Washington ( CNN ) -- The air we breathe in the United States is getting better -- at least by one recent measure . New data released by NASA this week show that at least one pollutant , nitrogen dioxide , has decreased substantially over the past decade . Areas with a high level of nitrogen dioxide have decreased an average of about 40 % , said Bryan Duncan , an atmospheric scientist at NASA 's Goddard Space Flight Center in Greenbelt , Maryland . `` It happened everywhere , not just in a few areas . It 's over the whole country . So this is a real success story for everyone in the U.S. , '' he said . The decrease is particularly prominent in the crowded Northeast , the Ohio River Valley , and some major cities . For example , NASA reported a 32 % decrease in New York City and a 42 % decrease in Atlanta between the periods of 2005-2007 and 2009-2011 . Duncan attributes the decrease to two factors : cars that are more efficient , and scrubbers on smokestack emissions that remove toxins . `` It means fewer hospital admissions , it means fewer emergency room visits , it means fewer doctor visits , '' said Janice Nolen , assistant vice president of the American Lung Association . Man-made sources emit an estimated 19.4 million metric tons of nitrogen oxides a year in the United States , produced primarily by vehicles and by coal-fired power plants , according to the American Lung Association . Nitrogen dioxide is a yellow-brown gas that can cause respiratory problems like coughing , asthma , and trouble breathing . It is one of six common air pollutants for which the Environmental Protection Agency has set limits . Nolen , who produces an annual report called State of the Air , pointed out that these pollutants are a separate concern from greenhouse gases , which are blamed for climate change , and were not measured in this report . And , she said , over 140 million Americans still live in areas with unhealthy levels of air pollution , according to the EPA . `` We have about half the nation who lives in counties that have unhealthy levels of air pollution , '' said Nolen . `` And what we 're learning is , levels that we thought were safe were not , and we need to clean up even more , to provide real protection . We still have a long way to go . '' The Supreme Court in April upheld an EPA rule intended to further reduce emissions of nitrogen oxide and sulfur dioxide that blow across state lines . The decrease cited in the NASA study is particularly prominent in the crowded Northeast , the Ohio River Valley , and some major cities . For example , NASA reported a 32 percent decrease in New York City and a 42 percent decrease in Atlanta between the periods of 2005-2007 and 2009-2011 . Click here for more cities .
`` We still have a long way to go , '' says American Lung Association official . The U.S. Environmental Protection Agency has proposed new rules that would require the nation's largest coal-fired power plants to cut emissions of carbon dioxide and other greenhouse gases by 30 percent from 2005 levels within five years. NASA scientists attributes reductions to more-efficient cars , smokestack emissions cuts . Nitrogen dioxide is separate from greenhouse gases , which were n't measured in this report .