NASA satellite images: Which are the most polluted countries in the world?

Story highlights

  • Nitrogen dioxide is a major pollutant that can play a role in asthma and other respiratory illnesses
  • New NASA maps show increases in China and decreases in U.S. and western Europe

(CNN)The United States and Europe are among the world's largest emitters of nitrogen dioxide -- but both have also shown the most dramatic reductions in these emissions between 2005 and 2014, according to new global NASA satellite maps.

Nitrogen dioxide is a yellow-brown gas that is a common emission from cars and industrial activity. It is a major respiratory pollutant in urban smog.