PM10 and PM2, 5 ambient particulate fractions collected in Amsterdam, Lodi, Oslo and Rome and the Dutch sea-side will be characterised by physical-chemical and immunological parameters. The samples will be screened for respiratory allergic and inflammatory potential and positive findings will undergo verification in vivo respiratory allergic and inflammatory models. Mechanisms underlying modulation of molecular and cellular functions of the immune system, of signalling pathways in rat and human epithelial lung cell cultures, and of in vivo respiratory immune responses will be clarified. Mechanistic information is important for extrapolating the results from the experimental models to the human situation. The outcome of these studies will be communicated to policymakers, the public and industry as basis for preventive action.