Diesel emissions: is more health research still needed?

It can legitimately be asked whether we need any more research on the health effects of diesel emissions. However, despite a research effort spanning at least 5 decades and the generation of a huge literature, there are still key uncertainties about the health impacts of present and future diesel emissions. This article briefly characterizes current knowledge and information gaps, and then proposes some key issues requiring further research. These issues include the adjuvant effect, the bioactivity of inhaled emissions at realistic doses, the toxicity of aged diesel exhaust particles, the importance of ultrafine particulate emissions, the need to improve our ability to predict the impacts of changes in emissions, and the placement of diesel health risks in context regarding other exposures.