Epidemiological studies have consistently shown relation between exposure to particulate matter in urban air and cardiovascular and pulmonary morbidity and mortality. Some recent population studies suggest that the ultrafine fraction of particulate matter generated by motorised vehicles is particularly damaging. Animal studies and cell culture experiments support this hypothesis and address the possible mechanisms of action. Inhalation of particulate matter, especially ultrafine particles, causes oxidative stress and inflammation in lung tissue, which affects the lung cells per se, as well as the cardiovascular system.