Current World Environment Vol. 9(2), 227-236 (2014) Assessment of Ambient Air Quality in Riyadh City, Saudi Arabia B. H. ALHARBI 1 *, M. J. PASHA 1 and N. TAPPER 2 1 National Center for Environmental Technology, King Abdulaziz City for Science and Technology (KACST), PO BOX: 6086, Riyadh 11442, Saudi Arabia. 2 School of Geography and Environmental Science, and Monash Weather and Climate Program, Monash University, Clayton, Victoria, Australia. http://dx.doi.org/10.12944/CWE.9.2.01 (Received: March 10, 2014; Accepted: May 05, 2014) ABSTRACT 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 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. Key words : Pollutants, AQI, Air Quality, Particulate matter, Health. INTRODUCTION Continuous monitoring and acquisition of knowledge of air pollutants are required not only to assess the air quality in a given location but also to understand and address several environmental issues. Air pollution is one issue of growing environmental concern because of its known adverse human health effects 1,2 . Globally every year, 4.6 million people die because of exposure to high level of air pollutants 3 . The correlations between exposure to air pollutants and the occurrence of respiratory and cardiovascular diseases and cardiopulmonary mortality are well documented in the literature 4-6 . For instance, ozone (O 3 ), a photochemical oxidant that helps to determine the atmospheric oxidizing capacity 7 , can cause direct, adverse effects on human health and ecosystems 8- 11 . Moreover, sulfuric and nitric acids as well as sulfate and nitrate aerosols are formed as a result of oxidation of sulfur dioxide (SO 2 ) and nitrogen oxides (NO x ). These fine aerosol particles can effectively cause visibility degradation, and modify the radiation budget by both absorption as well as scattering of solar radiation 12 . Furthermore, atmospheric particles contain a variety of hazardous inorganic substances including metals and sulfur compounds. Lung injury, bronchial-constriction and increased incidence of infections have all been related to metallic air pollution 13 . Asthma and chronic obstructive pulmonary disease have been