Air pollution causes health problems and is attributable to some 50,000 annual deaths in the United States, but not all air pollutants pack the same punch.
Scientists have tracked the scope of “PM 2.5” pollution over decades. PM 2.5 is a size of “particulate matter” that is less than 2.5 microns in diameter. But less information was available about its even tinier cousin, described as “submicron” or “PM 1” particulate matter, which is less than 1 micron in diameter. Why does that matter? Because the “little guys” might be the source of worse health effects.
With a study now published in The Lancet Planetary Health, researchers at Washington University in St. Louis have quantified the amount of PM 1 over the United States from the past 25 years.
The new dataset revealed another notable fact: pollution regulation does help. Across the contiguous U.S., average PM 1 levels in the air people breathe dropped sharply from 1998 to 2022, thanks to decades of environmental regulations like the Clean Air Act. However, this progress has slowed since 2010, mainly because of rising wildfire activity.
— source Washington University in St. Louis | Jun 16, 2025