The center noted that there are other factors that can affect air pollution—including weather. But it said the decline was expected to continue, given that traffic across the capital has increasingly disappeared and business has slowed.

“Air quality has started to improve in many U.K. cities, mirroring what has been seen in other countries that have restricted travel and levels of outdoor activity,” said Alastair Lewis, professor of atmospheric chemistry at the center, in a written comment. “This is primarily a consequence of lower traffic volumes, and some of the most clear reductions have been in nitrogen dioxide (NO2), which comes primarily from vehicle exhaust.”

London’s decline is expected to replicate the noticeable drop-off in air pollution occurring in other areas around the world that have been put under lockdown. Wuhan, China, and northern Italy—home to the industrial Po Valley, one of the most polluted areas in western Europe—have both seen dramatic drops in pollution, as measured by satellite monitoring systems.

The U.K.’s decline in pollution was expected to gain momentum as the country entered lockdown beginning Monday evening, following other European countries from Italy to Germany that have announced increasingly restrictive measures to keep people in their homes and stem the spread of the coronavirus.

In a televised address Monday evening, Prime Minister Boris Johnson announced that people were barred from leaving the house other than to get essential food and medicine and for daily exercise, essential work, and emergencies.

The government also moved to ban groups of more than two people who don’t live together from assembling, and said police would break up gatherings and fine those who do not comply.

“No Prime Minister wants to enact measures like this,” said Johnson. “I know the damage that this disruption is doing and will do to people’s lives, to their businesses, and to their jobs.”

Last week, the government pledged hundreds of billions of pounds to support the economy as businesses have been forced to close and companies have announced widespread layoffs. Those include a pledge to pay 80% of salaries for workers who would otherwise be laid off owing to the crisis, among other measures.

That dramatic economic hit is transforming Europe’s largest capitals, where pollution is often a consequence of the vast networks of road transport and their resulting traffic, particularly in regard to the most dangerous sources of air pollution: nitrogen dioxide (NO2) and particulate matter.

For London, road traffic accounts for roughly half of the city’s NO2, and about 26% of particulate matter, according to Breathe London, an air-quality monitoring project. Concentrations of those pollutants frequently exceed legal levels in high-traffic areas of the British capital.

Bringing down emissions has been a long-term focus of London Mayor Sadiq Khan. Under Khan, the city has instituted an Ultra Low Emission Zone in its center; dramatically expanded its fleet of low-emissions buses, including the red double-deckers the city is known for; and stopped licensing new diesel taxis.

One of the ironies of the dramatic recent reductions in emissions is that—under normal circumstances—any increase in air quality would have a measurable impact in terms of saving lives.

Outdoor air pollution is estimated to cost 400,000 lives per year in the U.K., according to figures from the Royal College of Physicians, noted Anna Hansell, professor in environmental epidemiology at the University of Leicester, in a written comment.

“Sadly we may not see reductions in air pollution translated into direct drops in mortality,” said Hansell. She noted that at least into 2021, the causes of many deaths—which can be exacerbated by stress, isolation, and an overburdened National Health Service system—will likely be difficult to parse.

In the meantime, Lewis said, the newfound quiet in London, and in other major cities, may offer something going forward: “The temporary reductions in emissions being seen at the moment provide us with some unique insight into what might be possible for future air quality in cities.”

It’s a view of how a city with dramatically lower emissions could look—under better circumstances—and feel.

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