Experts say air quality is improving in coronavirus quarantine countries, but it's far too early to talk about long-term changes.
NASA's images are clear: In February, the concentration of nitrogen dioxide (NO2) in Wuhan, China, the epicenter of the COVID-19 pandemic, dropped dramatically and went from an indicator that was red / orange to blue .
NO2 is mainly produced by vehicles, industrial sites and thermal power plants.
While China is peaking its crisis, recent European Space Agency (ESA) images show a resurgence in NO2 emissions.
A remarkable decrease has also been observed by ESA in northern Italy, which has been banned to combat the spread of the novel coronavirus that causes COVID-19.
The European Environment Agency (EEA) reports a similar change in Barcelona and Madrid, where the Spanish authorities issued arrest warrants in mid-March.
"NO2 is a short-lived pollutant with a lifetime in the atmosphere of about one day," said Vincent-Henri Peuch of the EU earth monitoring program Copernicus.
"As a result, this pollutant stays close to the emission sources and can be used as an indicator of the intensity of activity in different sectors," he told AFP.
Fei Liu, an air quality researcher at NASA's Goddard Space Flight Center, noticed the change in China and said, "This is the first time I've seen such a dramatic decline over such a wide area for a particular event."
Even during the economic crisis more than a decade ago, the decline in NO2 levels was "more continuous over time", according to EEA air quality specialist Alberto Gonzalez Ortiz.
In northern Italy, "the average NO2 concentration has almost halved on average," said Peuch.
The pollutant can cause severe inflammation of the respiratory tract.
In other countries or regions where residents have been asked to stay at home, particularly Argentina, Bavaria, Belgium, California, France and Tunisia, specialists are reviewing the data to determine if the trend is similar.
In the meantime, less NO2 doesn't necessarily mean cleaner air.
In February, pollution was caused by fine particles, the NASA Earth Observatory reported.
The air in Paris was also classified as moderately polluted on Friday due to the presence of fine particles and NO2, although the population had been confined at home for three days.
Peuch explained that the concentration of pollutants can vary with the weather.
"Some sources of emissions, such as power generation and consumption in residential areas, are unlikely to decrease noticeably when more people have to stay at home," he noted.
The concentration of so-called PM2.5 and PM10 particles and carbon monoxide (CO) should "also decrease over time," said Peuch.
This is a mixture of tiny solid particles and liquid droplets that are present in the atmosphere with diameters of 2.5 and 10 micrometers (micrometers).
So what can be expected in terms of health improvement, since air pollution causes around 8.8 million premature deaths each year, according to a recent study?
Fine particles irritate eyes and throat and inhibit breathing.
In extreme cases, the elderly and people with asthma risk death if not treated properly.
In the long term, air pollution can cause chronic breathing or heart problems or lung cancer.
"Less pollution is always good," said Gonzalez Ortiz.
According to a group of French doctors known as Air Sante Climate, detention measures protect in two ways by reducing the risk of COVID-19 infection and reducing traffic pollution.
However, it is difficult to know what the global population will actually benefit from, according to health experts, "long-term exposure will have more impact," said Gonzalez Ortiz.
(Except for the headline, this story was not edited by NDTV staff and published from a syndicated feed.)