Decades of U.S. air quality improvements may be slowing, and these areas have it the worst

Millions of Americans breathed unhealthy air in 2018.
smog over LA viewed from plane
Smog over LA used to be commonplace in the '90s steagnes06/Flickr

For decades, America has made progress on air quality. With emission regulations and advances in clean air technologies, the days of smog so thick it burned your eyes and lungs are virtually over.

But even with our gains, air pollution still contributes to one in every 25 early deaths. And our progress seems to be leveling off. Last week, a new report by U.S. PIRG Education Fund and Environment America Research & Policy Center found that in 2018, one-third of Americans lived in places with more than 100 days of degraded air quality. That’s 108 million people breathing polluted air for over three months—and 35 million more than than a similar report for 2016. “We focused on 100 days because it’s just unacceptable that for more than three months these communities were exposed to such bad air pollution,” says coauthor Morgan Folger, the clean cars campaign director at Environment America Research & Policy Center. “No one should even experience one day.”

The pollutants: ozone and PM2.5

The report focused on ground-level ozone and fine particulate matter (PM2.5). While ozone in the upper atmosphere blocks harmful ultraviolet rays, the same gas lower down irritates our lungs. Nitrogen oxides and volatile organic compounds, released from tailpipes and fossil fuel power plants, react with heat and sunlight in the atmosphere to form ozone. That’s why on hot, windless days, cities are the most hazy—ozone forms the visible smog that many city dwellers are familiar with.

bar graphs showing top 10 most populated areas with high pollution
Most recent data available are from 2018 Infographic by Sara Chodosh

Fine particulate matter, sometimes just called soot, is made up of tiny particles less than 2.5 micrometers in diameter. The particles include organic compounds, combustion particles, and metals. PM2.5 also comes from burning fossil fuels—especially diesel-powered trucks—and other sources like brake pads and wildfires.

These powerful pollutants can contribute to respiratory illnesses, mental health conditions, and cancer, and have been tied to many other conditions. Children growing up breathing polluted air are vulnerable to impaired lung development and long term function. Pregnant women and the elderly are also especially susceptible.

The most polluted regions

For the recent analysis, Folger and her team used EPA data collected at air quality sensors installed across the United States. To see how the pollution levels overlapped with population, they used census data. As for what constitutes “degraded air quality,” the report uses concentrations that the EPA describes as “moderate” air quality. Folger says this level is the point at which those more sensitive to air pollution—including people with respiratory illnesses or children—begin to experience health effects.

The report found that 89 urban areas and 12 rural counties had more than 100 days of polluted air in 2018. An additional 157 millions Americans experienced 31 to 100 days of impaired air. And every state, even Alaska and Hawaii, had regions that suffered at least a month of diminished air quality. “I think it is really surprising that there were way more metropolitan areas and so many more people [impacted by pollution] this year than there were in 2016,” says Folger. “It kind of shocks you—a third of the population could be impacted by this.”

Many places that had a problem with one pollutant also had elevated concentrations of the other. However, there’s some variation. Particulate matter has a lot of different sources, explains Folger. High levels in western states could be due to the rise of catastrophic wildfires in recent years. In other states, proximity to coal-fired power plants might instead determine how much soot pollution there is.

map showing areas of high pollution and population
Most recent data available are from 2018 Infographic by Sara Chodosh

Is air quality going backwards?

Don’t freak out and stay indoors just yet. “There are a lot of days that might be in the moderate range, but the vast majority of people really are not impacted by pollution at that level,” says Kevin Cromar, director of the air quality program at New York University’s Marron Institute of Urban Management, who was not involved with the report. Pollution levels at the lower end of what the EPA considers “moderate” are actually “pretty good air quality” for most people, says Cromar. However, he says there are still many health gains that we could make by addressing our pollution.

In the big picture, we’ve had immense progress in air quality, says Jason West, an atmospheric scientist at the University of North Carolina who was not involved with the report. And that’s largely thanks to the regulations we’ve enacted, such as the 1970 Clean Air Act. A 2018 study found that air quality improvements between 1990 and 2010 avoided approximately 35,800 particulate matter-caused deaths and 4,600 ozone-caused deaths.

But there are signs that this positive trend might be slowing down. “2018 had more days of pollution than each of the previous five years,” the authors of the new pollution report conclude. The American Lung Association’s most recent “State of the Air” report found a similar trend; between 2015 and 2017, more cities suffered from days of highly polluted air than between 2014 and 2016. Another study by researchers at Carnegie Mellon University reached a similar conclusion: while particulate matter pollution went down nationwide by 24 percent from 2009 to 2016, it rose by 5 percent from 2016 to 2018.

And while particulate matter is overall down from the previous decade, the same isn’t true for ozone. “In ozone, we haven’t seen that level of improvement,” says Cromar. “It’s remaining stubbornly high in most parts of the US.” As a recent report on air pollution-related deaths led by Cromar found, while the deaths attributable to particulate matter have gone down since 2008, the health impact of ozone pollution has remained about the same. And as populations in polluted areas grow, that means more people will be breathing that same level of ozone, leading to more pollution-related deaths.

The biggest polluters

Transportation—including passenger vehicles and shipping trucks—is a major contributor of air pollution. Vehicles burning gas and diesel release nitrogen oxides and volatile organic compounds into the air, which can form smog or contribute to particulate matter. While vehicles have become cleaner, some of those gains are offset by more people driving more miles every year. Other contributors include the fossil fuels we burn for power, smoke from wildfires, and industrial processes like chemical manufacturing.

As for the increase in air pollution in the past couple years, it’s impossible to say for sure what’s at play. West says that natural variability—such as in weather patterns, which can both disperse and concentrate pollutants—can make it hard to tell if there is actually an upward trend in emissions.

Folger has some suspicions. Driving miles are going up, and in the western states there’s been an increase in wildfires. She also says that the EPA has been lax on enforcing their own rules by not taking action when states are exceeding pollutant thresholds.

To make matters worse, climate change is expected to further reduce air quality. Increased temperatures speed up the reaction that forms ozone. Also, water-stressed plants release organic compounds that can also add to pollution under climate change. “We’ve seen 19 of the hottest years on record in the past two decades,” says Folger. “We’re definitely experiencing warmer and warmer days—that’ll mean that air pollution gets worse.” A 2017 study in Nature Climate Change estimated that, with our an emissions trajectory like the one we’re on now, we’ll have an additional 43,600 ozone-caused deaths and 215,000 PM2.5-caused deaths in 2100.

We need global climate action to protect local air quality

While climate change threatens to make air quality even worse, the flip side is that addressing the climate not only provides the benefits of slowing warming globally, but also improving air quality locally. Mandates for zero emissions vehicles, improved public transportation, and transitioning to renewable energy all achieve these dual goals. “If we take action to reduce greenhouse gas emissions, we reduce air pollution at the same time,” says West. “The air pollution benefits we would see would be immediate and local to where those actions take place.”

But the current administration is moving policy in the opposite direction, from weakening carbon emission rules for power plants to stopping states from setting their own tailpipe standards. “Those efforts are going to lead to worse air quality and health impacts,” says Cromar. “We need to be vigilant and adopt policies that improve air quality.”