Karachi, Pakistan is one of the largest and most polluted metropolitan cities in south Asia. Air quality was assessed using 24-hr fine-particulate matter (PM2.5) samples collected from two sites, Korangi (industrial/residential) and Tibet Center (commercial/residential). Spatial and temporal characteristics and sources of pollution were evaluated from August 2008 through August 2009 using samples analyzed for PM2.5, black carbon (BC), trace elements, and water-soluble ions. Elemental enrichment factors (EFs) and a receptor model/positive matrix factorization (PMF), were used to delineate the anthropogenic and natural sources. The mean 24-hr PM2.5 was 101 ± 45.6 μg m−3 at Korangi and 76.5 ± 38.4 μg m−3 at Tibet Center. Every 24-hr sample analyzed from both sites exceeded the WHO 24-hr guideline value of 25 μg m−3. Average PM2.5 levels were higher during winter, suggesting increased combustion activity and decreased air dispersion. EFs suggest significant contributions of trace elements from anthropogenic activities. Air pollution levels were higher at the Korangi site than Tibet Center, highlighting major contributions from industrial activities near the site. From PMF analysis, five sources and their relative contributions were determined at Korangi and Tibet Center, respectively: 1) oil combustion (25% and 21%); 2) soil and urban dust resuspension (28% and 25%); 3) vehicular emissions (23% and 5%); 4) sea spray (13% and 26%); and 5) industrial emissions (11% and 23%). The evaluation of health risk associated with exposure to air pollution indicated some major air pollution concerns in Karachi, stressing the need for the formulation and implementation of strict laws on PM2.5 emission.