Despite the progress that’s been made on cleaning up the nation’s air in recent decades, more than 40 percent of Americans still live in counties with levels of ozone or particulate pollution that exceed what are considered safe levels.

That’s one takeaway from the “State of the Air” report published today by the American Lung Association (ALA). The report has been grading counties in the US on their air quality for 19 years, and this year’s installment shows both continued progress on particle pollution and a worsening of ozone pollution, possibly caused by higher temperatures.

Since the passage of the Clean Air Act in 1970, the combined emissions of six key pollutants have dropped by 73 percent, according to EPA data, even as the US economy, population, and energy use have grown.

But these latest findings show that a warming climate may hinder continued improvements and underscore the importance of looking at pollution in tandem with climate change, say experts.

“Even with the continued improvements [in air quality] we’re seeing the evidence of the challenge of climate change,” says Janice Nolen, the ALA's assistant vice president of national policy. The association's latest report covers the 2014-2016 period, and 2016 was the second-warmest year on record. “With more days that get that kind of heat, it’s going to make [ozone pollution] more likely to form, and harder to clean it up," says Ms. Nolen.

Most ground-level ozone pollution is created when nitrogen oxides react with volatile organic compounds in sunlight. These chemicals are produced by many of the same sources, such as motor vehicles and power plants, that emit carbon and contribute to climate change.

While overall air pollution has been declining for decades, the number of people exposed to unhealthy air pollution levels is increasing. The ALA report puts the number at 134 million, up from 125 million during the 2013-2015 period. And of the 25 most ozone-polluted cities led by Los Angeles, 16 had ozone levels that were higher than the previous period.

“It’s important to recognize that in the US, air pollution has been going down. But that doesn’t mean the problem is solved,” says Tracey Holloway, an atmospheric scientist at the Nelson Institute for Environmental Studies at the University of Wisconsin-Madison. On a global basis, Professor Holloway notes, 1 in 8 deaths each year are caused by air pollution; in the US, nearly half of the population live in places considered unhealthy based on EPA standards.

Like Nolen, Holloway thinks it’s time to start decoupling the discussion of pollution from that of climate change.

“Something coming out of a tailpipe or a smokestack is a mixture of carbon dioxide and nitrogen dioxide or sulfur dioxide,” she says. “Some of [the emissions] warm the planet and some of them react with your lungs. The good news is almost anything you do to control climate and carbon also has major health benefits in terms of air quality. As we move to cleaner fuels, and reduce the use of fuels, it’s a win-win for climate as well as air quality.”

But the reverse, she notes, is not always the case. Some of the fixes that have helped reduce air pollution, like sulfur scrubbers on coal plants, not only don’t reduce carbon, they actually make plants less efficient and encourage them to burn more coal. We need to decide, she says, “if we are looking for single-win solutions, or win-win solutions.”

High readings in Fairbanks

The other main source of air pollution is particle pollution, the mixture of solids and liquid droplets suspended in the air that might include dust, fumes, soot, smoke, or aerosols.

The ALA grades counties both on year-round pollution and short-term pollution, both of which saw improvement. Still, 53 counties had too many days of unhealthy particle pollution, and at least 16 counties had year-round averages that were too high. Fairbanks, Alaska, which had sufficient monitoring for the first time ever, topped the list for the most-polluted city for year-round particle pollution.

That result, says Nolen, underscores the need for better monitoring.

“We have entire states with no data,” she says. “It’s not because they’re not trying, but it’s a complicated process.” Out of 3,007 counties, only about 900 have monitors, she says. A number of counties, including Los Angeles County and San Bernardino County in California, and the entire state of Mississippi, lacked data for year-round particle pollution. “People have a right to know if the air they’re breathing is healthy or not," she adds.

In the case of Fairbanks, the high reading has led the city to brainstorm on what changes they can make, particularly around home heating and wood-burning stoves and fireplaces, to improve the air quality, says Nolen.

Hot spots in the Golden State

For all three types of pollution – ozone, short-term particle, and year-round particle – many of the worst cities are in California. This is largely a function of population density, geography, and climate, especially in the Central Valley. But California is also making significant efforts to identify and address pollution.

One challenge in the past has been that localized “hot spots” – neighborhoods near power plants and freight corridors that are often home to poor communities of color who are also more vulnerable to health problems – have worse pollution. But they don't always get identified by regional monitoring, says John Balmes, a professor of medicine at University of California in San Francisco and the physician member of the California Air Resources Board.

California recently passed legislation designed to address these issues, requiring the Air Resources Board to work with communities to identify hot spots, put in advanced monitoring systems, and develop emission-reduction plans that local districts would carry out.

“It’s forward thinking, a paradigm shift,” says Dr. Balmes, noting California's history of climate leadership.

But Balmes, along with Nolen, worries about policymaking at the federal level, where the EPA has suggested rolling back fuel-efficiency targets and the Clean Power Plan, an Obama-era policy aimed at reducing carbon emissions from power plants.

“The two biggest sources of ozone and particle pollution are motor vehicles and power plants,” says Balmes. “We should as a society be moving aggressively to get away from the fossil fuel infrastructure and move toward cleaner vehicles and cleaner power.”

He also worries about efforts to devalue science and sow doubt about commonly accepted scientific findings, such as the association between fine particulate matter and increased cardiovascular mortality.

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The ALA cites six main threats at the federal level that could undermine the steady progress on air pollution that include cuts in funding and expertise for clean-air programs and a proposed waiver for “glider” trucks – old, dirty, engines in new truck bodies – to meet the same emission requirements as new trucks.

“These are battles we’ve fought for a long time," says Nolen. "But we’re seeing them come back again."