Response: to heavy pollution that can impact the health of travelers. Indeed, due to the excess pollution generated by Paris metro and RER traffic, the level of pollution within the rail transport network is high, on average 24 μg/m3 , almost 5 times higher than the recommendations of the World Health Organization (WHO).
These figures come from an investigation carried out by the team of journalists from Vert de Rage (France 5) which measured the concentration of PM2.5 particles (certain dangerous fine particles) on the 435 platforms of the 332 stations of the the entire metro and RER network (excluding zones 3, 4 and 5). As there is currently no standard for indoor air quality, the standard used here is based on the WHO recommendations for outdoor air, i.e. 5 μg/m3 of PM2.5 on average per year.
Full article (Fr) : Santé Respiratoire