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Studies were performed to compare the physical and chemical characteristics and the in vitro macrophage cytotoxicity of oil and coal fly ash. Sampling methodology was developed to collect size-fractionated particulate matter from the smokestack of either a coal-fired or an oil-fired power plant. Morphological studies demonstrated particle heterogeneity, although most coal fly ash particles appeared to be spherical. Oil fly ash contained two major morphologies; nonopaque amorphous particles and opaque amorphous particles. Elemental analysis indicates that the coal ash is predominantly composed of aluminosilicate particles, while the oil ash is predominantly inorganic sulfates and carbonaceous particles. In vitro macrophage assays demonstrate that the finest coal fly ash particles are the most cytotoxic; the cytotoxicity is significantly less than that of a-quartz, the positive control particle. In contrast, the oil fly ash particles are more cytotoxic than quartz. The cytotoxicity of oil fly ash is due to soluble components, possibly vanadium salts.Associated with the generation of electricity by fossil fuel combustion is the production and subsequent environmental release of primary particulate matter. This material, fly ash, presently poses a major solid waste disposal problem and a potential health hazard. The ash from coal combustion is primarily aluminosilicate in matrix and contains surface layers of vapor-deposited inorganic and organic compounds. Furthermore, interparticle chemical variability is such that individual particle composition may be quite different than the average composition (1).Although a number of studies have characterized the physical and chemical properties of coal fly ash (2), few detailed reports are available on oil fly ash. This is probably because of the relatively high ash content of coal and the large quantities combusted in power generation. It is estimated that 100 million tons of coal fly ash were produced in the United States in 1980 from combustion of coal averaging an ash content of 11% by mass (3). Particulate abatement technologies result in collection within the plant of 95-99% of the ash; thus 1-5% of the coal fly ash is released through the smokestack. Al-

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