Air pollution in Thailand is most severe in the Bangkok Metropolitan Region (BMR). In Bangkok, ambient levels of lead have been reduced significantly after lead phaseout was completed by 1995, while sulfur dioxide (SO2) , nitrogen dioxide (NO2) and carbon monoxide (CO) levels remain steady or have declined slightly. Particulates smaller than 10 microns (PM10) are now considered the priority air pollutant, and in recent times ozone levels have caused concern (Source:Thailand Environment Monitor 2000, Brown Agenda).