Gains in reducing America's smog problem have hit a dramatic slowdown

In the 1950s, Los Angeles residents raised signs protesting the region's filthy air. One woman's sign, at an anti-smog committee meeting in 1954, mocked that Los Angeles' San Gabriel Valley had been transformed into the inhospitable "Death Valley."

Largely due to car pollution, many American cities had indeed become ridden with highly visible air pollution, or smog, which grew worse over the next few decades until the U.S. Environmental Protection Agency (EPA) was created, along with the Clean Air Act, in 1970. The Clean Air Act, backed up by EPA enforcement, was strengthened in the decades that followed.

Since the 1970s and 1980s, U.S. air quality has unquestionably improved. However, new research, published Monday in the Proceedings of the National Academy of Sciences, shows that gains made in a critical component of smog, pollutants called nitrogen oxides, have slowed dramatically between 2011 and 2015. The exact cause of the slowdown is still uncertain, but finding the sources could prove more difficult under the current EPA, which has challenged independent scientific research -- including the new proposal that would exclude certain public health and air pollution studies when considering changes to federal air standards.

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"In a historically normal administration, this would be a conversation about putting resources into investigating the question of where these nitrogen oxide emissions are coming from," Joseph Goffman, executive director of Harvard University's environmental and energy law program, said in an interview. Goffman was not involved in the new study.

"Now, we're dealing with an EPA that is crippling its access to scientific resources," he said.

Air concentrations of nitrogen oxides -- which react with sunlight to form smog -- are no longer decreasing like they once were. Their downward trend slowed between 2011 and 2015 to declines of around 1.7 percent annually. This an over 75 percent slowdown when compared to the previous years, 2005 to 2009.

"Everything is declining, but it's not declining as fast as had been hoped for," Anthony Wexler, director of the UC Davis Air Quality Research Center, said in an interview. Wexler was not involved in the new study.

This new research follows the American Lung Association's 2018 State of the Air Report, released on April 18, which showed that many Americans are still exposed to unhealthy levels of smog. The greater New York City-New Jersey metropolitan area, along with eight cities in California, cracked the top ten of the Lung Association's list for the most polluted cities in America.

"Millions of people in these areas are being exposed to air pollution that is seriously reducing their lifespans," said Wexler, noting that Sacramento, close to where he works, received an "F" grade.

People overlooking smog in New York City, likely well prior to the passage of the Clean Air Act.Credit: The LIFE Picture Collection/Getty Images

Air pollution harms human health in two major ways, said Wexler. The tiny, microscopic bits of particulate matter emitted largely by cars have been repeatedly linked to heart disease, including a recent 10-year study that found living near heavily-trafficked areas speeds up the thickening of heart arteries. Meanwhile, smog also harms our lungs and causes breathing problems.

When researchers compared satellite maps that measured nitrogen oxide air pollutant trends between 2005-2009 with measurements taken between 2011-2015, the researchers found a glaring discrepancy.

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"We looked at the satellite data from these two different periods and thought, 'These are two very different maps'," said Helen Worden, a project scientist at the National Center for Atmospheric Research and a coauthor of the study, in an interview.

These satellite observations show the changes in nitrous dioxide (NO2) over two recent periods. From 2005 to 2009, NO2 levels declined sharply across much of the United States, as shown in the areas of darkest blue. But in more recent years, NO2 declined more slowly and even increased slightly in some areas (light red).Credit: Zhe Jiang, redrawn by Simmi Sinha, UCAR

Initially, Worden and her team thought pollution blowing over the Pacific Ocean from China was to blame. "But that wasn’t supported by the data," she said, noting that the greatest nitrogen oxide emission increases occurred in the eastern U.S., not the western half, where more Chinese pollution would be expected.

The pollution increase, it seemed, was coming from inside the country.

It's possible that some of these air pollutants are coming from automobile emissions. Wexler pointed out that sport utility vehicles are still hugely popular, and they pump out more nitrogen oxides than smaller cars. But cars are generally much less polluting than they used to be. This is largely thanks to the introduction of catalytic converters, which convert vehicle exhaust to much less toxic pollutants.

The emissions culprit, then, is likely industry, perhaps from industrial boilers and possibly large, dirtier diesel trucks too, Worden said.

This makes sense, as manufacturing began to ramp up again in 2011 after the economically devastating recession.

"In 2007 the economy just died, so that means you have less emissions," said Wexler. "And then in 2011, that's when everything started coming back again."

Satellite measurements (green line) diverge from the EPA's expected declines in nitrogen oxides (purple line).Credit: Figure by Zhe Jiang, redrawn by Simmi Sinha, UCAR

The trend in nitrogen oxide air pollutants will require further investigation, especially for many cities and local governments that still need to meet the EPA's 2015 rules for limiting smog (also known as ground-level ozone) that were made final during the Obama administration.

"Since these are the main ingredients in smog, then if they’re not reducing as expected, meeting that new standard could be challenging," said Worden.

Yet, the Trump-era EPA may hamper the investigation. The EPA has already removed scientists from its air quality advisory boards and proposed scientific transparency standards that could eliminate certain landmark public health studies linking smog to public health. Such studies didn't make some data public because it involves private medical records.

"The thing that worries me, of course, is federal agencies like NOAA and the EPA could be crippled by the current administration to pursue the investigation aggressively," said Harvard's Goffman. "Scott Pruitt seems to be on a campaign of suppressing science," he said, speaking of the EPA administrator.

Goffman finds the most recent proposal, of banning certain research because it contains medical data that isn't fully transparent, particularly problematic, as air pollution standards rely on medical data. These public health studies already meet the golden standard for scientific integrity, he said, because they're peer-reviewed by independent experts -- not industry interests.

But just because you can’t see it doesn’t mean it's not killing you.

"If you’re telling the scientific community that their decades-long peer review process isn’t adequate, you’re sending a signal that you’d rather hear from interested parties as opposed to independent scientists," Goffman said.

As the study shows, the U.S. air pollution problem might be much improved overall -- but it certainly isn't solved.

"It used to be when you drive in L.A. or the [California] Central Valley you would see this crap out there," said Wexler.

"It's not obvious anymore," he said. "But just because you can’t see it doesn’t mean it's not killing you."

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