Commuters wait for the train in the Gare de l'Est metro station in Paris.

Three Paris metro stops show "high particle pollution", according to a study released Monday, raising air quality concerns for underground transport months ahead of the Paris Olympics.

Activists have alleged high rates of air pollution in parts of the Paris metro system, mostly from the trains using mechanical breaks as they come in to each stop.

The Belleville, Oberkampf and Jaures stops in eastern Paris recorded high levels of PM10 airborne particles, a map produced by regional transport authority IDFM and air quality observatory Airparif showed.

PM10 particles, which measure 10 microns or less in diameter, are small enough to be inhaled and can aggravate respiratory and other health issues.

The new map was based on air quality readings made for at least a full week between 2015 and 2022 on platforms in 44 stations, IDFM said in a press release.

After an hour, the Belleville, Oberkampf and Jaures showed readings above the recommended national maximum for train hubs of 480 micrograms per cubic metre.

Paris is to host the Olympics from July 26 to August 11. Some 800,000 extra visitors are expected in the city.

The Respire ("Breathe") air quality activist group said the study should have included figures on microparticles with a diameter of 2.5 microns or less, which can be more harmful.

Sophie Mazoue, of the RATP transport authority for Paris, said there was no cause for alarm as no passenger stayed on metro station platforms for a whole hour.