The present paper is based on results collected from the air quality network, which was assembled for covering the Oporto metropolitan area, in Portugal. Among the SO 2 and PM 10 levels obtained for the last two decades, a particular unpolluted year was chosen to evaluate the health effects of concentrations lower than those that usually cause acute episodes. It was observed that even low levels of SO 2 could be related with the increase of obstructive chronic pulmonary diseases morbidity daily rates recorded in Oporto during the same year. When the relationship between levels of atmospheric pollution and human health is considered, climatic factors have a synergetic effect. The observation of respiratory symptoms for concentrations below the current ambient standards, in agreement with previous studies carried out in the same area and other countries, suggests that it is urgent the process of updating and revision of those values in order to protect in effect the health of populations. As particles have shown an important role on health impact, its characterisation was done, considering their respirable fractions. It was concluded that most of the total suspended particles are breathable. Moreover, more than 78% and 87% of the organic composition and of the carcinogenic agent benzo(a)pyrene content, respectively, are detected in particles liable to attain the respiratory tract.