“Nitrogen dioxide is one of the six common pollutants regulated by the U.S. Environmental Protection Agency (EPA) to protect human health. Alone it can impact the respiratory system, but it also New NASA Images Highlight U.S. Air Quality Improvementcontributes to the formation of other pollutants including ground-level ozone and particulates, which also carry adverse health effects. The gas is produced primarily during the combustion of gasoline in vehicle engines and coal in power plants. It’s also a good proxy for the presence of air pollution in general”.

“Air pollution has decreased even though population and the number of cars on the roads have increased. The shift is the result of regulations, technology improvements and economic changes, scientists say”.

 Reference: NASA, 2014, “New NASA Images Highlight U.S. Air Quality Improvement” retrieved at
http://www.nasa.gov/content/goddard/new-nasa-images-highlight-us-air-quality-improvement/