PARIS — Air pollution that has turned Paris skies a murky yellow and shrouded much of Belgium in smog for days forced drivers to slow down and gave a free ride Friday to millions in both countries who use public transportation.

The belt of pollution stretched from France's Atlantic coast hundreds of miles into Belgium and well into Germany.

There was no charge for public transit in the Paris region and two other cities from Friday through Sunday, while elsewhere in France and in Belgium's southern Wallonia, the free ride was only for Friday.

It was the worst air pollution France has seen since 2007, the Brussels-based European Environment Agency said.

The problem here is particularly severe because France has an unusually high number of diesel vehicles, whose nitrogen oxide fumes mix with ammonia from springtime fertilizers and form particulate ammonium nitrate. Pollutants from the burning of dead leaves and wood contribute as well.

Nearly all of France was under some sort of pollution alert, with levels in the Parisian region surpassing those in some of the world's most notoriously polluted cities Friday, 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," 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.

The website that keeps up-to-the-minute figures on the Paris region's air quality slowed to a crawl and was asking visitors to follow it on Twitter or Facebook rather than crash the site.

Foltescu said if everyone follows the government's advice "you will see an instant difference."

If not, he added, the pollution would last about as long as the unseasonably warm and sunny weather.