Paris has banned cars with license plates ending in even numbers from its roads today to reduce smog that last week briefly made the City of Lights among the world's most polluted places.

NPR's Eleanor Beardsley, who is reporting on the story for our Newscast unit, says the Paris Metro and other public transportation are free for the next few days to encourage people to use them. The ban on cars doesn't extend to electric, hybrid or emergency vehicles.

"Pollution spiked in the Paris region and northern France," Eleanor says. "According to one company that measures air quality, the French capital briefly dethroned [New] Delhi and Beijing as the world's most polluted city."

The steps are not unprecedented. The city took similar measures on March 17, 2014, when pollution from smog reached dangerous levels.

The company mentioned in Eleanor's story is Plume Labs. Last Wednesday, it said that Paris' air quality index was 125 — higher than figures for the Indian and Chinese capitals. Plume Labs, which monitors 60 cities worldwide, considers an air quality level of above 100 "harmful" and above 150 is considered "critical."

But the methodology has its critics.

Karine Leger, assistant director of Airparif, which monitors air quality for the French Environment Ministry, told France 24 that while Paris does have pollution issues, "It's the wrong idea to compare a city at a certain moment when you have meteorological conditions that could make the pollution worse at that point."

France 24 adds that the weather is partly to blame: "March has been a dry month and as the spring weather warms up, more dangerous fine particles have filled the air."

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