Short-term exposure to PM2.5 and ozone were significantly associate with mortality risk without evidence of a threshold, in a study on the entire Medicare population. Researchers at the Harvard/MIT ACE center examined all-cause mortality risk associated with same-day and prior day exposure to PM2.5 and ozone (adjusting for simultaneous exposure) in a case-crossover design applied to entire Medicare population between 2000 and 2012. Exposures were estimated using models based on land-use, chemical transport, and satellite remote sensing data (at 1×1 km resolution), so that exposures estimates were available even for rural populations not covered by the air pollution monitoring network. A short-term increase of 10 mg/m3 in PM2.5 (adjusted by ozone) was associated with a statistically significant 1.05% increase in daily mortality rate; an increase of 10 ppb of ozone (adjusted for PM2.5) was associated with a 0.51% relative increase. These relative risk increases correspond to 1.42 and 0.66 additional death per million people at risk, for PM2.5 and ozone respectively. In the associated editorial in JAMA, the significance of the finding for policy review of the NAAQS is emphasized, particularly because the lack of evidence of a threshold effect suggests that no level of exposure to these pollutants is entirely safe. The evidence presented in the study also benefits from the large study population, the robustness of the design, and the consideration of multi-pollutant effects.