How low can you go? Air pollution affects mortality at very low levels.

The World Health Organization (WHO) recently released new guidelines for outdoor fine particulate air pollution (PM _2.5 ) recommending an annual average concentration of 5 μg/m ³ . Yet, our understanding of the concentration-response relationship between outdoor PM _2.5 and mortality in this range of near-background concentrations remains incomplete. To address this uncertainty, we conducted a population-based cohort study of 7.1 million adults in one of the world's lowest exposure environments. Our findings reveal a supralinear concentration-response relationship between outdoor PM _2.5 and mortality at very low (<5 μg/m ³ ) concentrations. Our updated global concentration-response function incorporating this new information suggests an additional 1.5 million deaths globally attributable to outdoor PM _2.5 annually compared to previous estimates. The global health benefits of meeting the new WHO guideline for outdoor PM _2.5 are greater than previously assumed and indicate a need for continued reductions in outdoor air pollution around the world.