Airborne particles are responsible for large numbers of premature deaths in Europe. To reduce particulate matter concentrations, direct emissions of particles from, e.g., motor vehicles need to be curtailed. But a large fraction of airborne particles is formed in the atmosphere, through reactions of ammonia, sulfur dioxide and nitrogen oxides. So emissions of these gases need to be reduced as well. This comment highlights the importance for human health of exposure to secondary inorganic particles, and the importance of controlling ammonia emissions to prevent secondary inorganic particles from being formed.