A general view shows the Eiffel Tower and the Paris skyline through a small-particle haze March 18, 2015. REUTERS/Gonzalo Fuentes

Winter air pollution in Paris is the worst it has been in a decade, according to the Airparif agency, which is charged with measuring the levels. In response, the city has made public transportation free for the second day in a row and has limited which cars are allowed to drive, the Independent reported Wednesday.

The city has effectively attempted to cut in half the number of drivers in the city to fight the smog. Only drivers with even-numbered license plates were allowed to drive Tuesday, while Wednesday was limited to odd-numbered license plates only. The regulation applied to both drivers in the city and motorists in Paris' surrounding suburbs, reported the Local. However, the rules reportedly did not apply to hybrids, electric vehicles, emergency vehicles, food delivery vehicles and cars carrying three or more people.

Police have reportedly set up some 140 checkpoints to catch drivers breaking the rules. Paris Mayor Anne Hidalgo tweeted about the issue, sharing an image of the smog-covered city, saying that it was all the proof needed to reduce vehicle usage.

Officials blamed the especially bad smog on three factors: vehicle emissions, domestic wood fires and very little wind, which has caused pollutants to stay in place and not disperse. Air pollution can be especially dangerous for people who live in major cities, such as Paris.

More than 80 percent of people who live in urban areas are exposed to air past the acceptable limits for pollution, according to a report released this year by the World Health Organization (WHO). Cities in Europe, the Americas and the Western Pacific did have lower levels of pollution than the poorest cities in the world in the Eastern Mediterranean and Southeast Asia, the group found.

Paris took center-stage in the fight against global warming and climate change when a group of countries came together in the city and agreed to a deal to reduce emissions last year. The goal was to be more transparent in how much pollution countries were accounting for and to reduce emissions in an effort to keep the global temperature from rising 3.6 degrees Fahrenheit above pre-industrial levels.

The deal could be in perilous shape, however, since President-elect Donald Trump — who has in the past called climate change a hoax invented by China — has threatened to withdraw from the agreement.