At the initiative of the transport organizing authority Ile-de-France Mobilités (IDFM), with the support of Airparif, a precise mapping of pollution in the Paris metro and RER was carried out. The concentration of fine particles, produced when trains brake, can cause respiratory difficulties or illnesses, particularly in vulnerable people. The three stations concerned are Belleville, Jaurès and Oberkampf, all located in eastern Paris. In the meantime, IDFM intends to ask RATP and SNCF operators to deploy an action plan to improve air quality in most polluted stations.