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DIESEL EXHAUST RESEARCH: WHAT HAS IT TOLD US ABOUT AMBIENT ORGANIC PM TOXICITY.
MADDEN, M. C. DIESEL EXHAUST RESEARCH: WHAT HAS IT TOLD US ABOUT AMBIENT ORGANIC PM TOXICITY. Presented at Health Effects of Organic Aerosols Workshop, Palo Alto, CA, October 24 - 25, 2007.
Summary of diesel exhaust research.
Diesel exhaust is a complex mixture of components which includes organic gaseous and particulate material. Sources of the exhaust are derived from both on road and off road engines. Use of diesel fuel continues to increase in the US and globally, though the development and use of new technology and implementation of stricter regulatory standards are combining to lower emissions of PM and some gas phase components. Diesel exhaust contributes to ambient outdoor PM and gases, both in a localized manner (e.g., construction sites) and a more ubiquitous fashion (e.g., onroad vehicles). A portion of exhaust can migrate into the indoor environment. As such, research into the health effects induced by diesel exhaust could shed light on possible effects induced by ambient organic aerosols. The health effect usually associated with repeated diesel exposures is lung cancer. For acute exposures, most of the evidence for diesel exhaust-induce effects are derived from controlled exposures with healthy adolescent volunteers. Pulmonary inflammation, symptoms (e.g., irritation), and increased immunoglobulin E levels (IgE; related to allergy processes) have commonly been observed. Changes in lung physiology, such as lung function decrements, are typically not observed with controlled diesel exposures with human subjects. Effects of diesel exhaust on human cardiac function has not yet been thoroughly evaluated, while ambient PM exposure studies have revealed alterations in cardiac electrophysiology, such as heart rate variability changes. Vascular dilation in response to various agonists have been shown to be attenuated by diesel exhaust exposure suggesting alteration of endothelial function. There is an extensive literature showing diesel exposure induces increased severity of lung viral and bacterial infections in animal and cell models through several mechanisms. Other effects observed in animal models need further confirmation in diesel exposed-humans; similarly potential health effects revealed by epidemiological studies need further confirmation in controlled toxicological studies. The types of diesel exhaust used in health effect studies can lead to variable effects, and magnitude associated with an effect. Additionally, potentially susceptible populations can be identified via controlled exposure studies. To date susceptibility factors identified for sensitivity to diesel exhaust-induced effects include humans with certain genetic polymorphisms related to glutathione-S-transferases. Other possible factors that may indicate susceptibility are other genetic polymorphisms (e.g., P450 enzymes), age, diet, obesity, diabetes, chronic obstructive pulmonary disease. Mild asthmatic individuals as a group do not exhibit a greater degree of responses such as lung neutrophilia; however the variability seems to be greater within asthmatic subjects possibly suggesting a subpopulation of sensitive individuals. Changes in emissions of certain classes of diesel exhaust through technological innovations can elucidate whether health effects are altered, and indicate which sources that contribute to organic aerosols to limit. Some lines of evidence suggest that exhaust from the combustion of biofuels such as biodiesel may not be less toxic than that from petroleum diesel. The classes of diesel exhaust components that may be involved in observed health effects and responses seem to include PAH and PAH-like compounds, carbonyls, and interactions of the diesel particle with transition metals probably via complexation. Controlled exposures of humans to diesel exhaust as well as concentrated ambient particulate matter, and use of other approaches (in vitro screening, epidemiological studies) together can assist in identifying health effects of organic aerosols. Attention should be directed towards identifying potentially unique ambient exposure situations where levels of airborne organic material are at least temporarily changed, thereby allowing examination of health effects and biological responses at different exposure concentrations. [This abstract may not reflect official U.S. EPA policy.]