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The Clean Air Act, the sweeping federal law designed to control air pollution, has been celebrated for saving millions of human lives since it was first enacted in 1963 and expanded four more times. Its checks on industrial and tailpipe pollutants are expected to prevent more than 230,000 deaths in 2020 alone, along with millions of asthma attacks and thousands of ER visits during a year of a raging pandemic.

But people were not the only ones to benefit from the act. A recent study led by the Cornell Lab of Ornithology finds that that the policy, designed to protect human health, has also saved as many as 1.5 billion birds in the past 40 years. That’s about 20 percent of the total estimated North American bird population.

"It is the first study that quantifies how changes in air pollution in the United States—or really anywhere—impacts birds over such a large area," says Tracey Holloway, an atmospheric scientist at the University of Wisconsin-Madison who wasn't involved in the research. 

The research, published in November in the, is an early attempt to study how air quality regulations affect bird health on a nationwide scale. The researchers focused on ground-level ozone—the primary ingredient of smog—which is known to cause asthma and respiratory illness in people and damage plant and animal life.  have connected ozone pollution to declines in bird health in the past, but none attempted to study the correlation across such a big geographic area, or to connect it to national policy. "It opens a whole other area of research to refine what we can say about the impacts of air pollution on nature, human health, and bird populations," Holloway says. 

Cornell’s Lab of Ornithology operates the eBird database, a community science platform that tens of thousands of people use to track and report bird sightings all over the United States. Researchers used data from eBird to approximate bird abundance levels across the country, while controlling for other variables, such as birders who are more active in particular locations and on fair-weather days. They teased out how local bird abundance related to ground ozone pollution levels and air quality regulations in more than 3,000 counties during the past 15 years, and found areas with higher ozone pollution had fewer birds.

In particular, the authors of the study found that the EPA’s NOx Budget Trading Program—which limits emissions of nitrous oxide, a pollutant that contributes to ozone’s formation in the atmosphere —“significantly bolstered” the migratory bird population in the eastern United States. The results, they wrote, suggest that a “large decline in average U.S. ozone concentrations over the past several decades”—thanks to the Clean Air Act—had “averted the loss of potentially billions of birds.” 

Ivan Rudik, a Cornell environmental economist and a co-author of the study, says that putting numbers to the law’s life-saving benefits to birds, as this study attempts to do, helps support the case for air quality regulations. Federal regulators typically weigh the economic costs of a new environmental rules against their benefits to inform decisions, and birds might be included in that equation. “All policies in the United States have a cost-benefit test,” he says. “These are benefits that are not captured yet.”

Migratory land birds, such as finches and warblers, seemed to be especially affected by ozone pollution. There may be several reasons for that, says study co-author Amanda Rodewald, an ecologist at Cornell. “The physiological demands of migration are high, so they're going to be vulnerable,” she says. The birds’ respiratory systems also require a continuous flow of air, which holds polluted air in their lungs for longer periods. Plus, migratory land birds tend to eat insects; high levels of ozone pollution compromise the plant habitats that those insects rely on, thus diminishing the birds’ food source.

Just last year, the Lab of Ornithology and collaborators published a major study that found overall North American bird population levels had dropped by around 3 billion birds compared to 1970, though that study didn’t examine causes of the declines. The new research makes the case that even more birds would have been lost were it not for the Clean Air Act.

In the waning days of the Trump administration, however, the EPA is pushing regulatory policies in the opposite direction, particularly to the detriment of individuals and marginalized communities who are most vulnerable to the effects of air pollution. In December, the agency refused to shore up limits on ozone and fine particulate pollution (also known as soot—the country’s most deadly form of air pollution) to the levels supported by. 

Beyond that, the agency also recently finalized a new rule that could weaken future Clean Air Act decisions by changing how the costs of air regulations are weighed against their benefits. The new procedures allow EPA to more easily dismiss “indirect” benefits of pollution controls on the health of people, wildlife, and ecosystems, making it potentially easier for industries to successfully argue against strong air pollution standards. (The incoming Biden administration could choose to "essentially sidestep the rule while it works to undo it," .)

Despite these worrying efforts, environmental advocates see hope on the horizon in President-elect Joe Biden’s pick for EPA chief, Michael Regan, who was an air quality and energy regulator during the Clinton and Bush administrations. His selection has been by the scientific community. The study’s authors hope their findings will be an important step toward considering the case for environmental policies more holistically. “We're at a point where we just don't have the ability to have policies attack one challenge at a time,” Rodewald says.