Airborne fine particles of PM 2.5-10 and PM 2.5 in Bangkok, Nonthaburi, and Ayutthaya were measured from December 22, 1998, to March 26, 1999, and from November 30, 1999, to December 2, 1999. Almost all the PM 10 values in the high-polluted (H) area exceeded the Thailand National Ambient Air Quality Standards (NAAQS) of 120 µg/m 3 . The low-polluted (L) area showed low PM 10 (34-74 µg/m 3 in the daytime and 54-89 µg/m 3 at night). PM 2.5 in the H area varied between 82 and 143 µg/m 3 in the daytime and between 45 and 146 µg/m 3 at night. In the L area, PM 2.5 was quite low both day and night and varied between 24 and 54 µg/m 3 , lower than the U.S. Environmental Protection Agency (EPA) standard (65 µg/m 3 ). The personal exposure results showed a significantly higher proportion of PM 2.5 to PM 10 in the H area than in the L area (H = 0.80 ± 0.08 and L = 0.65 ± 0.04).Roadside PM 10 was measured simultaneously with the Thailand Pollution Control Department (PCD) monitoring station at the same site and at the intersections where police work. The result from dual simultaneous measurements of PM 10 showed a good correlation (correlation coefficient: r = 0.93); however, PM levels near the roadside at the intersections were higher than the concentrations at the monitoring station. The relationship between ambient PM level and actual personal exposures was examined. Correlation coefficients between the general ambient outdoors and personal exposure levels were 0.92 for both PM 2.5 and PM 10 .Bangkok air quality data for 1997-2000, including 24-hr average PM 10 , NO 2 , SO 2 , and O 3 from eight PCD monitoring stations, were analyzed and validated. The annual arithmetic mean PM 10 of the PCD data at the roadside monitoring stations for the last 3 years decreased from 130 to 73 µg/m 3 , whereas the corresponding levels at the general monitoring stations decreased from 90 to 49 µg/m 3 . The proportion of days when the level of the 24-hr average PM 10 exceeded the NAAQS was between 13 and 26% at roadside stations. PCD data showed PM 10 was well correlated with NO 2 but not with SO 2 , suggesting that automobile exhaust is the main source of the particulate air pollution. The results obtained from the simultaneous measurement of PM 2.5 and PM 10 indicate the potential environmental health hazard of fine particles. In conclusion, Bangkok traffic police were exposed to high levels of automobilederived particulate air pollution. . The PM 2.5 to PM 10 ratio was ~80%, suggesting a high contribution of automobile exhaust. Roadside PCD monitoring station data showed 24-hr average PM 10 exceeded the NAAQS on 13-26% of days per year. These results suggest that Bangkok traffic police were exposed to high levels of automobile-derived particulate. Evaluation of the health effects of the particulate air pollution is recommended. Roadside Particulate Air Pollution in Bangkok Jinsart et al.