An aerial picture taken aboard an helicopter on July 20, 2010 shows a smoggy view of the Eiffel tower (L) and the Tour Maine-Montparnasse in Paris. Boris Horvat/AFP/Getty Images

Air pollution that has turned the skies over Paris a murky yellow and shrouded much of Belgium for days forced drivers to slow down Friday and gave millions a free ride on public transportation.



The belt of smog stretched for hundreds of miles, from France's Atlantic coast to Belgium and well into Germany. It was the worst air pollution France has seen since 2007, the European Environment Agency said.



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



To combat the smog, public transit around Paris and in two other cities was free Friday through Sunday. Elsewhere in France and in Belgium's southern Wallonia area, the free ride was only for Friday.

Starting Monday morning, the city will enact a system of “alternating traffic,” permitting only vehicles with either odd or even license plate numbers to drive on alternate days.

A statement from the office of French Prime Minister Jean-Marc Ayrault said that a decision on whether to extend the measure will be announced on Tuesday, when forecasters expect pollution levels to rise again.

The smog is particularly severe here 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.

One environmental group complained earlier this week, denouncing the "inertia of the government," saying it was putting lives in danger.

There's no question that pollution can be an immediate health hazard, especially for the very young and old and for anyone with respiratory or cardiac disorders, said European Environment Agency air quality manager Valentin Foltescu.

"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.

But the website that keeps up-to-the-minute figures on the Paris region's air quality slowed to a crawl and asked 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 region's unseasonably warm and sunny weather.

Al Jazeera and The Associated Press