The outdoor air quality is one of the most important determinants of health. In this paper we present the example of cooperation of sanitary engineers in a multidisciplinary research project in the model area of Zasavje region, Slovenia. We have focused on the results of ecological time-trend study, by which we have assessed the temporal association between outdoor air pollution and respiratory diseases in children. The methodology of linking health and environmental data was recommended by the World Health Organization. Positive temporal association was registered in particulate matter of 10 micrometres in diameter in the municipalies of Zagorje ob Savi and Trbovlje and ozone in the municipality of Trbovlje. The study presents an example of good collaboration between experts in the field of environment and health. In the future major challenges for experts in the field of outdoor air represent upgrading of presented methodology for exposure assessment and study of the impact of ultrafine particulate matter on health.