Air pollution has fallen in urban areas across Europe during lockdowns to combat the coronavirus, dramatic satellite images show.

Average levels of noxious nitrogen dioxide fell in cities such as Brussels, Paris, Madrid, Milan and Frankfurt from March 5-25 (compared with the same period in 2019), according to Sentinel-5 satellite images released on Monday.

That coincides with lockdowns in many European countries that have curbed road transport – the largest source of nitrogen oxides – and slowed output at gas-emitting factories.

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The new images, released by the European Space Agency and analysed by the non-profit European Public Health Alliance, show the changing density of nitrogen dioxide, which can cause respiratory problems and cancer, like heat maps.

Daily weather events can influence atmospheric pollution so the satellite pictures took a 20-day average and excluded readings where cloud cover reduced the quality of the data.

Data from the European Environment Agency showed a similar trend from March 16-22. In Madrid, average nitrogen dioxide levels decreased by 56 per cent week-on-week after the Spanish government banned non-essential travel on March 14.

However, the EPHA said people living in polluted cities might be more at risk from COVID-19 because prolonged exposure to bad air can weaken immune systems.

“That connection is very likely,” EPHA clean air policy manager Zoltan Massay-Kosubek said. “But because the disease is new, it still has to be demonstrated.”

Air pollution can cause or exacerbate lung cancer, pulmonary disease and strokes.

It’s not the first sign of reduced pollution amid coronavirus lockdowns. Early in March, residents of Venice said the abrupt drop in tourist numbers because of Italy’s shutdown meant the city’s famous canals were cleaner than they had been for years.

China also recorded a drop in nitrogen dioxide pollution in cities in February, when the government imposed draconian lockdown measures to contain the raging coronavirus epidemic.

“This is the first time I have seen such a dramatic drop-off over such a wide area for a specific event,” Fei Liu, an air quality researcher at NASA, said.

A composite image of comparative levels of nitrogen dioxide over Wuhan. Photo: AAP

In some regions of Poland, however, nitrogen dioxide levels remained relatively high during the period despite its lockdown, perhaps due to the prevalence of coal-based heating.

Countries that went into lockdown later – such as Britain, which did so on March 23 – are likely to enjoy a similar pollution reprieve in coming weeks, EPHA said.

-with AAP