With traffic dramatically down in recent months, the United States is in the middle of an accidental experiment showing what happens to air pollution when millions of people stop driving.
The air is clearer. But the pollution declines aren't nearly as large as early indications suggested, according to an NPR analysis of six years of Environmental Protection Agency data.
In some cities, the amount of one pollutant, ozone, has barely decreased compared with levels over the past five years, despite traffic reductions of more than 40%. Ground-level ozone, or smog, occurs when the chemicals emitted by cars, trucks, factories and other sources react with sunlight and heat.
NPR analyzed more than half a million air pollution measurements reported to the EPA from more than 900 air monitoring sites around the country. We compared the median ozone levels detected this spring with levels found during the comparable period over the past five years.
Our analysis revealed that, in the vast majority of places, ozone pollution decreased by 15% or less, a clear indication that improving air quality will take much more than cleaning up tailpipes of passenger cars.