In vitro exposure to combustion exhaust: Effects of exhaust characteristics

Combustion generated particles are a major contributor to ambient urban particles that are associated with increased morbidity and mortality. However, the biologic mechanism of how these aerosolized particles cause human health effects and what physical and chemical characteristics are important in the response are still unclear. Controlled in vitro exposure to aerosolized particles is a useful first approach to investigate the health effects of particle characteristics. The objective of this work is to develop an in vitro exposure to aerosolized particles at an air-liquid interface (ALI), and use this method to investigate specific characteristics of combustion exhaust particles and gases that are important in causing a toxic and inflammatory response.;Initial experiments were conducted to validate the response of the human bronchial epithelial cell line (16HBE14o) used in this work and the exposure methodology. Cells were exposed by the conventional method to aqueous suspensions of diesel particles from a single cylinder diesel engine. The suspensions were nontoxic, but elicited an inflammatory response at high doses, in agreement with comparable studies reported in the literature. Likewise, cells exposed to aged and diluted diesel exhaust at the ALI exhibited toxic and inflammatory responses that agree with similar exposures in the literature. The number of particles that deposit at the ALI is approximately the same as that in the tracheobronchial region of the human lung. The mean diameter of particles depositing at the ALI is approximately four times larger than that predicted for the lung. Cells exposed by the ALI method had similar toxic and inflammatory responses compared to cells exposed by the conventional method to particle suspensions. However, the suspension dose is about 20 times larger than that at the ALI. The increased responsiveness of the ALI method is likely from the avoidance of collection and resuspension artifacts inherent in the conventional suspension exposure.;The ALI exposure was used to investigate the characteristics of diesel exhaust that contribute to toxic and inflammatory effects. The cells display a linear dose response with increasing exposure duration. Filtering the particles before exposure revealed that exhaust gases, such nitrogen oxides, sulfur dioxide, and hydrocarbons are responsible for approximately half of the toxic and inflammatory response from a loaded engine. Removal of the gas phase and a portion of the semi-volatile organic carbon compounds from unloaded exhaust reduces the inflammatory response to unexposed levels. Exhaust from a loaded engine causes an inflammatory response that is three times greater than that from an unloaded engine.;Oxidant gases present in the atmosphere, such as ozone and nitrogen dioxide, interact with combustion generated particles and modify their toxicity. The effects of oxidative aging were investigated by oxidizing flame generated particles with ozone. Suspensions of oxidized particles are six times more toxic to cells than suspensions of unoxidized particles. The oxidized particle size and morphology are unchanged from the unoxidized, but the surface chemical composition is modified. Reaction with ozone leads to the formation of oxygen containing functional groups on the particle surface, which is believed to be the source of increased toxicity.