Revision of ambient air quality standards for PM?

Recent epidemiologic findings gave rise to new approaches in setting ambient air quality guidelines for particulate matter in various countries. Because of the complexity of this system defined by many parameters like size distribution, chemical composition, geometry, formation, hygroscopy, or concentration the toxicologic base for acute and chronic effects, especially in the low dose range, is not yet adequate for the formulation of air quality guidelines. Open questions as there is existence of a threshold, shape of the dose-response curve, causative models for the mechanisms and health effects outcome in connection with relatively low doses and groups at higher risks are still under discussion at an international level, thus making political decisions very difficult.