Improving air quality in underground railway areas

Millions of people use the metro and other underground transport systems to get around seven large urban areas of France. Improving air quality inside underground railway areas is one of the objectives of the National Environmental Health Action Plan. Having received a request from the public authorities, ANSES confirms the need to continue taking action to limit the exposure of users by reducing concentrations of airborne particulate matter. To that end, the Agency is proposing monitoring indicators to strengthen air quality monitoring in these environments.

Higher particulate matter concentrations in railway areas than outdoors

France's underground railway networks are spread out over seven large urban areas (Paris, Marseille, Lyon, Lille, Toulouse, Rennes and Rouen). The Ile-de-France network is by far the largest in France and one of the busiest in the world. Since the early 2000s, air quality measurements have shown that on average, concentrations of airborne particulate matter (PM10, PM2.5 in µg/m3) in railway areas in France are three times higher than in urban outdoor air.

However, its composition is different, with high levels of metallic elements, including iron, and elemental and organic carbon. Specific to underground railway activity, this pollution is caused by the wear and tear of materials due to the braking of trains, by contact between the rolling stock and the track, and by the resuspension of dust due to the moving of trains.

A need to continue undertaking preventive actions

ANSES has updated its review of knowledge from its 2015 opinion on the health effects of exposure to particulate matter in the air of underground railway areas. Its analysis shows that the body of evidence remains too limited to enable firm conclusions to be drawn concerning such health effects.

However, the epidemiological and toxicological data suggest that there may be cardiorespiratory effects, given the biological changes observed in terms of inflammation, oxidative stress and cardiac autonomic function. In view of these observations, the Agency confirms the need to reduce particulate pollution in underground railway areas and therefore pursue actions to that end such as renewing rolling stock, using braking systems that emit less particulate matter, and improving ventilation in these areas.

New indicators for monitoring air quality inside railway areas

The Agency is proposing indicators to characterise air quality in these environments.

The proposed indicators correspond to concentrations of suspended particulate matter (PM10 and PM2.5) that should be targeted in underground railway areas. They have been determined so that they can be applied to each network according to the characteristic duration of its users' journeys. These indicators have been calculated in such a way that considers a day of exposure in various environments (at home, at work and in transport) and the regulatory limit values or guidance values in terms of daily concentrations not to be exceeded, which are defined for particulate matter in ambient air”, explains Matteo Redaelli, coordinator of the expert appraisal at ANSES.

These indicators are meant to be used as benchmark points of comparison for the levels of particulate pollution measured or modelled in the air of each network in France. They will be used to assess the effectiveness of source reduction actions.

Given the expected increase in train traffic in a context where alternatives to cars are being favoured, the Agency recommends in particular strengthening air pollution monitoring systems in the various environments of railway areas: platforms, stations, trains, etc. The Agency also recommends improving knowledge of the specific health effects of suspended particulate matter in these areas, which could enable reference values to be defined.

More generally, the Agency stresses that reducing air pollution, especially in urban areas, is a public health priority. Shifting from motorised road transport to other less polluting modes of transport, including rail transport, should therefore be encouraged.