Introduction Several studies have shown a correlation between air pollution and mortality from cancer and respiratory and cardiovascular diseases in highly populated urban areas and in proximity to industrial settlements. Ultrafine particles in the aerodynamic diameter range of 1 nm - 10 µm have been suggested as the most relevant risk factor.

Methods During March 2014 – September 2015, we conducted outdoor air monitoring at the entrance of elementary schools and kindergartens located in 4 highly populated Italian urban areas and 5 towns in close proximity to oil refinery plants. We used a Electric Low Pressure Impactor (ELPI+ Dekati, Tampere, Finland), which allows to measure absolute number, mass and surface area of particles, in 14 classes of aerodynamic diameter within the range 6 nm – 10 µm.

Results Although within a broad range, median values of particle count were highest in two towns nearby large oil refineries in the wintertime (54,200/cm³) and in the summertime (53,502/cm³) compared with the other urban areas. Overall, the median ultrafine particle count in the towns nearby the oil refineries ranged 7,385–54,200/cm³, while it ranged 16,908–47,417/cm³ in the most populated urban areas, with top readings equal to 292,238 and 928,179/cm³, nearby the industrial settlements and in the urban areas respectively. Results were similar when considering mass units or surface area. Air concentrations were significantly affected by the weather conditions.

Conclusions Our results show that airborne ultrafine particle levels overlap between residential areas nearby oil refineries, and populated urban areas. Apart from industrial emissions, urban traffic and traditional heating systems do appear to contribute, as suggested by significantly higher counts in the wintertime. Monitoring airborne ultrafine particles all year long in more numerous urban and industrial locations might allow to better characterise environmental exposure and its relation to the health status of the resident population.