1-year long field campaign was carried out in Milan, in order to investigate the presence and behaviour of some of the most toxic micropollutants in the aerosols, such as polycyclic aromatic hydrocarbons (PAH) and polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F). The presence of PAHs in the atmospheric aerosols was hardly depending on seasons changing and the highest values were detected in winter, whilst the lowest ones were reached in spring and summer, when PAHs can undergo degradation processes depending on temperature and photooxidative conditions. This kind of evaluation can lead to underestimate the long term toxic effects, because the year target value of 1 ng/m3 is based only on the benzo(a)pyrene. The other heavier and carcinogenic PAHs, such as benzo(a)anthracene and dibenzo(a,h)anthracene, can remain for longer time in the atmosphere and at higher concentration. For a better characterisation, the BaPE index has been proposed. N-IPA concentration was similar than in other Italian and European cities (Table 1). Nitrated pyrenes and fluoranthenes were found to account for a large part; since the reactions leading to their presence in ambient air are isomer-specific, it is possible to discriminate between in-situ generation and direct emission by introducing the R1 ratio (2-nitrofluoranthene]/[1-nitropyrene). As far as PCDD/F are concerned, n summertime the behaviour is different from PAHs and this confirm that the organochlorinated compounds are not related to traffic emission but come from other sources, such as natural fires or industrial emissions