Recently, the U.S. Environmental Protection Agency adopted a more stringent annual-average air quality standard for PM2.5 (9 μg/m3). Here, we demonstrate that 44% of urban areas exceeding this new standard─encompassing ∼20 million people─would remain undetected because of gaps in the current PM2.5 monitoring network.

Crucially, we find that “uncaptured” hotspots, which contain 2.8 million people in census tracts that are misclassified as in attainment of the new PM2.5 standard, have substantially higher percentages of minority populations (i.e., people of color, disadvantaged communities, and low-income populations) compared with the overall U.S. population.

To address these gaps, we highlight 10 priority locations that could reduce the population in the uncaptured hotspots by 67%.

Overall, our findings highlight the urgent need to address gaps in the existing monitoring network.