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Atmospheric concentrations of PM2.5 and PM10 were measured in Beirut, Lebanon, for a period of 12 months. The daily average concentrations of PM10 and PM2.5 were found to be 51.3 ± 33.1 and 30.3 ± 9.4 µg.m-3, respectively, with corresponding maximum values of 359.7 and 208.6 µg.m-3. The annual average concentrations of PM10 and PM2.5 exceeded the World Health Organization's standards by 150% and 200%, respectively. The mean concentration of coarse particles (PM10-2.5) was found to be 41% of the average PM10, suggesting that the site was also influenced by re-suspended surface dust and soil. The mean PM2.5/PM10 ratio for the entire study period was 0.61 ± 0.12. This indicates that in Beirut, PM2.5 accounts for about 61% of PM10. Such a large fraction of fine particles could have considerable effect on health; thus, it is necessary to quantify its impact. Daily concentrations of PM10 and PM2.5 exceeded the upper threshold limit on 133 and 129 days, respectively, representing 39% and 38% of the entire sample, respectively. These findings indicate the important role dust events play within this area. Concentrations of PM2.5 were highly correlated with NO2, whereas concentrations of PM10 and PM10-2.5 were not associated with any gaseous pollutant. Regression analysis showed that 93% of PM2.5 and 43% of PM10 particle mass concentrations were derived from road traffic exhaust in Beirut. [ABSTRACT FROM AUTHOR]