Check out this map generated by Aaron van Donkelaar at Dalhousie University. It uses NASA satellite imagery to calculate the amount of air pollution throughout the world. More specifically, it is looking at levels of fine particulate matter less than 2.5 microns in diameter (PM-2.5). These are particles that are small enough to go deep in the lungs and cause health problems. Usually these are monitored with surface-based monitoring systems, but many developing countries don't have such systems in place. The authors of this study used satellite measurements taken between 2001-2006 and computer modeling to calculate the surface levels of PM-2.5 throughout the world.
Notice the huge area of pollution in North Africa, the Sahara and South Asia. This is where about 80% of the world's population lives, so the health impact is potentially great. Much of the particulate matter comes from unfiltered coal-burning plants, automobiles and agricultural burning. However not all of it is man-made. In the Saharan desert many particles of mineral dust are picked up by strong winds.
van Donkelaar, A., Martin, R., Brauer, M., Kahn, R., Levy, R., Verduzco, C., & Villeneuve, P. (2010). Global Estimates of Ambient Fine Particulate Matter Concentrations from Satellite-Based Aerosol Optical Depth: Development and Application Environmental Health Perspectives, 118 (6), 847-855 DOI: 10.1289/ehp.0901623