See NASA's Dazzling Proof that U.S. Air Quality Has Improved

Traffic moves on Interstate 5 in Portland, Oregon on Nov. 17, 2005. Vehicles are among the biggest source of nitrogen dioxide pollution.

Image: Greg Wahl-Stephens/Associated Press

It's summer in the U.S. â€" the time of year when the humidity skyrockets, air stagnates, and the air quality deteriorates, especially downwind of the coal-fired power plants and manufacturing plants of the Ohio Valley.

During hazy and hot summer days in the big cities along the I-95 corridor, the sky often looks like the visual accompaniment to white noise â€" with the horizon indistinguishable from the milky sky.

Yet air quality has actually been steadily improving over the years, largely thanks to the Clean Air Act, along with a drop in coal use and dramatic changes in vehicle fuel efficiency and emissions. On Thursday, NASA released beautiful images demonstrating that people in major U.S. cities from Los Angeles to New York are breathing less nitrogen oxide, a main component of smog. The data comes from the Ozone Monitoring Instrument aboard the Aura satellite.

Nitrogen dioxide is one of the six common pollutants the EPA regulates to protect human health. It can harm the respiratory system and also combines with other pollutants to form smog. Nitrogen dioxide is mostly produced by burning gasoline in vehicle engines as well as from burning coal.

Animation showing how nitrogen dioxide concentrations during spring and summer months, averaged from 2005-2007, compare to the average from 2009-2011.

Image: NASA Goddard Institute of Space Studies

According to NASA, citing EPA figures, 142 million people still live in areas in the U.S. with unhealthy levels of air pollution. Also, high levels of air pollution are a growing issue elsewhere, especially in parts of China, India and other developing nations.

Nitrogen dioxide concentrations during spring and summer months, averaged from 2005-2007.

Image: NASA Goddard Institute for Space Studies

Nitrogen dioxide concentrations during spring and summer months, averaged from 2009-2011.

Image: NASA Goddard Institute for Space Studies

"While our air quality has certainly improved over the last few decades, there is still work to do â€" ozone and particulate matter are still problems," said Bryan Duncan, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, in a press release.