Atmospheric pollution from industrial activity has fallen across Europe as countries shutdown their economies to fight the coronavirus pandemic.
Images from the Copernicus programme's Sentinel-5P satellite clearly show a drop in nitrogen dioxide concentration in France, Italy and Spain throughout March.
In February, NASA showed how similar kinds of emissions from road traffic drastically dropped in Wuhan, epicentre of the epidemic. From red/orange, China's map turned to blue.
The European Space Agency, which oversees the Earth observing satellites in the Sentinel fleet, has observed the same phenomenon in Italy, France and Spain, with a particular drop off in pollution along the Po Valley in northern Italy, where cities have been effectively at a standstill since early March.
According to the European Environment Agency, the Spanish cities of Madrid and Barcelona are following the same pattern, and have also been under strict confinement measures since mid-March.
Such radical falls are unprecedented. "It is the first time that I have seen such a significant change in such a large region linked to an event," said Fei Liu, a researcher at NASA, about China.
Even during the economic crisis in 2008-2009, the decrease "had been more continuous over time", added Alberto González Ortiz, an air quality specialist at the European Environment Agency.
In northern Italy, "the average NO2 concentration levels have reduced by half," noted Vincent-Henri Peuch, from European Earth monitoring program Copernicus.
For other countries or regions that have decided to go under lockdown - Belgium, Argentina, California, Tunisia, Bavaria, Colombia - researchers are waiting to see developments.
However, this does not mean that the air is clean: in China, Beijing experienced episodes of fine particle pollution in February, according to the NASA Earth Observatory.
Ditto in Paris, which recorded an average index pollution despite the lockdown - once again because of fine particles.
The concentration of pollutants can indeed vary depending on the weather, explained Peuch to AFP. "Some emission sources, such as energy production and those related to housing, do not visibly decrease when more people stay at home," he added.
However, PM2.5 and PM10 particles as well as carbon monoxide "should also decrease over time" due in particular to the reduction of transport and industry, he said.
What is the impact of such a break over the population?
According to a recent study, air pollution causes 8.8 million premature deaths per year across the world. So what is the impact of these abnormal circumstances?
"Any drop in pollution is good to have," said the French pulmonologist Bruno Housset.
In the short term, pollution with fine particles causes eye and throat irritations and respiratory discomfort.
For elderly or asthmatic people, treatment may be necessary for respiratory or cardiovascular pathologies in the days or weeks following the exposure.
In the longer term, it can induce chronic, respiratory or cardiovascular diseases or lung cancer.
Lockdown can therefore "reduce the inflammatory effects", Housset explained, all the more since the quality of the air inside houses depends a lot on the air coming from the outside.