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Ambient fine particulate matter (PM2.5) is composed of a complex mixture of solids and liquids (smaller than 2.5 microns in aerodynamic diameter) derived from diverse sources (e.g., human activities, including fossil fuel combustion and industrial activities; and natural sources, including volcanic ash and pollens) that varies in space and time due to atmospheric chemistry, weather, and interactions between it and other pollutants in the atmosphere1. The composition of particulate matter, therefore, varies between and within regions of the world, countries, and urban and rural areas, and is influenced by such factors as climate, proximity to an ocean, agricultural activities, transportation activities, and kinds and quantities of point source emitters(2). The proximity to sources also affects the nature of the mixture of constituents composing PM2.5 (mixing state) with a greater propensity for external mixtures of particle types closer to sources, and more homogeneity among particle types as an air mass ages and the particles undergo a greater degree of atmospheric processing. The major components of PM2.5 typically consist of: sulphate; nitrate; ammonium; chloride/sea salt; carbon - described variously as elemental carbon, organic carbon, and black carbon; crustal material, including dust and minerals; and, biological materials and organic mass(2). The level of toxicity associated with PM2.5 is strongly affected by its mass and number concentrations, in addition to particle size, shape, chemical composition, and mixing state.