Enlarge / Exhaust from fossil fuel vehicles is a significant contributor to PM2.5 pollution in the US. (credit: Ina Fassbender/picture alliance)

At the outset of the pandemic, when lockdowns were widespread, a remarkable phenomenon occurred. Places that had been saturated with pollution suddenly cleared. The sky over Los Angeles turned blue. Snow-capped mountain ranges that were normally obscured by pollution glistened on the horizon. The white marble of the Taj Mahal, which is normally wrapped in smog, shone brightly against an azure sky. Even in regions that aren’t typically considered polluted, the air just smelled fresher.

The reason, of course, is that hardly anyone was driving. Burning fossil fuels, whether under the hood of an automobile or in a home furnace or power plant, produces copious amounts of fine particulate pollution, also known as PM2.5—particulate matter smaller than 2.5 microns. For years, researchers didn’t understand the impact of these pollutants.

But as scientists have started digging, they have discovered that particulates have an outsize impact on our health. Previous research showed that PM2.5 from burning fossil fuels kills more than a million people a year, mostly in heavily polluted regions of Asia. But now, a new study from the Health Effects Institute shows that even in the US, where the air is comparatively clean, PM2.5 current Environmental Protection Agency guidelines may not be low enough to prevent unnecessary deaths.