Air quality has seriously deteriorated as a consequence of population growth and urbanisation and respiratory diseases increased among residents of urban areas in Chiang Mai Province, Thailand. An International Study of Asthma and Allergies in Childhood (ISAAC) study was conducted among children attending schools located in the selected sites to assess the potential impacts of air pollution on health. The results showed that the prevalence of asthma was similar in all of the schools (approximately 5%; p = 1.000) but that the prevalence of rhinitis [24.3% (CI = 19.4-30.1) vs. 15.7% (CI = 10.3-23.0); p = 0.029] and atopic dermatitis [12.5% (CI = 9.1-16.8) vs. 7.2% (CI = 3.7-12.6); p = 0.093)] was higher in the urban schools, which were exposed to more pollution. Logistic regression analysis identified factors that may be involved, including air pollution, some components of the diet and contact with animals.