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.