As the coronavirus pandemic swept through Asia, many countries imposed strict lockdowns to stop the disease from spreading, especially in major cities.
From Beijing in China to Hanoi in Vietnam, cities suspended transport and ordered millions of people to stay at home, leaving roads unusually empty.
The reduction in traffic on the roads had an unintended consequence – scientists observed a drastic drop in air pollution levels in early 2020 over cities and industrial areas in Asia and elsewhere.
Now, as countries in Asia cautiously ease their restrictions, traffic has returned to the roads and pollution is spiking.
“There were really marked reductions in air pollution across Asia,” Paul Monks, professor of air pollution at the University of Leicester, told the BBC. “What we’re seeing now is a return to pre-COVID levels.”
Satellite data from Nasa and the European Space Agency showed that, in the first three months of 2020, levels of nitrogen dioxide were far lower than in the same period in 2019.