Raising standards to lower diesel emissions

California policies protect vulnerable communities the most and should be adopted nationwide Air pollution from fine particulate matter (PM 2.5 ) is increasingly driving the global burden of disease ( 1 ), and diesel-powered vehicles are substantial contributors. Recognizing the public health impacts of diesel PM 2.5 (DPM) ( 2 ), many countries have reduced emissions of DPM from both on- and off-road mobile sources over the past three decades. The previous US federal administration, however, changed course by eliminating or weakening policies and standards that govern these emissions. In contrast, the State of California has continued to reduce mobile-source DPM emissions using the state's long-standing authority under the Clean Air Act (CAA) to regulate air pollution more stringently than the federal government. Our analysis of mobile-source DPM emissions suggests that many California sector-based policies have been highly effective relative to the rest of the US. To improve health in communities disproportionately affected by these emissions, we point to opportunities to further reduce DPM emissions in California, in the US more broadly, and in parts of the world where countries have less aggressive vehicle emissions policies than the US ( 3 ).