Recent epidemiological studies have shown a consistent association between ambient levels of inhalable particles (PM10) and exacerbation of respiratory diseases as well as cardiovascular morbidity and adult mortality in high risk groups. The particles responsible of the observed health effects are unknown; it seems that different particles could be related to different effects, depending on the deposition pattern in the airways and on the chemical reactivity. Larger particles could be more related to upper airway and tracheobronchial effects while the smaller carbonaceous particles seem to be preferentially involved in inflammation and cardiopulmonary injury. It has been proposed that the possible biological mechanisms of action of PM could be related to its radical activity and the induction of oxidative stress and lung inflammation followed by systemic low grade inflammation and pro-coagulant state.