The smog in Paris, France, is so thick today that city officials have opened up the city's public transportation system to everyone, free of charge, for the next three days. The thick air, prompted by unseasonably warm weather, is some of the worst air pollution in the city in recent memory

As of late Friday afternoon, the air quality index in the city center was an unhealthy 158. By comparison, the AQI in Beijing (which has a notorious, chronic, air pollution problem) was 159. The city does see increases in smog as the weather warms seasonally, but as the New York Times notes, this week's smog is way too early, and way too severe, to be dismissed as business as usual.

AP. The center of Paris is seen through the smog, Thursday March 13, 2014, as pollution over the French capital is at high levels.

The Atlantic Cities has some good background on why experts believe this is happening now: "France’s air has become a sort of toxic stew," they explain, after a week of what would otherwise be wonderful weather. A week of sunny, warm days is great, except for when those days are followed by cold nights and low winds: the cold air traps exhaust fumes in a "bubble," and the pollution hangs around in the air Parisians are supposed to breathe. The smog has clogged the city's air for about a week so far, and is expected to hang around for the weekend as well. 

Paris officials are opening up public transport because they want commuters to leave their cars at home in order to starve the smog of a major source of particles: the exhaust fumes from diesel-powered vehicles, which are much more common in France than they are here, mainly because of their fuel efficiency. French officials have tried to reduce the number of those vehicles on the road permanently, with mixed results. 

Reuters. An electronic road sign reads "Pollution, speed limit 60kms" on the Paris ring road, March 14, 2014, as warm and sunny weather continues in France.
Reuters. Panorama of Paris. The Eiffel Tower is barely visible.