Fewer people might have died in China this year than expected because of the coronavirus pandemic.
The ironic outcome is the result of improved air quality, the lockdowns enacted to halt the virus' spread temporarily clearing the notoriously polluted Chinese skies.
According to new research published in journal The Lancet Planetary Health, 24,200 "premature deaths associated with particulate matter" were prevented between February and March in China, where the virus was first detected. Over that same time, Chinese authorities reported just 3309 deaths from COVID-19.
Air pollution in the sprawling nation - often called the world's factory - dropped 29.7 percent in February and March, University of Notre Dame scientists said, compared to measurements taken over the previous four years.
By the end of March, COVID-19 had claimed about 40,000 lives worldwide.
Each year, the World Health Organization says around 7 million people die from exposure to air pollution. Most of them - about 4.2 million - are killed by ambient air pollution outside the home.
The WHO in 2016 said China's PM2.5 particulate levels - the most dangerous kind - were on average four times worse than its maximum recommended exposure.
In Europe air pollution fell about 17 percent through February to May, saving an estimated 2190 lives. Europe typically has much better air quality than China, so the gains were fewer.
"These results underline the severity of air quality issues in some areas of the world and the need for immediate action," said study author Paola Crippa.
Dr Crippa said if Europe and China had to lock down for all of 2020, hundreds of thousands of lives lost to pollution would be saved.
"This unique, real-world experiment shows us that strong improvements in severely polluted areas are achievable even in the short term, if strong measures are implemented."
A separate study out this week found an 8.8 decrease in carbon dioxide (CO2) emissions in the first half of 2020, compared to 2019 - most of it down to a 40 percent drop in transport emissions as people worked from home.
"In April, at the height of the first wave of corona infections when most major countries shut down their public life and parts of their economy, emissions even declined by 16.9 percent," said author Zhu Liu of Tsinghua University, Beijing.
The drop is the biggest since the outbreak of World War II, the researchers said, dwarfing the 1.4 percent drop seen as the global financial crisis hit in 2008.
In total 1551 million tonnes of carbon dioxide was put into the atmosphere. While this sounds like a lot, as soon as lockdowns were lifted emissions shot back up to normal - and the researchers said even the lowered rates of emissions during lockdown, if sustained, wouldn't be enough to stop disastrous climate change.
"While the CO2 drop is unprecedented, decreases of human activities cannot be the answer," said co-author Hans Joachim Schellnhuber of the Potsdam Institute for Climate Impact Research. "Instead we need structural and transformational changes in our energy production and consumption systems. Individual behavior is certainly important, but what we really need to focus on is reducing the carbon intensity of our global economy."
The authors of the University of Notre Dame study said different approaches would be needed in different countries.
"In China, we saw that lockdowns implied very significant reductions in PM2.5 concentrations, which means that policies targeting industrial and traffic emissions might be very effective in the future," Dr Crippa said. "In Europe those reductions were somewhat smaller but there was still a significant effect, suggesting that other factors might be considered to shape an effective mitigation strategy."
About 1.1 million deaths have been attributed to COVID-19 to date, heading for an annual toll - at least in its first year - of about 1.5 million, significantly less than the WHO's estimated toll for air pollution.
The Potsdam Institute for Climate Impact Research study was published in journal Nature Communications.
