Paris and London are both choking under heavy levels of smog this week. The French tabloids are blaming "German air," while the British tabloids are blaming "French air" - which demonstrates, above all, how air pollution has no borders.
Paris has banned all cars with odd-numbered license plates from driving in the city today (06.12.2016), in an effort to halve the automobile traffic that is the main cause of pollution. Mayor Anne Hidalgo has made all public transport free for the day.
In London, Mayor Sadiq Khan has issued an air pollution alert and urged drivers to voluntarily leave their cars at home. The British newspaper "The Evening Standard" has reassured citizens that relief from the "filthy French air" will come soon, "as Atlantic air arrives."
The high concentration of dirty air in London is actually the result of home-grown pollution combined with pollution blowing in from across the channel. A lack of wind at the moment means the still air is trapping the pollution in place.
This week's incident comes exactly 64 years after the "great smog of London." That four-day-long incident is estimated to have killed 4,000 people and made 100,000 ill.
Extra grey winter
It's no accident that today's incident comes at the same time of year. "Warm air is lighter than cold air - it always tries to go up, while cold air stays down," explains Alberto González Ortiz, an air quality expert with the European Environment Agency.
"In winter we have thermal inversion, with the cold air closer to the ground. So the pollution that's emitted into the atmosphere stays there, rather than going up."
Smog is created through the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere, which generates airborne particles and ground-level ozone.
Winter smog in particular is caused by a toxic mixture of cold air, still wind, and increased traffic and heating. Most of the pollution is caused by cars - particularly diesel cars, which emit up to 95 percent of nitrogen dioxide from exhaust. That is why Mayor Hidalgo has ordered half the cars off the roads of Paris today.
"These are short-term solutions when you have these episodes of very high concentrations," González Ortiz told DW. "But we should aim for long-term solutions - not removing cars only on the days when you have very high concentrations, but not having cars in general."
Banning diesel cars
Hidalgo has just such a long-term plan in mind. Last week, she announced a plan to ban diesel vehicles from her city by 2025 - along with the mayors of Athens, Madrid and Mexico City.
This will be a particularly difficult task in Europe, where diesel has been favored by public policy because it produces less carbon emissions than petrol, and is therefore better in the fight against climate change.
But diesel produces far more air pollution - up to 15 times more than petrol (or unleaded gasoline).
"Europe set much looser tailpipe emission standards for diesel vehicles than for petrol 20 years ago - while the US was technology-neutral, and regulated both the same," explained Simon Birkett, founder of the campaign group Clean Air in London.
Diesel cars currently make up almost 50 percent of the automobile market in Europe, compared with 3 percent in the United States. As a result, Europe's three largest cities have higher nitrogen oxide levels than their American equivalents.
Even weaker diesel standards are not being complied with, Birkett told DW - as has been demonstrated by the "Dieselgate" scandal, in which VW was shown to be cheating on emissions tests for nitrogen dioxide.
"There is only one thing we can do to solve nitrogen dioxide pollution: ban diesel as we banned burning coal [for home heating and transport] so successfully 60 years ago," he said.
Winds of change blowing to London?
Notably absent from the press conference announcing the planned bans at the C40 Mayors Summit in Mexico City last week was London Mayor Khan. He has not yet signed London up to the pact to ban diesel cars - but Birkett believes he will soon.
"The mayor hasn't got around to it yet, but he hasn't been very long in the job," he said. "I reckon it won't be far behind, as he won't like that he wasn't in that picture with the four other mayors in Mexico City."
Birkett notes that Europe's air pollution policy has generally been a success story - but emphasizes that it must keep adapting as situations change.
This attributes this in part to an enormously changed understanding of the health effects of air pollution. "In the last 20 years we've discovered that everyone is affected by air pollution, all the time. That long-term exposure to invisible air pollution is what we're fighting now."
Winds are expected to blow into both cities from the west on Tuesday night, relieving them from this short-term air crisis.
But campaigners hope that the experience of the past two days will spur more ambitious long-term action, and result in greater public support for Hidalgo's diesel ban in Paris.