NASA Satellite Images: Which Are The Most Polluted Countries in the World?

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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.

(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.

The latest images show how pollution levels have changed in the last decade in various regions and 195 cities across the world. But the space agency has gone a step further and looked at how factors on the ground, such as large power plants, have impacted the findings.

China, the world’s growing manufacturing hub, saw an increase of between 20-50% in nitrogen dioxide much of it occurring over the North China Plain, a densely populated area that runs from Beijing south to Nanjing.

Expert: Changes ‘aren’t random’

However, three major Chinese metropolitan areas — Beijing, Shanghai, and the Pearl River Delta — saw nitrogen dioxide reductions of 40%. On December 8, Beijing’s city government issued its first red alert for pollution, forcing it to close schools and construction sites and restrict traffic.

“These changes in air quality patterns aren’t random,” said Bryan Duncan, an atmospheric scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who led the research. “When governments step in and say we’re going to build something here or we’re going to regulate this pollutant, you see the impact in the data.”

Exposure to nitrogen dioxide can play a role in asthma, bronchial symptoms, lung inflammation and reduced lung function, according to the World Health Organization. The major sources of emissions of the gas caused by human activity are combustion processes, such as heating, power generation and engines in vehicles and ships.

Nitrogen dioxide levels have decreased from between 20-50% in the U.S. and by as much as 50% in western Europe. And researchers put these changes largely down to the effects of environmental regulations that require technological improvements to reduce pollution emissions from cars and power plants.

In the Middle East, researchers suggest that the increased nitrogen dioxide levels since 2005 in Iraq, Kuwait and Iran were down to economic growth in these countries. But in Syria, the decrease since 2011 is most likely because of the civil war, which began in that year, and has since left more than 300,000 people dead and forced 10.6 million — nearly half the population — to flee.

On December 12, history was made in Paris at the COP21 meeting, where government ministers from 195 countries agreed to abide by a legally binding agreement to combat climate change and keep global warming below two degrees Celsius (3.6 degrees Fahrenheit).