Epidemiological studies conducted in different countries have reported associations between increases in PM concentration and increases in morbidity and mortality, particularly among those people with respiratory or cardiovascular diseases. However, there are still important unanswered questions concerning potential biologic mechanisms of PM effects, the identification of the factors responsible of the adverse health effects (size and or chemical composition), the groups of people that may be particularly sensitive to the effects of PM. The most recent hypotheses indicate that ultrafine particles (UF) (< 0.1 microm) and transition metals (like Fe) may play an important role in the induction of toxic effects. Results obtained during last years in some sites located in the city center of Rome, showed mean annual levels of PM10 and PM2.5, that were often higher than 40 microg/m3, with peak values during winter months. UF particles concentrations showed similar seasonal trends, and daily trend correlated with increasing traffic fluxes, with peak levels of 10(5) p/cm3.