And Hubei wasn't alone. Satellite images released 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."