US Air Pollution Progress Is Slowing. Researchers Are Looking at Cities

Air pollution is falling, but the declines have started to lag. To examine how pollution varies across communities, a new project is pursuing more detailed measurements.

(Bloomberg) — At first glance, the backpack — army green with a white North Face logo — looks at home on the campus of City College of New York. That is until you notice the tubes rising three feet into the air from its front-right corner. Those are for monitoring air pollution. 

The backpack’s three latched-together monitors, developed by the National Oceanic and Atmospheric Administration and 2B Technologies, track GPS information alongside measures of ozone and a type of air pollution known as PM2.5. Between late July and mid-August, City College volunteers donned the backpack to walk, bike and subway across pre-planned routes in Manhattan and the Bronx, mapping pollution along the way. Their goal: improve researchers’ understanding of how air pollution varies across communities and in different kinds of spaces. 

The data could also help solve a mystery. While air pollution is falling in the US — PM.2.5 pollution is down by about 40%, on average, over the past two decades — those declines have recently started to slow. And since air pollution tends to be hyper-local, the backpack could help researchers figure out why.

The US gold standard for monitoring air pollution are the more than 4,000 stationary monitors overseen by the Environmental Protection Agency. Eight of those are in the New York City area, says Yoshira Ornelas Van Horne, an assistant professor at Columbia University’s Mailman School of Public Health; “which, if you think about the size and the amount of people, that’s not enough.” 

Air pollution tends to be localized, which means even communities next to each other can experience dramatically different levels of it. By measuring block by block, the backpack is part of a broader NOAA mission to better understand air pollution in cities.

The first type of pollution it’s tracking, PM2.5, gets its name from its type (particulate matter) and its size: 2.5 microns or less. One grain of sand is roughly 500 times larger than a single particle of PM2.5 pollution. That diminutive size makes the particles adept at lodging themselves in our lungs, and then our bloodstream, where they can cause everything from heart attacks to stroke. The World Health Organization estimates PM2.5 pollution causes roughly 4.2 million premature deaths every year. 

The second type of pollution targeted by the backpack, ozone, forms due to chemical reactions (triggered by heat and sunlight) that occur between the nitrogen oxides formed when we burn fuel and what are known as volatile organic compounds, or VOCs. VOCs can arise naturally, but the VOCs that trigger ozone formation usually come from fossil fuels or petrochemicals. Sources can include building materials, car interiors and even perfume.

“You don’t see ozone; it’s completely transparent. But it does affect health,” says Audrey Gaudel, a research scientist at the Cooperative Institute for Research in Environmental Sciences, and co-lead on the New York study. CIRES is a partnership between NOAA and the University of Colorado Boulder.

For decades, the largest urban source of air pollution has been cars. The 1970 Clean Air Act required the EPA to set national quality standards for six common air pollutants, including ground-level ozone and particulate matter. The EPA also passed regulations forcing oil companies to clean up their fuel supplies and automakers to adopt emissions-limiting technologies like catalytic converters. 

Since then, regulations have gotten even stricter, and air pollution in the US has been falling. Between 2000 and 2022, PM2.5 pollution dropped by more than 40%, on average, according to EPA data. Since 1990, US levels of nitrogen dioxide (a precursor to ozone) have declined by more than half. 

Rising average temperatures now risk undoing that progress. “The regions in the northern hemisphere are doing a good job of cleaning their air locally,” Gaudel says. “But we are still impacted by high levels of pollution just because of climate change.”

As one example, Gaudel points to the Canadian wildfire smoke that blanketed much of the northeast this summer. In June and July, New York and Chicago saw more “very unhealthy” and “hazardous” air quality days for PM2.5 pollution than in the same months every year since the EPA began tracking PM2.5 nationally in 2000, a Bloomberg CityLab analysis of federal data found. Research shows that climate change made the fire conditions in Quebec, where the smoke originated, twice as likely. 

Rising temperatures can also speed up the chemical processes that produce ozone, a key component of smog. “Heat is why in many places smog is worse in the summer than in the winter,” says Patrick Hayes, associate professor of Environmental and Analytical Chemistry at the University of Montreal, who is not involved in the NOAA research.

Meanwhile, studies increasingly suggest that air pollution can harm human health even at levels previously considered safe. In January, the EPA reduced its public health guidelines for PM2.5 pollution to 10 micrograms per meter cubed, from 15. “But we’re still finding associations with a lot of health outcomes even below those levels,” says Ornelas Van Horne, “such as cardiovascular diseases [and] respiratory health.”

The dramatic fall-off in air pollution seen after the 1970s started to slow a few years ago. Researchers are now trying to understand what exactly is behind the stall, and the degree to which climate change is a culprit. Changes in atmospheric chemistry associated with rising temperatures are one possible factor. One backpack survey during a heat wave in New York City this summer showed “that more ozone was basically produced that day,” Gaudel says. 

Another answer could be an evolution in the sources of air pollution: One 2018 study found that as the levels of VOCs from cars has declined, the relative importance of VOCs from products such as pesticides, printing inks, cleaning supplies and personal care products has increased. 

Part of the challenge in pinning down cause and effect, Hayes says, is that the relationship between pollutants and pollution isn’t linear. During the pandemic, for example, “nitrogen oxides levels dropped, but in some locales ozone went up,” he says. 

In New York, the backpack volunteers aligned their routes around rush hour — when air pollution levels are usually higher — and to complement existing pollution-monitoring that’s done via plane and a satellite. “Our goal was to see how well the street-level data we’re getting is mapping to the satellite images,” says Van Horne. 

While the satellite can more easily cover a given area, the snapshots it takes of pollution are expansive, covering up to two square miles (five square kilometers). Pairing on-the-ground observations with those types of high-level readings could lead to improved models for air pollution. One quirk the backpackers noticed, for example, was that ozone and PM2.5 pollution don’t necessarily climb in tandem. 

The hope is that the volunteers’ research will contribute to a better understanding of air pollution, which could in turn shape regulatory policy. But gathering data on the ground can be tedious work: Each backpack route took about two hours to complete. The volunteers also biked some routes — bike lanes are interesting “because that’s where people breathe so intensively,” Gaudel says — and lugged the backpack through multiple subway trips. 

“Walking the streets is a long task,” Van Horne says. “It takes a lot of man and woman power, a lot of time, to be able to go street by street.” 

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