Aerosol mass (PM(10) and PM(2.5)) and detailed elemental composition were measured in monthly composites during the calendar year of 2007 at a site in Lahore, Pakistan. Elemental analysis revealed extremely high concentrations of Pb (4.4microg m(-3)), Zn (12microg m(-3)), Cd (0.077microg m(-3)), and several other toxic metals. A significant fraction of the concentration of Pb (84%), Zn (98%), and Cd (90%) was contained in the fine particulate fraction (PM(2.5) and smaller); in addition, Zn and Cd were largely (>or=60%) water soluble. The 2007 annual average PM(10) mass concentration was 340microg m(-3), which is well above the WHO guideline of 20microg m(-3). Dust sources were found to contribute on average (maximum) 41% (70%) of PM(10) mass and 14% (29%) of PM(2.5) mass on a monthly basis. Seasonally, concentrations were found to be lowest during the monsoon season (July-September). Principle component analysis identified seven factors, which combined explained 91% of the variance of the measured components of PM(10). These factors included three industrial sources, re-suspended soil, mobile sources, and two regional secondary aerosol sources likely from coal and/or biomass burning. The majority of the Pb was found to be associated with one industrial source, along with a number of other toxic metals including As and Cr. Cadmium, another toxic metal, was found at concentrations 16 times higher than the maximum exposure level recommended by the World Health Organization, and was concentrated in one industrial source that was also associated with Zn. These results highlight the importance of focusing control strategies not only on reducing PM mass concentration, but also on the reduction of toxic components of the PM as well, to most effectively protect human health and the environment.