Several epidemiological studies have shown that in the human population ambient particulate matter (PM) is associated with adverse health effects. Little is known, however, about the relative effects of aerosol constituents. Since 2002, diurnal samples of ambient PM2.5 were analysed by automated methods for the quantification of particle-associated organic compounds (POC). Data on chemical composition have been investigated in epidemiological and biological effect studies. As a result of these studies, the associations found between PAH concentration and symptoms of myocardial infarction survivors suggest a major influence of combustion sources on cardiovascular health effects. The correlations found between formation of reactive oxygen species and the presence of specific organic compounds suggests an important influence of biomass combustion particles in PM2.5-associated oxidative stress.