Paris authorities are making public transport free for three days to encourage drivers to leave their vehicles behind due to severe pollution caused by unusually warm weather.
The French capital has been under maximum pollution alert for days, and the air is expected to remain exceptionally unhealthy until the end of the weekend.
The pollution has also affected other nearby countries including Belgium, where authorities have reduced the maximum speed allowed on main roads in a bid to reduce the strong concentration of polluting particles in the atmosphere.
Jean-Paul Huchon, head of the STIF organisation that oversees transport in Paris and neighbouring areas, said transport would be free from yesterday to tomorrow night due to the "significant risks to the health of residents" posed by the pollution.
By way of comparison, the overall air quality index in central Paris late on Thursday was about the same as that of Beijing, one of the world's most polluted cities.
The northern city of Caen also decided to make public transport free for three days due to the pollution. Reims, a city in France's northeast, and Rouen in the northwest, were doing the same yesterday.
A lack of wind, coupled with cold nights followed by balmy days, has contributed to the severe air pollution.
At night, the ground cools while the atmosphere heats up during the day, which means polluting particles emitted by cars or heating become stuck under a warm layer of air and cannot rise up or be blown away.
As a result, monitoring centres have reported a particularly strong concentration of particles with a diameter of less than 10 microns - so-called PM10 particles - in the air.