There is a certain kind of environmentalist who seems unable to acknowledge any good news about the environment, because it might create complacency about remaining issues. I'm not a fan of this approach. When successes are denied, credibility diminishes. And if there's never been an environmental success to celebrate, I'm more likely to be discouraged about the future than energized. In that spirit, here are some figures from an Environment Protection Agency annual report, Our Nation's Air.
This figure shows the decline in what are often called the "criteria" air pollutants. The horizontal line shows the U.S. National Ambient Air Quality Standards. At a national level, all of the pollutants are below the dashed line. The percentages in the upper right corner of the figure show the decline in the concentration of each category of air pollution since 1990s.
In thinking about how to reduce pollution further, it's worth noting that different pollutants tend to come from different sources. For example, highway vehicles (green bar) are bigger producers of carbon monoxide and nitrogen oxides, while stationary fuel combustion (think power plants, blue bar) produce a large share of the particulates and sulfur dioxide, and industrial processes contribute most to emissions of volatile organic compounds (VOC) and ammonia (NH3).
It's worth noting that the overall decline in air pollution in the last half-century or so is happening at a time of rising US population, rising GDP, vehicle-miles traveled, and overall energy consumption.
And yes, of course, these criterion air pollutants, by tradition, don't include carbon dioxide and other greenhouse gases. I offered some thoughts on these issues last week in "Economics of Climate Change: Three Recent Takes" (August 14, 2018). Here, I would just note that actions to reduce conventional air pollutants, both in the US and around the world, can be viewed as part of a "co-benefits" approach, which would lead also to lower carbon emissions.
Timothy Taylor is an American economist. He is managing editor of the Journal of Economic Perspectives, a quarterly academic journal produced at Macalester College and published by the American Economic Association. Taylor received his Bachelor of Arts degree from Haverford College and a master's degree in economics from Stanford University. At Stanford, he was winner of the award for excellent teaching in a large class (more than 30 students) given by the Associated Students of Stanford University. At Minnesota, he was named a Distinguished Lecturer by the Department of Economics and voted Teacher of the Year by the master's degree students at the Hubert H. Humphrey Institute of Public Affairs. Taylor has been a guest speaker for groups of teachers of high school economics, visiting diplomats from eastern Europe, talk-radio shows, and community groups. From 1989 to 1997, Professor Taylor wrote an economics opinion column for the San Jose Mercury-News. He has published multiple lectures on economics through The Teaching Company. With Rudolph Penner and Isabel Sawhill, he is co-author of Updating America's Social Contract (2000), whose first chapter provided an early radical centrist perspective, "An Agenda for the Radical Middle". Taylor is also the author of The Instant Economist: Everything You Need to Know About How the Economy Works, published by the Penguin Group in 2012. The fourth edition of Taylor's Principles of Economics textbook was published by Textbook Media in 2017.