California and Oregon are among the U.S. hotspots for dangerous submicron (PM1) air pollution—those with particles less than a micron, or a millionth of a meter wide—according to new research.
Researchers at Washington University of St. Louis have been quantifying the amount of submicron particles in the air across the country over the last 25 years.
The data, which calculated submicron estimates based on known rations of what makes up PM 2.5 particles (those less than 2.5 microns wide), suggest that many areas in the east of the contiguous U.S. contain hotspots.
Because PM1 pollution is much smaller than PM2.5 (and at least six times smaller than human blood cells), it has the potential to lead to even worse health effects, as it is small enough to slip past the body’s defenses.
Parts of Arizona, California, Idaho, New Mexico, Oregon and Texas all appear to have higher concentrations of submicron particulate matter, shown by darker red areas on the researchers’ map, as below.
Conversely, parts of Colorado, Montana, Utah, and Wyoming appear to have far fewer dark patches on the map, suggesting much lower levels of submicron particulate matter.
Parts of Arizona and New Mexico showed high concentration levels in some areas, and much lower levels among other regions of their respective state.
Higher concentrations of PM1 were found in major urban and industrial areas and areas affected by wildfires, highlighting the role of combustion sources in producing harmful particles. This contrasts with lower concentrations being prevalent across the arid west.
The World Health Organization (WHO) suggests that 99 percent of the global population breathes in air containing high levels of pollutants and exceeds the WHO guidelines. The combined effects of ambient air pollution with household air pollution are associated with 7 million premature deaths every year.
Both outdoor and indoor air pollution can cause respiratory diseases, as the small particles are able to get deep into the lungs or potentially even into the bloodstream.
Exposure to particle pollution can cause heart attacks, irregular heartbeat, aggravated asthma, decreased lung function, and difficulty breathing, according to the Environmental Protection Agency.
According to Chi Li, assistant research professor and first author of the study, the small particles identified tend to come from direct air emissions, such as black carbon particles from diesel engines, or the smoke from wildfires.
PM1 can also form through secondary processes however, when sulfur dioxide or nitrogen oxides are spit out through fuel combustion and burning coal, Li noted.
The significance of pollution regulation should not be overlooked, as the dataset reveals that PM1 levels dropped sharply from 1998 to 2022, as a result of regulations such as the Clean Air Act of 1970.
Newsweek has contacted Li via email for comment.
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Reference
Li, C., Martin, R. V., Donkelaar, A. van, Jimenez, J. L., Zhang, Q., Turner, J. R., Liu, X., Rowe, M., Meng, J., Yu, W., & Thurston, G. D. (2025). Estimates of submicron particulate matter (PM1) concentrations for 1998–2022 across the contiguous USA: Leveraging measurements of PM1 with nationwide PM2·5 component data. The Lancet Planetary Health, 9(6). https://doi.org/10.1016/S2542-5196(25)00094-4
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