It’s an antidote to the cooped up, post-COVID-19 world: a walk or run to get some sun and breathe the spring air.
And yes, it’s no illusion born of captivity, the air is actually fresher.
Pollution — in a remarkably short time — has abated. In the past few weeks, satellite measurements have found that emissions from cars, trucks, and airplanes have declined in metropolitan Boston by about 30 percent, while overall carbon emissions have fallen by an estimated 15 percent.
Such a sudden drop has few precedents in the modern era, a testament to the scale of societal disruption caused by the virus.
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“It was like a magic wand was waved,” said Lucy Hutyra, a Boston University associate professor who studies emissions. “But viruses are not a very good solution for climate change.”
Hutyra and her colleagues at BU, along with scientists at Harvard and the Massachusetts Institute of Technology, use satellite data to monitor environmental measures such as air quality. They also found a reduction in another dangerous type of pollution — fine particles from vehicle exhausts, power plants, and other sources. The World Health Organization estimates these particles are responsible for an estimated 7 million deaths every year around the world, including 77,000 in the United States.
In Boston, the amount of such harmful particulate matter has declined over the past two weeks by nearly 20 percent, compared to the prior two-week period. Levels of carbon monoxide and nitrogen dioxide have declined by similar amounts, according to an analysis of three federal air quality monitoring stations in Boston.
Those decreases are primarily the result of a 50 percent drop in traffic on local highways and a 75 percent plunge in passenger traffic at Logan Airport. These mirror declines recorded in other cities around the world where there has been a similar cessation of normal life, including on the West Coast, in Northern Italy, and in China.
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“We have cleaner air now, but this isn’t how we want cleaner air,” said Priyanka deSouza, an air quality data analyst at MIT’s City Scanner project, which measures emissions and other pollution in urban areas.
She and others emphasized that pollution levels are likely to climb back, and quickly, when the shutdown ends and America’s love of the car asserts its grip again.
These air emissions specialists also raised concerns that pollution could get worse.
One reason: The Trump administration this week announced that it was relaxing a range of environmental rules, allowing factories, power plants, and other large emitters to decide on their own whether to comply with federal requirements for air and water pollution.
After being lobbied by the oil and gas industry, officials at the Environmental Protection Agency on Thursday said they would waive enforcement of a range of environmental protections. More broadly, the EPA has sought to roll back dozens of regulations that the administration has deemed harmful to businesses, even if beneficial to the environment.
In a statement, EPA Administrator Andrew Wheeler said the waiver would be temporary and date back to March 13.
“EPA is committed to protecting human health and the environment, but recognizes challenges resulting from efforts to protect workers and the public from COVID-19 may directly impact the ability of regulated facilities to meet all federal regulatory requirements,” Wheeler said.
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The move was denounced by environmental advocates, who accused the administration of taking advantage of the pandemic to ease enforcement of laws such as the landmark Clean Water Act.
“We are adding to the public health emergency here with this relaxation of what are really national public health protections,” said Jack Clarke, director of public policy at Mass Audubon.
Ben Hellerstein, director of Environment Massachusetts, said the EPA’s move put the nation’s air and water “at the mercy of polluters.”
“The whole framework of the Clean Air Act depends on monitoring and reporting, without which we have no idea what facilities are releasing into the air we breathe,” he said. “Under this reckless new policy, the EPA could also let facilities off the hook for actual instances of excessive pollution.”
There are others reasons to be concerned that pollution could spike after the shutdown ends.
Some worry that continued fears of the coronavirus, combined with a plunge in gas prices, could lead to fewer people riding the MBTA — ridership is down by nearly 90 percent from usual — and eventually mean even more traffic on the region’s already congested roads.
“For some period of time, we’ll probably see that people are slow to come back to transit,” said Chris Dempsey, director of Transportation for Massachusetts, a statewide advocacy group that promotes public transportation. “But two years from now, I think we’ll be right back where we were.”
He and others also raised concerns about potential delays in the Transportation and Climate Initiative, which aims to curb transportation emissions — the nation’s largest source of greenhouse gases — that Massachusetts and 11 other Northeastern and mid-Atlantic states had planned to complete this spring.
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The initiative would cap vehicle emissions from Maine to Virginia and require hundreds of fuel distributors in participating states to buy pollution permits for the carbon dioxide they produce.
“We were expecting action on TCI soon, but at this point, given that governors’ attention is elsewhere, I think we’re unlikely to have an announcement this spring,” said Jordan Stutt, carbon programs director for the Acadia Center, an environmental advocacy group in Boston.
Stutt remained optimistic that states will ultimately look to TCI with a “renewed sense of urgency,” as the program could serve as a source of much-needed revenue and jobs to a region with surging unemployment claims and depleted financial reserves.
“It’s a public health program and an economic stimulus program wrapped in one,” he said. “The billions of dollars generated could be invested in infrastructure programs and high quality jobs.”
State environmental officials said there has been no decision yet to delay the initiative, which was slated to take effect in 2022.
“The Transportation and Climate Initiative states ... continue to hold meetings and collaborate through e-mail, telephone, and videoconference,” said Katie Gronendyke, a spokeswoman for the state’s Executive Office of Energy and Environmental Affairs.
While some inspections and other work of the state’s environmental agencies have been halted, state officials from the Department of Environmental Protection continue to “perform inspections to respond to threats to public health and the environment, such as complaints about ongoing discharges,” she said.
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The Environmental Police also continue to process boat and recreational vehicle registrations and enforce hunting and fishing laws throughout the state. They’re also increasing their presence at state parks to remind visitors of social distancing guidelines, she said.
But it’s unclear what will become of those agencies in the coming months. The intense need for government support to bolster the economy could mean that other vital programs that curb emissions and other pollution fall by the wayside.
The state’s lauded energy efficiency program, for example, has now been suspended, and many solar and wind projects have been postponed.
It could also mean more cuts at agencies such as the Department of Environmental Protection, which saw substantial declines in its staff and ability to enforce state laws after the 2008 financial crisis.
“We recognize that the budget is going to be in tough shape, and that environmental agencies are often the first to get cut," said Elizabeth Turnbull Henry, president of the Environmental League of Massachusetts. “These are uneasy times, but we can’t afford to do that again.”
David Abel can be reached at david.abel@globe.com. Follow him @davabel.