47 percent of Americans live in places with unhealthy levels of air pollution

How bad is air pollution in the United States? Things are much, much cleaner than they were a few decades ago. But roughly half the country still lives in areas where ozone or particulate pollution can reach unhealthy levels — putting them at risk for lung or cardiovascular problems.

That’s according to the American Lung Association’s new “State of the Air 2014” report. Here are the four most striking points:

Here’s the bad news: Roughly 47 percent of the country — 147 million Americans — live in places where either the level of ozone, the short-term readings of particulates, or the year-round exposure to particulates can reach levels that are harmful to human health.

Depending on the type of pollution, that can create a variety of problems. Short-term spikes in particulates (say, from a sudden surge in wood-burning during winter) can increase the risk of strokes or heart attacks. Long-term exposure to particulates (say, from living near a busy highway) can damage lung function or lead to asthma. And ozone can cause shortness of breath and increase the risk of lung infections.

Many cities had at least one of these of problems in a given year. But 27 million Americans live in counties that saw unhealthy levels of all three types of pollution.

Here’s the ALA’s list of the top 10 most polluted metropolitan areas:

Why does southern California dominate the list? One reason is that tailpipe emissions from all those cars and trucks interact with high heat and bright sunlight to create ozone pollution. An especially hot summer made things worse. So did the geography of the Central Valley, whose weather patterns tend to trap pollutants in the region.

By the way, the four cleanest cities in America are Bangor, Maine; Bismarck, North Dakota; Cape Coral-Fort-Myers, Florida; Salinas, California. So if you don’t want any unsafe air pollution, check out Bangor.

The report notes that many US cities have made strong progress in cutting down on particulate pollution. One reason? Many utilities have been switching from coal to cleaner natural gas during the fracking boom. And many truckers are switching to cleaner diesel engines.

On the flip side, ozone pollution actually rose in 2013 — likely for weather reasons. Ozone pollution, or smog, is created when pollutants like nitrogen oxide interact with heat or sunlight. So ozone readings tend to spike during particularly hot years. That’s a concern if global warming continues to drive up temperatures in the United States in the years ahead.

By the way, this report comes as the Environmental Protection Agency is considering a stricter nationwide standard on ground-level ozone, or smog. The EPA is supposed to review its ozone standards every five years — and levels are currently set at 75 parts per billion over an 8-hour period. Back in 2011, the Obama administration rejected a proposal to lower that standard to between 60 and 70 parts per billion. A scientific panel is expected to propose new standards this summer.

That all said, it’s worth putting this in historical perspective. As the the chart above shows, six major air pollutants have all fallen 72 percent since 1970 — due, in large part, to the Clean Air Act.

Over that same period, the economy has grown 219 percent, the number of miles we drive has grown 165 percent, and the amount of energy we use has grown 47 percent. So it’s certainly possible to drive down air pollution and still get much, much richer.