Air pollution that has turned the skies above Paris a murky shade of yellow and also shrouded much of Belgium in smog has forced drivers to slow down and given a free ride to millions in both countries who use public transport.
The belt of pollution stretched from France's Atlantic coast hundreds of miles into Belgium and well into Germany.
There will be no charge for public transport in the Paris region and two other cities from today until Sunday, while elsewhere in France and in Belgium's southern Wallonia, there is free travel today.
It is the worst air pollution France has seen since 2007, the Brussels-based European Environment Agency said.
The problem is particularly severe because France has an unusually high number of diesel vehicles, whose nitrogen oxide fumes mix with ammonia from springtime fertilisers and form particulate ammonium nitrate. Pollutants from the burning of dead leaves and wood contribute as well.
Nearly all of France is under some sort of pollution alert, with levels in the Parisian region surpassing those in some of the world's most notoriously polluted cities including Beijing and Delhi.
An environmental group brought a complaint earlier this week denouncing the "inertia of the government" that it says has put lives in danger.
There is no question that pollution can be an immediate health hazard, especially for the very young and old, and anyone with respiratory or cardiac disorders, European Environment Agency air quality project manager Valentin Foltescu said.
"Some people will, unfortunately, die," Mr Foltescu said. "There is a high correlation of pollution of this kind and mortality."
Speed limits dropped in France and Belgium, and electronic billboards in Paris dispensed advice and emergency information.