Lille: High air pollution in 2018

In Lille, the air is bad. Nothing new – therefor, the city introduced an environmental zone in 2017. But Lille exceeded limit value for air pollution by particulate matter more than 60 times in 2018. According to WHO, these limits should not be exceeded more than 3 times a year. Despite that heavy air pollution, the temporary environmental zone “ZPA Lille” has never been activated in 2018. Are there other reasons for the air pollution?