A systematic review of the health effects associated with the inhalation of particle-filtered and whole diesel exhaust
Diesel exhaust is a complex mixture comprised of gases and particulate matter and is a major contributor to ambient air pollution. To reduce health risks, recent changes in diesel engine technology have significantly altered the composition of exhaust, primarily by lowering levels of particulate matter. However, animal toxicological studies continue to report health effects following exposure to whole diesel exhaust from engines employing particulate filters. The cause of these effects remains unclear. To gain an understanding of the role of both particle-filtered and whole diesel exhaust on specific health outcomes, we conducted a systematic review in which we examined animal inhalation and controlled human exposure studies that included a comparison between the effects of particle-filtered and whole diesel exhaust on any health endpoint. We identified 23 studies that met both the inclusion and study evaluation criteria. For studies that examined respiratory and immune system outcomes, as well as those that included gestational exposure, exposure to the gaseous fraction of diesel exhaust was associated with significant adverse effects. For cardiovascular, nervous system, and endocrine outcomes, the impacts from filtered and whole exhaust were endpoint specific. Results from this systematic review demonstrate that exposure to the gaseous and particulate fractions from newer diesel engines can have distinct and independent health effects. Thus, to better inform future risk-mitigation strategies, research to further elucidate health effects from diesel exhaust should include filtration to examine both fractions, rather than whole diesel alone.