Concentrations of airborne particulate matter with an aerodynamic diameter less than 10 µm (PM 10 ) and five gaseous air pollutants (O 3 , CO, NO 2 , SO 2 and H 2 S) were measured over a period of approximately six years (October 1999-June 2004 at five air quality monitoring network stations of King Abulaziz City for Science and Technology (KACST) in Riyadh city, Saudi Arabia. The main objective of this study is to evaluate the quality of ambient air in relation to its possible effects on human

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... health in the urban area of Riyadh city using the U.S. Environmental Protection Agency (USEPA) Air Quality Index (AQI) and break down analysis of five criteria pollutants (O 3 , CO, NO 2 and SO 2 and PM 10 ) and Hydrogen Sulphide (H 2 S). The concentrations of selected pollutants in ambient air has shown upward trends except for sulfur dioxide (SO 2 ) and hydrogen sulfide (H 2 S), which exhibited decreasing trends over the time. Using the AQI based on a health perspective, a breakdown analysis was conducted. The results confirmed that 71% of the time Riyadh city air is of "Good" quality using the AQI and causes almost no health impacts on city inhabitants. The remaining 29% of more problematic air quality is caused by PM 10 (74%) and SO 2 (~24%). The study has revealed that both ozone (O 3 ) and carbon monoxide (CO) have little contribution to Riyadh air pollution at 2% and 0.52%, respectively.