A Chinese man wears a protective mask as he passes near the CCTV building in fog and pollution during rush hour in the central business district on February 13, in Beijing, China. Kevin Frayer/Getty Images

Factories were shuttered and streets were cleared across China's Hubei province as authorities ordered residents to stay home to stop the spread of the coronavirus.

It seems the lockdown had an unintended benefit -- blue skies.

The average number of "good quality air days" increased 21.5% in February, compared to the same period last year, according to China's Ministry of Ecology and Environment.

And Hubei wasn't alone. Satellite images by NASA and the European Space Agency show a dramatic reduction in nitrogen dioxide emissions -- those released by vehicles, power plants and industrial facilities -- in major Chinese cities between January and February. The visible cloud of toxic gas hanging over industrial powerhouses almost disappeared.

"This is the first time I have seen such a dramatic drop-off over such a wide area for a specific event," says Fei Liu, an air quality researcher at NASA's Goddard Space Flight Center. "I am not surprised because many cities nationwide have taken measures to minimize the spread of the virus."

Coal consumption dropped dramatically. Major coal-fired power stations saw huge drops in consumption, likely because electricity demand is down.

Burning fossil fuels like coal emits carbon dioxide -- and emissions were down by at least 25% from February 3 to March 1, according to air pollution research group Center for Research on Energy and Clean Air (CREA).

But some are worried that once the coronavirus threat passes, China will be solely focused on restarting its economy, and the pollution will return in full force.

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