Air quality across the United States, especially in major cities, has been improving as demonstrated by 10 years of images from satellites, NASA says.

The space agency has released a series of images confirming a substantial reduction in levels of air pollution.

"After 10 years in orbit, the Ozone Monitoring Instrument (OMI) on NASA's Aura satellite has been in orbit sufficiently long to show that people in major U.S. cities are breathing less nitrogen dioxide -- a yellow-brown gas that can cause respiratory problems," the agency said in a release.

Nitrogen dioxide, identified as a significant human health risk, comes mostly from emissions from coal-fired power plants and combustion engine exhaust.

The Environmental Protection Agency strictly regulates levels of allowable nitrogen dioxide emissions in the United States.

NASA says the satellite data shows levels have been reduced, especially over the East Coast of the United States, where levels have historically been high, even though the number of exhaust-emitting cars on America's roads has increased.

The satellite images released by the agency show a reduction in the period 2009 to 2011 compared with levels in 2005-2007.

Still, NASA scientists acknowledge, there's a long way to go.

"While our air quality has certainly improved over the last few decades, there is still work to do -- ozone and particulate matter are still problems," says atmospheric scientist Bryan Duncan at the space agency's Goddard Space Flight Center.

That's confirmed by the EPA, which reports an estimated 140 million people are still living in regions of the United States that experience unhealthy air pollution levels.

And the problem is a global one. The World Health Organization estimates that just 10 percent of the world's city dwellers breathe air that meets recommended quality levels.

Precise knowledge of where pollution exists and at what level is vital in efforts to reduce the risks it presents to human health, researchers say.

"You can't control what you don't measure," says Russ Dickerson of the University of Maryland.

"NASA measurements of air quality have value to the people with the authority to control emissions and develop policy," says Dickerson, a member of the NASA Air Quality Applied Sciences Team.

The team was set up in 2011 under the NASA Applied Sciences Program to assist U.S. air quality management by providing scientific data.

NASA visualization experts have color-coded the released satellite images to show concentrations of nitrogen dioxide in the troposphere, the lowest layer of the atmosphere. Green and blue colors indicate lower concentrations, while red and orange colors indicate areas of higher concentrations.

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