When Paula Brooks stands in front of her two-bedroom house on Camp Street in Indianapolis and glances south, she sees brightly painted historic homes and towering oaks. When she glances north, she sees traffic. All day, cars, ambulances, buses, delivery trucks, dump trucks, and semi-tractor trailers rumble by. At rush hour, the traffic crawls as commuters squeeze onto nearby Interstate 65.
“The majority of those heavy vehicles are using diesel, so when it gets congested, you know cars idling, then it really hits you, the fumes,” says Brooks, who grew up in this historic African American neighborhood known as Ransom Place.
Those diesel engines spew microscopic bits of pollution called fine particulate matter, which are 30 times smaller than the width of a human hair, roughly 2.5 microns in diameter. (Scientists call this pollution PM2.5) Researchers have linked the particles, which can lodge deep in the lungs and enter the bloodstream, to cardiovascular disease, diabetes, autism, asthma, chronic obstructive pulmonary disease, and even Alzheimer’s disease.
In theory, a network of Environmental Protection Agency monitors should help track this pollution in Brooks’ neighborhood—and similar places across the country. The most common EPA monitors consist of a metal box with a protruding device at the top to draw in air. Inside are instruments that carefully control the volume of air flow and deliver it to a filtration system that captures pollution particles. Under the Clean Air Act, state regulators are required to put these monitors around geographical regions—usually counties. The regulators then use the monitors’ data to show they are complying with the National Ambient Air Quality Standards, which limits how much of a certain pollutant is allowed in a cubic meter of air.
In practice, many experts and advocates say, that system is broken. Data collected by researchers at the nonprofit Resources for the Future, along with studies from at least one academic group, suggests that actual levels of some air pollutants in Ransom Place and other US neighborhoods are higher than what the monitors indicate. There are simply too few monitors, they say, to give an accurate picture of local pollution levels.
Indeed, in Marion County, where Indianapolis is located, there are five PM2.5 monitors to cover 400-square miles. None are in the urban core, where traffic is among the heaviest. “Our stationary monitors are not well suited to pick up our worst emitters,” says Gabe Filippelli, a biogeochemist at Indiana University-Purdue University Indianapolis who studies air pollution.
Some researchers suggest that state regulators may have an incentive to keep the limited supply of monitors away from pollution hotspots. And gaming the regulatory system may not be limited to monitor placement. Some recent work suggests that regulators take advantage of intermittent monitoring, an EPA practice that allows regulators to turn on monitors just once every three or six days. Other research shows how facilities may exploit a Clean Air Act provision to avoid penalization by claiming any excess emissions—above what their EPA permit allows—were unintended.
The result of all this overlooked pollution, the researchers say, is that people get hurt—mostly people of color. A 2021 study in Science Advances shows racial-ethnic minorities in the United States are disproportionately exposed to higher levels of PM2.5. The authors modeled pollution concentrations from vehicles, construction, industry, and more, and found Black, Asian, and Hispanic people breathe dirtier air than White people.
This unequal exposure is due to racism, says Julian Marshall, study coauthor and a civil and environmental engineer at the University of Washington. He points to historic discriminatory housing and land-use policies, including mortgage redlining, which made it so Black Americans and other people of color could not get government-backed home loans where they lived. City planners viewed these redlined neighborhoods as good places for industry and highways because redlining rendered the land cheaper; residents also lacked the political clout to fight such developments. These explicitly racist policies from decades ago still impact the people who breathe the dirtiest air today, says Marshall: “The past is still present.”
For years, the EPA’s ground monitors were the only way to measure pollution. Then in 1999, NASA launched a rocket from California’s Vandenberg Air Force Base—and with it, a new system for gathering pollution data. Hitching a ride on that rocket was a satellite that flies over every inch of the planet once per day. Along with another satellite launched in 2002, it gathers data on the concentration of pollution particles by comparing solar radiation at the top of Earth’s atmosphere with how much is reflected back from Earth’s surface—the more particles, the less radiation is reflected back.
Researchers have now been analyzing satellite data for two decades to get a more complete picture of Earth’s pollution, but they are still refining the statistical tools needed to interpret the data. Because satellite measurements are not direct samplings of pollution particles, researchers must use a statistical conversion process that at least one study showed can involve a lot of error. This means the data should be viewed with some caution, according to University of California, Berkeley professor Meredith Fowlie, a co-author on the study. In an email to Undark, she explained that the EPA monitors provide a direct measure of pollution; whereas, the satellite-based estimates are “informed guesses coming out of a very challenging prediction exercise.”
Still, Fowlie and others see value in looking at satellite pollution estimates to fill in the data gaps inherent among the nation’s ground monitoring network. The satellite estimates reveal, for instance, that the monitors may be missing pollution. And some researchers, including Alan Krupnick, a senior fellow at Resources for the Future, and Corbett Grainger, an environmental economist at the University of Wisconsin-Madison, say that may be strategic. Both have been comparing satellite data with ground monitor data and have found evidence that in some counties, including where Brooks lives, monitors may sit just outside pollution hotspots—perhaps because state regulators want to avoid costly investments necessary to bring down pollution levels.
EPA officials disagree, pointing to guidelines on where their monitors should be sited. And Barry Sneed, a spokesperson for the Indiana Department of Environmental Management, told Undark via email that his agency follows those guidelines as best they can, adding that “there are limitations to locations that may otherwise be ideal such as vegetation, structures, or the lack of access to power.”
In other parts of the country, regulators may be taking advantage of possibly unintended air monitoring loopholes, such as an EPA policy that allows regulators to monitor pollution on an intermittent basis. (EPA data show that the annual cost to operate a monitor that is turned on every day is about $41,000; whereas, to turn it on once every six days is about $21,000.) The one-in-six-day schedule ensures that the monitor will be actively measuring pollution on different days of the week; the EPA says the pollution captured should then average out to a representative number.
But according to recent research by Eric Zou, an economist at the University of Oregon, local regulators may have found a way to skirt this rule. Zou says local regulators are tasked with gathering data to forecast local pollution levels and thus know ahead of time when upcoming air quality may be bad. They also know the day that intermittent monitors will be turned on (the EPA publishes a public schedule). And, critics say, local regulators should know that if they issue a pollution advisory—a warning to residents to stay indoors and drive less—they may be able to quickly lower pollution.
Zou compared daily NASA satellite PM2.5 data to one-in-six-days measurements from ground monitors in hundreds of counties. He found that pollution levels dropped on the day the monitors were turned on, then rose again once they were turned off. He also found that local regulators were 10 percent more likely to issue an advisory, which have names like “Spare the Air,” on the days the monitors were turned on. “If you look at when these warnings are issued, look at the timing,” Zou says. “You can see a pile up of these issuances exactly on the days when the federal monitors are monitoring.”