US monitors fail to capture certain air pollution

In February, the US Environmental Protection Agency lowered its fine particulate matter (PM_2.5) standard from 12 to 9 μg/m³. The EPA said it would track which areas comply with the new standard using data collected from a network of about 1,000 air-monitoring stations spread throughout the country.

But a new study, which compared existing monitoring data with air quality models and satellite data, found that 44% of urban areas that exceed the new standard either do not have a monitoring station or have monitors that fail to capture the full extent of the areas’ PM_2.5 pollution (Environ. Sci. Technol. Lett. 2024, DOI: 10.1021/acs.estlett.4c00605). If the EPA lowered the standard further to match the World Health Organization’s PM_2.5 guideline of 5 μg/m³, that percentage would increase to 60%.

When the researchers zoomed into specific US census tracts in those urban areas, they calculated that 2.8 million people live in PM_2.5 hot spots not covered by the air-monitoring network. Fifty percent of these people are American Indian, Asian, Black, Latino or Hispanic, Pacific Islander, or multiracial, and 39% of them are low income—high percentages compared with the groups’ share of the overall US population.

“It’s well known in the US that we have large socioeconomic, and especially racial and ethnic, disparities in air pollution,” says Joshua Apte, a professor of civil and environmental engineering at the University of California, Berkeley, and one of the coauthors of the study. “Better monitoring can help advance environmental justice by helping protect a population that is disparately exposed to higher levels of air pollution.”

Previous studies have also found monitoring gaps (Environ. Res. Lett. 2022, DOI: 10.1088/1748-9326/ac548f). “The current configuration of the EPA network reflects the legacy of US air quality policy and regulation, primarily from the 1990s, when many of these stations were deployed,” says Makoto Kelp, a postdoctoral researcher at Stanford University who authored the 2022 study. “The network was intended to monitor area-wide pollution sources rather than specific hot spots or neighborhoods.”

But as part of the rule that lowered the PM_2.5 standard, the EPA required that monitors be placed in communities at-risk for high pollution exposure. Apte and his team found that adding just 10 monitors could shrink the size of the uncaptured hot spots, thus reducing the number of people living in them by 67%. They could also reduce the number of marginalized racial and ethnic groups living in these monitoring gaps by 20%.

The new monitors would be located in the biggest cities that had census tracts that exceeded the new standard but are not currently captured by monitors.

Kelp says the 10 proposed locations make sense “given the authors’ goal of reducing the number of people not captured by the current monitoring network under a stricter annual standard.”

Alan Krupnick, a senior fellow at Resources for the Future, a nonprofit that studies environmental and natural resource issues, agrees that adding monitors is necessary, but he says more work needs to be done to determine their optimal locations. The EPA “would need to do their own analyses and then decide on where to locate new monitors,” Krupnick says.

And Apte points out that while adding the 10 monitors would be beneficial, “they would not be sufficient to help us understand what disparities exist.” Doing that, he says, requires other tools, such as low-cost sensors or satellite-based measurements.