Although the majority of the on-road vehicle fleet in the United States is fueled by gasoline, diesel combustion makes up an overwhelming share of vehicle air pollution emissions. Air pollution emissions can be affected by blending biodiesel into diesel fuel. Studies have found that biodiesel combustion results in lower emissions of particulate matter (PM), carbon monoxide (CO), and hydrocarbon (HC), but increases the formation of nitrous oxide (NOx). The U.S. Environmental Protection Agency concluded in a 2002 report that, overall, biodiesel combustion does not worsen air quality compared to conventional diesel and reaffirmed that conclusion in a 2020 proposal and subsequent rulemaking. This study presents a meta-analysis of air pollution changes from vehicles and engines running on biodiesel blends in the United States relative to a conventional diesel baseline.