For the third day in a row, air pollution blanketed Paris, which authorities called the worst bout for at least 10 years. The city imposed driving restrictions and made public transit free.

Unusually calm air failed to disperse vehicle emissions and particulates from wood fires, creating conditions that have veiled the Eiffel Tower in a gray haze.

Paris has instituted a system based on alternating odd or even license plate numbers to ban certain vehicles from city streets, effectively cutting traffic in half each day. This is just the fourth time in 20 years that Paris has taken this step, and the first time it has been in place for consecutive days.

"Cars are poisoning the air," Paris city hall transport official Herve Levife told Reuters. "We need to take preventive measures."

"We want these bans to automatically take effect when the pollution exceeds a certain level, not have to negotiate them with the government each time," Levife added.

More than 1,700 drivers were issued tickets for violating the ban on Tuesday, which carries a fine of 35 Euros, or about $37.42. Hybrid and battery electric vehicles, as well as those carrying three or more passengers, are exempt.

All public transit was made free, putting a strain on commuter systems as crowds piled onto trains and buses. The city's bike-share system was also free to use.

Along with Paris, the French cities of Lyons and Villeurbanne were expected to impose similar measures.

Air quality index readings reached or exceeded 150 on Thursday, considered a "critical" level. Air Pollution in Paris: Real-time Air Quality Index Visual Map

Readings of particulate matter exceeded 80 micrograms per cubic meter. The European Union has set a maximum daily average of 50. Particulate matter, due to its small size, can be inhaled deeply into lungs. High exposure can cause asthma, lung cancer, cardiovascular disease, respiratory diseases, birth defects and premature death.

Beginning July 1, Paris banned all cars 20 years or older. Longer-term, Paris and three other cities—Athens, Madrid and Mexico City—will ban diesel engines by 2025 as announced earlier this week. Diesels area major emitter of particulate matter pollution.

In March 2015, the air quality index in Paris briefly made it the worst polluted city in the world.