the odds ratio from 2.10 to 1.65. On the contrary, the regional differences in recent asthma symptoms were not explained by HDM allergy differences nor by any other factor under study. This urban-suburban comparison indicated that house dust mite allergy is a major determinant of prior (childhood) asthma, whereas factors contributing to higher urban prevalence of present asthma symptoms could not be identified. Furthermore, our results indicate that it may be inappropriate to combine data from neighbouring areas, when their similarity has not been verified.