Airborne silica levels in an urban area.

In order to evaluate the exposure levels of the general population we studied the concentrations of silica particles in the inhalable particulate fraction (PM10) in different meteorological-climate periods in an urban area of Rome. In order to determine the concentration and the granulometric spectrum of silica particles, PM10 sampled by a cascade impactor was analysed by X-ray diffractometry (XRD) and by scanning electron microscopy equipped with a thin-window system for X-ray microanalysis (SEM/EDX). Over the period September 2004-October 2005 the abundance of silica particles as evaluated by SEM/EDX ranged from 1.6 to 10.4% of the total PM10 particulate, with a weight concentration of free crystalline silica, evaluated by XRD, in the range 0.25-2.87 microg/m3. The mean diameter of silica particles ranged from 0.3 to 10.5 microm, with more than 87% of particles having a diameter of less than 2.5 microm. The correlations between SEM/EDX and XRD data seem to suggest that the airborne silica particles in the urban location studied were mainly in the form crystalline silica. A strong relationship was found between the meteorological-climate conditions and the concentration level of free crystalline silica. This result suggests that the Southern winds from the Sahara desert carry an important amount of silica particles into Mediterranean Europe.