The metro, a means of transport that is both more ecological and more polluted? In the cities that have them, these underground trains are still used for both practical and ecological reasons.

Indeed, since they have their own tracks, metros are not subject to traffic jams or the vagaries of traffic, and they generally allow you to get from one end of a city to the other more quickly.

Some users also choose this means of transport for its ecological side. By bringing together many people on the same train, metros allow each passenger to avoid using their own vehicle. This drastically reduces greenhouse gas emissions, since the number of vehicles in circulation is therefore lower.

However, a study carried out by the National Health Security Agency (Anses) has just revealed that inside metro trains, the air is on average three times more laden with fine particles than outside. The agency therefore recommends stepping up actions to improve air quality. For this, it recommends in particular a renewal of equipment, and better ventilation, reports Capital.

According to ANSES, this high level of fine particles is due to friction with the rails, mainly when braking, which releases large quantities of dust.

In France, 7 cities have metros and are therefore concerned by this study. Find out which ones in our slideshow.