Motorists drive under a traffic board reading « Air pollution, slow down » near Rennes, western France, on March 21 , 2015. Smog blowing in Europe, mixing with home-grown pollution, has been sending air pollution levels soaring, as French authorities will on March 23 put in place emergency traffic-limiting measures in Paris, as the City of Light and much of northern France suffers from a choking smog. AFP PHOTO / DAMIEN MEYER