Do users of metropolitan metros breathe harmful air? The question agitates scientists, associations, and obviously travelers using daily underground transport in Paris, Marseille, Lyon, Lille, Toulouse, Rennes and Rouen. This Wednesday, a new and thick publication of the National Agency for Food, Environmental and Occupational Health and Safety (ANSES), takes stock of the various scientific works published on the issue. With a first warning message: in underground railway enclosures, fine particle pollution is three times greater than outside. This expert report focused on fine particles (PM10: less than 10 microns) and very fine particles (PM2.5: less than 2.5 microns). In the underground passages of the metro, these particles are different from those of the outside air. “These are metallic particles composed mainly of iron, or manganese. There is also organic carbon there, particularly in connection with the braking of trains and the friction of organic carbon soles”, explains Mattéo Redaelli, expert in the evaluation of air-related risks at ANSES.
Is there a level from which these micro-elements become dangerous? In 2019, the agency was seized by the Directorate General of Health to establish “guide values for indoor air associated with the exposure of users in underground railway enclosures”. An approach that has come up against a lack of data. “We could not have enough studies to have firm conclusions in terms of health effect and therefore on the construction of a guide value as consolidated as this approach requires”, warns Eric Vial, director of evaluation. risks at ANSES. Although ANSES refuses to establish a guideline value for indoor air pollution in the metro, it nevertheless emits concentrations not to be exceeded. Based on the values of the World Health Organization, it proposes a maximum concentration of 50 μg/m3, for a maximum duration of 2 hours per day, or 140 μg/m3, for a maximum daily duration of attendance 30 minutes.
Biological markers identified
Since the 2000s, measurements of air quality in underground transport have been carried out, but studies on the subject are still limited. “Air pollution is known for outdoor ambient air but that of users in indoor air is much less so”, explains Eric Vial. “In total, all of the studies on the toxicity of particles from underground railway enclosures represent eighteen in vitro studies and three in vivo studies”, details the report, which notes that “the toxicity of particles from EFS remains poorly documented and existing studies focus only on short-term effects.
The approach of ANSES scientists is therefore cautious. Metro air quality is a sensitive subject. In March 2021, the Respire association, which specializes in air monitoring, filed a complaint against the RATP for “aggravated deception” and “involuntary injuries”, questioning the levels of air pollution in the Paris metro. . On this issue, the Agency warns that the limited body of study does not allow conclusions to be drawn. “The epidemiological results available do not show an increased risk of bronchopulmonary cancer”, underlines for example ANSES, but they are based “on the basis of a single occupational exposure study”, nuance-t- she.
“However, studies show that certain biological markers related to oxidative stress, the body’s pro-inflammatory response and the body’s ability to control heart rate variability may have been modified in relation to exposure to particles”, emphasizes Matteo Redaelli.
Further studies required
Professionals, scientists and associations are asking this time for further studies to be carried out to better understand the risks of exposure to these particles. “Outdoor air pollution is very well documented, and we know that these particles have health consequences and we can imagine that it is also in indoor air, but it is not supported by disease figures. We therefore need a real strategy for measuring pollution and its consequences”, agrees Jean-Baptiste Renard, research director at the CNRS in Orléans, specializing in air pollution and who has participated in studies on the Paris metro.
On the side of the Respire association, the general manager Tony Renucci welcomes this new report, which “puts the subject of indoor air quality back on the table and makes for the first time a proposal for a particle threshold to be avoided. not exceed into indoor air”. “To make progress on this subject, we would need to have standards and legal thresholds for exposure limits in underground railway enclosures, of course, but also in other establishments open to the public”, he adds.
The RATP, which concentrates the largest share of users of this underground transport in France, says it has initiated “an action plan with the financial support of Île-de-France Mobilités, to improve knowledge of particles and informing the public”. New sensors should thus be installed in the Parisian metro and RER stations, allowing in particular to detect extrafine particles (less than 0.5 microns). “The second line of work is to work on rolling stock,” explains Sophie Mazoué, sustainable development unit manager at RATP. “To do this, we are testing new brake linings which have had promising results on a test bench and which would make it possible to reduce the emissions of particles by 90%”. On the network, new ventilation systems should also see the light of day to avoid air confinement and the accumulation of particles as much as possible.