SHANGHAI - Air and water quality in China and elsewhere have improved dramatically after coronavirus lockdowns cut traffic and forced polluting factories to close.
But on the 50th annual Earth Day, an event intended to show support for protecting the environment, residents in some of China's most pollution-prone cities said they were worried the blue skies would not last.
Wildlife has returned to cities around the world, with wolves, deer and kangaroos spotted in roads usually teeming with traffic. Residents of some Indian cities reported seeing the Himalayas for the first time in decades.
NASA satellite imagery has also shown significant air quality improvements across Europe and Asia.
But with lockdowns being relaxed there were fears it would soon be smog as usual.
"In the second half of the year, when the epidemic eases, the weather will slowly be worse after factories reopen," said Tang Zhiwei, 27, a resident of Shanghai. "Try your best to enjoy the blue sky now."
With millions staying at home, concentrations of small lung-damaging floating particles fell by nearly 15 percent in more than 300 Chinese cities in the first 3 months of the year.
Shanghai saw emissions fall by nearly 20 percent in the first quarter, while in Wuhan, where the pandemic originated, monthly averages dropped more than a third.
"The fall in air pollution in the first quarter of course means that air quality targets for the winter period were met comfortably, and meeting the targets for 2020 will not require improvements during the rest of the year," said Lauri Myllyvirta, lead analyst with the Centre for Research on Energy and Clean Air, a think-tank.
A study this week in the Science of the Total Environment journal said many of the pre-existing health conditions that make COVID-19 so dangerous - including hypertension and heart disease - are aggravated by long-term exposure to pollution.
Another paper estimated the recent decline in smog could have reduced the number of premature pollution-related deaths in China and India by 7,400, and the number of pediatric asthma cases by 6,600 in the first 2 weeks of the lockdown alone.
Dan Greenbaum, President of the Health Effects Institute, which tracks the impact of air pollution on mortality, said pollution was a long-term health risk that won't be solved by one-off events, but the recent improvements could at least build support for cleaner air worldwide.
"The perception of what it means to have clean air is likely changing radically right now: being able to go to the India Gate in Delhi and see blue skies," he said.
"I can't imagine that people with asthma in China, or people whose children have asthma, aren't seeing a dramatic improvement."
However, experts worry the decline could give China leeway to turn a blind eye to pollution in order to stimulate the economy, which declined for the first time on record in the first quarter.
There are already signs air quality has started to deteriorate in April compared to last year, Myllyvirta said.
"As investment levels are ramped up to offset the economic weakness caused by the virus, this pattern will spell trouble," he said.