Cass R. Sunstein is a Bloomberg Opinion columnist. He is the author of “The Cost-Benefit Revolution” and a co-author of “Nudge: Improving Decisions About Health, Wealth and Happiness.” Read more opinion LISTEN TO ARTICLE 5:24 SHARE THIS ARTICLE Share Tweet Post Email

Photographer: Saul Loeb/AFP/Getty Images Photographer: Saul Loeb/AFP/Getty Images

In terms of public health, one of the worst air pollutants is fine particulate matter. From 2009 to 2016, average levels of these particulates in the ambient air in the U.S. plummeted by 24.2 percent. That’s the good news.

The bad news is that from 2016 to 2018, average levels jumped by 5.5 percent.

As a result of that increase, 4,900 Americans died prematurely in 2017, and 9,700 died prematurely in 2018, according to the government’s own estimates of the likely effects of exposure to fine particulate matter.

In short, the air got a lot cleaner during the years when Barack Obama was president (preventing tens of thousands of premature deaths), and has become a lot dirtier under President Donald Trump. But instead of scoring political points or assigning blame, let’s try to understand what is happening, with the help of new research from economists Karen Clay and Nicholas Muller of Carnegie Mellon University.

Reductions in air pollution are among the spectacular success stories in American government. Since 2000, there have been significant decreases in fine particulate matter, technically referred to as PM 2.5, and the progress occurred under President George W. Bush as well as Obama – a 39 percent decline in the national average. Fine particulate matter comes from many sources, including motor vehicles, coal-fired power plants and forest fires. The decreases are attributable, in significant part, to regulatory restrictions.

If you look at other pollutants and longer time frames, you can find even more impressive achievements. For example, ambient concentrations of nitrogen oxide have fallen by 61 percent since 1980, and ambient concentrations of lead have declined by a spectacular 99 percent since that time.

The Clean Air Act, first signed into law by President Richard Nixon in 1970, has almost certainly prevented tens of thousands of deaths -- and perhaps as many as 230,000. It has also prevented many thousands of cases of nonfatal heart disease; tens of thousands of emergency-room visits; and as many as two million cases of exacerbated asthma symptoms.

But Clay and Muller focus on the surprising bad news: the deterioration in air quality in 2017 and 2018. They use a database with over 1.8 million daily readings from 653 monitors in the U.S. What happened in 2017 and 2018?

The researchers explore three possibilities. The first is an increase in wildfires. That may be a contributor, but at most it’s a part of the picture. If you drop the main wildfire season from the data, you get the same general pattern: big decreases in fine particulate matter from 2008 to 2016 and significant increases from 2017 to 2018.

Maybe an increase in economic activity is responsible? Clay and Muller think so. In particular, they suggest that the rise in the use of natural gas and increases in vehicles miles traveled are “likely contributors to the increase in PM2.5.”

To accept this conclusion, we would need some fine-grained evidence, showing big spikes in natural gas use and motor vehicle miles in 2017 and 2018. Clay and Muller do not provide that evidence. Still, increased economic activity is a probable contributing factor.

A third possibility is a decrease in enforcement activity by the Environmental Protection Agency. Clay and Muller note that enforcement activity – understood as the number of penalty actions brought by the agency for violations of the Clean Air Act -- has indeed been variable over time.

The most important finding is a significant decrease in the number of enforcement actions from 2016 to 2018, the very time when pollution levels have been increasing. (There was a modest increase in the number of enforcement actions from 2011 to 2013; a significant fall from 2013 to 2015; and a modest increase from 2015 to 2016.)

It’s hard to know whether the decline from 2016 to 2018 is a result of increased levels of compliance (and hence a reduced need for enforcement) or instead demonstrates that the EPA has been sitting on the sidelines. But Clay and Muller delicately note that in view of significant increases in PM 2.5 after 2016, the recent decline in enforcement activity is “concerning.”

The timing is suggestive here. As of Jan. 21, 2017, did some polluters think they wouldn’t need to worry so much about the EPA – and might be able to get away with violating regulatory standards (some of them coming from the previous administration)? Maybe.

With respect to public health, the most important question is simple: What can be done to reverse the recent trend? Increased enforcement activity would be an excellent idea. If the goal is to protect public health, the EPA should target the dirtiest areas with the largest populations.

To the extent that the recent deterioration in air quality comes from motor vehicles, there is even more reason for the EPA, along with the Department of Transportation, to step back from the disastrous plan to soften existing standards for automobiles and light-duty trucks.

Those standards would cut a variety of pollutants, including fine particulate matter, as well as greenhouse gases. Weakening the rules would increase the number of premature deaths.

Whatever the source of the recent spike in fine particulate matter, it happened on the Trump administration’s watch. It’s the government’s job to respond, and sooner rather than later.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.