Pollution stood at three times the safe level last week.
Parisian authorities ordered public transportation remain free for the fourth consecutive day on Thursday, as the city looked to combat the worst pollution in a decade.
The level of fine airborne particles stood at 80 micrograms per cubic meter of air particles, and it reached 144 micrograms per cubic meter one week ago, Agence-France Presse reported. The daily maximum should never exceed 50 micrograms per cubic meter, according to guidelines from the European Union.
The city government also ordered half of the cities cars off the road each day, alternating between odd-numbered and even-numbered license plates.
By making public transportation free of charge, the city aimed to incentivize people to take subways and buses over cars. The move is costing the city around 4 million euros, or about $4.3 million, per day.
City Hall announced in a tweet that the free public transport services would continue into Friday.
Cold temperatures coupled with low winds and a heavy reliance on diesel vehicles contributed to the stagnant pollution. Since hot air rises, the cold air is trapped underneath.
“It’s like a lid is placed on this air,” John Abraham, a professor of thermal and fluid sciences at University of St. Thomas in Minnesota, told Travel + Leisure.
“Paris is experiencing a double whammy of many high polluting vehicles combined with a week of weather that [has] not blown the pollutants away,” he said.
France has long relied on diesel fuel, particularly after the government began subsidizing it in the 20th century. The high volume of diesel cars in Paris is seen as one of the city’s major pollution obstacles.
Making public transportation more convenient and energy-efficient will be the most sustainable solution going forward, according to Abraham.