A view of foggy London, which has seen high levels of pollution this December.
When a thick cloud of air pollution settled in over London last week, experts warned those with health problems to avoid strenuous exercise. The advice to Londoners essentially boiled down to this: breathe less.
Last week, London mayor Sadiq Khan rolled out a new system of air quality alerts at bus stops, Tube stations and roadsides, warning those who experience symptoms from air pollution to reduce strenuous activity. The mayor also announced a doubling of funding for reducing pollution.
But he does not have the legal authority to institute a ban, and has demanded the government take urgent action, including a diesel scrappage scheme.
Cities around the world are confronting problems similar to London's. Some have been more aggressive than others, but overall, their experience shows that concerted steps to improve air quality do work, and they save lives.
Meanwhile, as Paris suffered a similar pollution episode - its worst in a decade - officials swung into action, waiving charges for public transport and restricting the number of cars allowed on roads, alternately barring those with odd and even license plates.
At the same time Paris mayor Anne Hidalgo joined officials from Madrid, Athens and Mexico City in announcing plans to get all diesel vehicles off the roads by 2025.
Diesel is highly polluting, emitting far greater amounts of dangerous nitrogen dioxide and tiny pollution particles than petrol, and can cause cancer to heart attacks.
Despite the health damage it wreaks, governments across Europe, including Britain's, have offered motorists tax incentives that effectively encourage the use of diesel, on the assumption – now being questioned - that it produces less planet-warming carbon dioxide than petrol.
Berlin is a notable exception to the story of the diesel disaster gripping much of western Europe. It has cleaned up its own fleet, installing pollution filters on buses and garbage trucks, and imposed tough rules on heavy goods vehicles. A strict emission zone bars older diesel vehicles, and rates of car use, which are already among the lowest in Germany, have dropped even further in recent years. Public transport is efficient and easy to use, with a two-hour pass costing just €2.70 (£2.25).
In 2008 German authorities established strict emission zones in Berlin, Hanover and Cologne.
As a result, levels of the tiniest, most dangerous particles, known as ultrafines, fell 70% in just three years, says Axel Friedrich, former head of transport and noise at the federal environmental agency, and an adviser to government and advocacy groups.
Kraków has the worst air in Poland - one of Europe's most polluted countries.
After a long legal fight, the city is now moving forward with a ban on burning coal for home heating, to take effect in September 2019.
New York has also targeted heating systems. After an analysis found that 1% of buildings burning the dirtiest kinds of fuel oil, were producing more soot than all the city's traffic, officials made plans to gradually ban their use and to help landlords convert.
The changeover is already credited with saving hundreds of lives each year. It's just one piece of New York's air quality strategy, which also aims at slashing greenhouse gas emissions 80% from 2005 levels by 2050, says Mark Chambers, director of the mayor's Office of Sustainability.
Los Angeles, the city where American car culture reached its zenith, has also pushed hard to clean up its air.
"We've made incredible progress, we can see the mountains in Los Angeles, when those of us who grew up here never could when we were young," says Joe Lyou, president of California's Coalition for Clean Air.
The dramatic improvement is the result of the most stringent air quality regulation in America. A statewide crackdown on dirty diesel lorries and a push to expand use of zero-emission vehicles are also a big part of the story.
In addition to the decades of regulation that have made American cars 99% cleaner than they were 40 years ago, cities like New York and LA have benefitted from American motorists' distaste for diesel, which accounts for only about 2% of cars in the US.
Beijing has used license plate restrictions to limit the number of cars and set out plans to keep the oldest and most polluting vehicles off roads when air is especially bad. More importantly, the government has harnessed public anger over pollution to plough billions of dollars into wind and solar power, becoming the world's biggest investor in renewable energy. Officials have even begun cancelling plans for new coal-fired power stations - a move with repercussions for the health of those living in Chinese cities, and for the planet.
In November New Delhi experienced dangerously polluted air.
Last month, Delhi's 20 million people suffered through the worst smog episode in 17 years, according to the Centre for Science and Environment. Officials temporarily shuttered a coal-fired power plant, halted all construction and demolition work and shut down many diesel power generators.
In a sign that Delhi has begun to acknowledge the problem, officials also closed 1,800 schools for three days, as particulate levels soared to 28 times the recommended maximums.