Photochemical Products in Urban Mixtures Enhance Inflammatory Responses in Lung Cells

Abstract

Complex urban air mixtures that realistically mimic urban smog can be generated for investigating adverse health effects. “Smog chambers” have been used for over 30 yr to conduct experiments for developing and testing photochemical models that predict ambient ozone (O₃) concentrations and aerosol chemistry. These chambers were used to generate photochemical and nonirradiated systems, which were interfaced with an in vitro exposure system to compare the inflammatory effects of complex air pollutant mixtures with and without sunlight-driven chemistry. These are preliminary experiments in a new project to study the health effects of particulate matter and associated gaseous copollutants. Briefly, two matched outdoor chambers capable of using real sunlight were utilized to generate two test atmospheres for simultaneous exposures to cultured lung cells. One chamber was used to produce a photochemically active system, which ran from sunrise to sunset, producing O₃ and the associated secondary products. A few hours after sunset, NO was added to titrate and remove completely the O₃, forming NO₂. In the second chamber, an equal amount of NO₂ and the same amount of the 55-component hydrocarbon mixture used to setup the photochemical system in the first side were injected. A549 cells, from an alveolar type II-like cell line grown on membranous support, were exposed to the photochemical mixture or the “original” NO₂/hydrocarbon mixture for 5 h and analyzed for inflammatory response (IL-8 mRNA levels) 4 h postexposure. In addition, a variation of this experiment was conducted to compare the photochemical system producing O₃ and NO₂, with a simple mixture of only the O₃ and NO₂. Our data suggest that the photochemically altered mixtures that produced secondary products induced about two- to threefold more IL-8 mRNA than the mixture of NO₂ and hydrocarbons or O₃. These results indicate that secondary products generated through the photochemical reactions of NO_x and hydrocarbons may significantly contribute to the inflammatory responses induced by exposure to urban smog. From previous experience with relevant experiments, we know that many of these gaseous organic products would contribute to the formation of significant secondary organic particle mass in the presence of seed particles (including road dust or combustion products). In the absence of such particles, these gaseous products remained mostly as gases. These experiments show that photochemically produced gaseous products do influence the toxic responses of the cells in the absence of particles.

Although the research described in this article has been funded wholly or in part by the U.S. Environmental Protection Agency through cooperative agreements CR829522 with the Center for Environmental Medicine, Asthma, and Lung Biology and CR829762 with the Department of Environmental Science and Engineering at the University of North Carolina at Chapel Hill, it has not been subjected to the agency's required peer and policy review and therefore does not necessarily reflect the views of the agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.