A bus carries commuters as it travels over Waterloo Bridge in London. (Toby Melville/Reuters)

It’s Christmastime on Oxford Street. Brilliant displays of white lights rain from above. Decked-out shoppers dash from one gaudy sale to the next. And Johnny ­Conquest breathes in poison. 

“The air is horrible. The taxis stop right here, and when they take off, boom, you can taste it,” says the 67-year-old as the heavenly smell of the caramel peanuts he hawks from a humble street stall mingles with the sickly stench of diesel. “I’m on the worst corner in London.”

In at least one important respect, it may be the worst in the world. 

London has come a long way since the days when its infamous coal-fired pollution shrouded Sherlock in a permanent haze or struck at least 4,000 residents dead in less than a week.  

But the city’s overreliance on diesel-powered vehicles has given it a dubious distinction: a global leader in nitrogen dioxide, a particularly noxious pollutant that shortens the lives of thousands of Londoners a year. 

(Griff Witte,Karla Adam/The Washington Post)

Here and in cities across environmentally minded Europe, NO2 levels are substantially higher than in North America, or even in Asian and African megacities whose names have become bywords for dirty air. And that is all because of decades of government incentives designed to spur the purchase of supposedly cleaner diesel cars and trucks.

“It’s a complete policy failure,” said Gary Fuller, who directs an air-quality-study center at King’s College London. “No one could defend this.” 

Rather than try, European mayors are declaring war on diesel, hoping to give their cities a clean start. 

This month, mayors of three major European capitals, plus Mexico City, announced ambitious plans to ban all diesel vehicles within the next decade. 

“We can no longer tolerate air pollution and the health problems and deaths it causes, particularly for our most vulnerable citizens,” said Paris Mayor Anne Hidalgo, who was joined in the pledge by the mayors of Athens and Madrid.

London’s new mayor, Sadiq Khan, has not gone as far — yet. But he has made reducing air pollution a central pillar of his young administration, more than doubling funding for clean-air campaigns with a billion-dollar commitment and announcing plans that will radically transform the city’s fleet of iconic — but diesel­-dependent — taxis and buses.

London is shrouded in fog on Dec. 1. (Dan Kitwood/Getty Images)

“With nearly 10,000 Londoners dying early every year due to air pollution, tackling poor air quality is a public health emergency,” Khan said in announcing the moves.

The scale of the challenge has been on display recently in cities across the continent. Paris this month experienced its worst air-pollution episode in a decade, with a thick blanket of ugly air smothering the City of Light for days. Municipal leaders temporarily made public transit free to cut cars from the roads.

In Milan, safe limits on dangerous fine particles were breached each day for a week, prompting the city council to stiffen a ban on the worst-polluting diesel vehicles. Wood-burning fires also were forbidden, a decision that echoed that of a town near Naples, which last year outlawed pizza-making in a bid to cleanse its choking air.

London, too, has been feeling the effects, with air-quality-monitoring stations this month showing some of the worst pollutant levels in recent years. The forecast for the coming days prompted one green activist to quip that Santa should take care not to exert himself during his rounds in Britain.

But as a habitual London visitor, Santa will have seen worse over the city’s long history.

Air quality has been a problem in London since at least the Middle Ages. Rapid industrialization and urban growth lent a chronically smoky backdrop to literature throughout the Victorian period. In December 1952, coal-belching homes and factories enveloped London in smog so thick that air, rail and road traffic was halted for five days as cows dropped dead in their fields and people suffocated on the streets. 

That event — known as the Great Smog — inspired the world’s first clean-air legislation four years later. The regulation of furnaces and fireplaces, plus the banning of coal in key areas, ushered in dramatic improvements. But it also gave lawmakers the illusion that the problem of urban air pollution had been solved.

A fateful bet on diesel has brought it back with a vengeance. Governments across Europe have aggressively promoted diesel vehicles, reasoning that diesel’s lower carbon-dioxide output makes it gentler on the planet than gasoline. In London, the streets are filled with diesel-powered buses and taxis. Continent-wide, diesel accounts for about half the car market.

But diesel has one glaring disadvantage: It is a major source of NO2, a pollutant that stunts lung growth and has been linked to a range of respiratory and cardiovascular diseases. The diesel push has meant that although air in Europe is far cleaner overall than in many parts of the globe, it still can be — and often is — deadly.

“It’s been a public health catastrophe on an unimaginable scale,” said Simon Birkett, founder and director of the advocacy group Clean Air in London. “We’ll probably never know the full extent of the impact.”

In particularly traffic-swollen areas of central London, it took just the first eight days of 2016 to breach the European Union’s NO2 limits for the entire year. 

Samantha Walker, policy director for Asthma UK, said that such high concentrations of pollutants can bring on an attack in minutes and that prolonged exposure among children can cause health impacts that last a lifetime.

Citing those health costs, Khan, London’s mayor since May, has launched plans to expand zones in the capital where only low-emission vehicles can tread, and to replace thousands of diesel-powered buses and taxis with hybrid and fully electric vehicles.

Birkett, the activist, said Khan deserves plaudits for such moves. But he also urged the mayor to go further by joining Paris, Madrid, Athens and Mexico City in pledging to eliminate diesel vehicles altogether by 2025. 

“There’s an opportunity here to re-engineer cities,” Birkett said. “We need to ban diesel just like we banned coal 60 years ago.” 

Fuller, the King’s College scientist, said an outright diesel ban would be “a huge challenge” for any city given the dependence on diesel for public transit and delivery trucks. 

Air-quality solutions, Fuller said, need to be “holistic,” with a focus not just on what comes out of a vehicle’s tailpipe but also on persuading people to abandon their cars altogether in favor of public transit, walking and biking.

In a sprawling and ancient city such as London, that is not always easy. For years, officials have batted around the idea of pedestrianizing Oxford Street, London’s blinged-out central shopping district. There is just one problem.

“There’s nowhere else for the traffic to go,” said Conquest, the sidewalk peanut vendor.

Instead of a walker’s paradise, Oxford Street remains a vehicle-clogged dystopia, with some of the world’s worst NO2 levels. Buses and taxis chug along emitting diesel fumes day and night, while tall buildings trap the noxious gases for pedestrians to breathe. 

Conquest, a slight and spry man who has been selling his wares on Oxford Street for 50 years, said he has been lucky. He stays in shape and has been spared the health effects that have hobbled so many others. But he says he does not doubt that decades spent breathing toxic air have taken their toll.

“I run marathons,” he said. “I would have won a few of them if I hadn’t been standing on this ­corner.”