Since the early 2000s, air quality measurements have been taken in underground railway areas (RER and metro stations in Paris, Lille, Lyon, Marseille, Rennes, Toulouse) which have shown concentrations of PM10 particles that are much higher than those in outdoor air. The main substances identified include several metals: iron, elemental carbon and organic carbon. Other chemical pollutants, found in concentrations sometimes higher than those found in outdoor air, were also identified, including aromatic hydrocarbons (toluene, phenanthrene, fluoranthene, anthracene and pyrene), and more rarely benzene, nitrogen dioxide and benzo(a)pyrene.
Our understanding of the toxicity of airborne particles in underground railway areas appears to be much less extensive than that of urban air pollution particles. Because of this, ANSES was asked by the Directorates General for Labour, Health and Risk Prevention to conduct an expert assessment of the chemical pollution of the air in underground railway transport areas and the associated health risks for workers.
The first objective is to produce a review of the state of knowledge regarding chemical air pollution in underground railway areas, the toxicity associated with these pollutants, especially airborne particles, individual worker exposure, the health effects linked to this exposure, and the effectiveness of current management policies.
Based on this, the second objective is to assess the health risks to workers exposed to this pollution.
If necessary, a third objective will be to suggest avenues for research and management ideas to reduce this pollution and the associated health risks for workers through discussions of ways to integrate such measures into the management of transportation user exposure.
Air pollution in underground railway areas and health risks for workers
In underground railway areas, air pollution is mainly due to airborne particles. The concentration of particles measured in the air is often expressed as PM10 and PM2.5. These particles penetrate the respiratory apparatus, and the finest among them (PM2.5) can settle in the pulmonary alveoli.
Mass concentrations of PM10 and PM2.5 particles measured in underground railway areas in France and abroad are much higher than those measured in outdoor air and in air inside housing. The main source of these particles with a high concentration of metallic components, mainly iron, and of carbon, is the wearing down of equipment through wheel-brake friction in passenger trains, followed by wheel-rail contact and the contact between rolling stock and the power supply system.
Air particles in these areas have physico-chemical properties that are different from outdoor air particles. On an epidemiological level, the data available regarding workers in underground railway areas are not sufficient to clearly establish the long-term risks. Although their toxicity currently remains insufficiently documented, the available toxicological data suggest that at equivalent mass concentrations they are at least as toxic in the short term as ambient outdoor air particles.
The Agency's expert assessment has concluded that an inflammation of the respiratory tract and the subsequent impact of this inflammation are probably due to chronic exposure to particles from underground railway areas. By analogy with the well-documented health risks for ambient outdoor air, their harmful effects are expected to impact both cardiovascular and respiratory health. Those concerned include workers in the following sectors: railway transportation operations, transportation and service organisation, shops, police, security, prevention and social work. The health risks are likely to be higher for workers in charge of the maintenance of infrastructures, due to emissions from Diesel engines and from maintenance work.
In this context, the Agency recommends implementing and pursuing measures for preventing and reducing exposure by all of these categories of workers, and in particular those who are in charge of infrastructure maintenance.
With regard to the lack of scientific data on the long-term toxicity of the particles present in the air of underground railway areas, and to the corpus of epidemiological studies which is still insufficient to assess the health risks of chronic exposure by this worker population, ANSES recommends that specific studies be conducted in order to acquire further knowledge.
ANSES also suggests establishing a management limit value and reinforcing the air quality surveillance scheme for the networks in their entirety in order to assess the effectiveness of measures to improve air quality and reduce worker exposure.
Reducing overall ambient air pollution remains the top priority
The Agency has stressed that the first priority with regard to public health is the reduction of overall ambient air pollution. Because of this, the use of means of transportation that are less polluting than road transportation, including railways, is to be encouraged.
The air pollution that users of road transportation are subjected to appears of higher concern for health than the pollution of underground railway areas, due to high road traffic levels of numerous pollutants with proven toxicity, including: high concentrations of fine particles, black carbon (soot), gases such as carbon monoxide, nitrogen dioxide, benzene and toluene.