Human Cell Mutagens in Los Angeles Air

Abstract

The human cell mutagenicity of particulate air pollution samples collected in southern California is measured. The human cell mutation assay used in this study tests mutagenic activity at the thymidine kinase locus in h1A1v2 cells using a 72-h exposure. Throughout 1993, airborne fine particle samples were taken at a regional background site on San Nicolas Island upwind of Los Angeles and at four urban sites:  Long Beach, central Los Angeles, Azusa, and Rubidoux. The Long Beach site is in close proximity to direct emissions from industrialized sources including power plants, petroleum refineries, and the Los Angeles−Long Beach harbor complex. Central Los Angeles was chosen because of its dense vehicle traffic, railroad yards, and proximity to the central business district. Azusa and Rubidoux are photochemical smog receptor sites located generally downwind of the highest density of primary emissions sources. No systematic seasonal variation of the mutagenic potency (mutagenicity per unit organic aerosol mass) is observed at any of the urban sites. This suggests that the important human cell mutagens are not dominated by a seasonal emission source such as wood combustion and that if the atmospheric transformation products of photochemical air pollution are involved, then these reactions must occur during the winter as well as during the summer photochemical smog season. No significant spatial variation of annual average mutagenic potency of the aerosol was observed between three of the four urban sites; while the average mutagenic potency of the Long Beach aerosol was slightly higher than elsewhere in the air basin. This similarity of mutagenic potency values across widely separated monitoring sites suggests that the mutagenicity of the aerosol is due largely to ubiquitous emission sources (e.g., motor vehicle traffic or stationary source fuel combustion) rather than to proximity to isolated point sources of unusual mutagenic organics. The mutagen concentration per cubic meter of ambient air was computed by weighting the mutagenic potency values of the aerosol according to the mass concentration of organics present at each monitoring site. The human cell mutagen concentration in Los Angeles urban air was found to be 1 order of magnitude greater than at the background site studied upwind of the city, showing that the city is indeed a source of human cell mutagens.

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