Paris today enters its fourth consecutive day of urgent measures taken in an attempt to ease the heaviest pollution the city has experienced in over a decade.

Only cars with odd licence plate numbers are permitted to drive around the Paris region today, following three days of alternating permission between odd and even plates. Public transport has been made free of charge across the city since Tuesday, along with public vehicle services such as Velib bikes and Autolib electric cars, encouraging residents to ditch their cars and emit less.

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

This is only the fourth time this license plate restriction has been enforced in 20 years: previously in 1997, 2014 and 2015. However, it’s the first time it has ever been extended for more than one consecutive day. There are some doubts over the effectiveness of the driving ban in place, since many drivers have chosen to flout the ban on their cars, with over 1,700 defiant drivers fined on Tuesday alone.

The prolonged spike in pollution is a consequence of vehicle emissions and increased heating, especially home wood-burning, combined with a continuing lack of windy conditions, said AirParif, Paris’ air quality monitoring organization. They triggered a pollution alert once ambient levels of PM10 exceeded their threshold of 80 micrograms per cubic meter for several days, beginning from over a week ago. However, even this “danger” threshold far exceeds the recommended limits of PM10 set by the World Health Organisation, at a much lower limit of 20µg/m3 annual mean, and 50µg/m3 for 24-hour exposure.

Similar measures to tackle air pollution focusing on vehicle use are also being introduced in French cities Lyon and Villeurbanne for the first time from today. The Paris ban follows previous moves by mayor Anne Hidalgo to lower air pollution in the capital, such as implementing a driving ban on inefficient old vehicles across the city. Last Friday, Hidalgo, also the new Chair of the C40 Cities Climate Leadership Group was one of 4 major city-mayors to further pledge to remove all diesel vehicles from their capitals by 2025, along with Mexico City, Madrid and Athens.