Air pollution causes millions of premature deaths worldwide each year, with fine particulate matter (PM2.5) identified as a major culprit. In response, countries from Ireland to China have promoted low-smoke or smokeless fuels as a clean alternative to traditional bituminous coal, peat, and wood. However, the health risks of ultrafine particles (UFPs, PM0.1, <100 nm) produced by low-smoke fuels have long been overlooked.