Innovating for better air quality in underground railway networks

With 63,000 employees operating 4.8 billion trips per year worldwide, of which 1.5 billion outside the Ile-de-France region, the RATP Group is a major player in mobility. The long-established leader in passenger transport in the Ile-de-France region works every day to improve its impact on air quality, in an effort to meet the demands of a sustainable city and to reduce its environmental footprint. Sophie Mazoué, the Sustainable Development Manager of the RATP Group, describes the initiatives the company has taken to monitor and improve air quality


Air pollution is a worldwide issue resulting from many different human activities, including industry and transport. What are the RATP’s current initiatives in this field?

In our activity as a bus and rail network operator, we must pay close attention to air quality outdoors and indoors. The RATP launched the energy transition of its bus network with the “Bus 2025” project, which impacts our fleet of 4,700 buses. By 2025, 100% of the fleet will be clean, ultimately with two-thirds of electric buses and one-third running on bioNGV . This project will enable us to keep our promises on energy and the climate (a 50% reduction of our greenhouse gas emissions and a 20% reduction of our energy consumption between 2015 and 2025) and to also cut polluting emissions.  

We face different air quality issues in our underground rail networks. Even if gaseous pollution is low, the braking systems of the rolling stock emit particles. This phenomenon exists on every rail network in the world.  Consequently, we have taken measures to identify and characterise these particles, whether they are large, fine or ultrafine. The action plan to improve air quality includes the development of electric braking systems, innovative air filtering solutions and the improvement of ventilation.

The occupational health department also conducts epidemiological surveys. Today, their results are very reassuring, because they show no increase in the prevalence of cardiovascular or respiratory symptoms amongst the staff members who work underground. 

What means does the RATP deploy to monitor the impact of urban transport on air quality in the Paris region?

We conduct numerous measurement campaigns. First, we have a network that continuously measures the different parameters of air quality: temperature, relative humidity, carbon dioxide, particles and different gases, such as nitrogen oxides. These instruments are installed in three stations that are representative of the metro and regional express networks: Franklin Roosevelt on line 1, Châtelet on line 4 and Auber on the regional express line A. We also make spot measurements using portable equipment to have a complete vision of the air quality in the entire underground network. In addition to the parameters measured by our 24x7 monitoring network, we also measure other parameters, such as ultrafine particles or polycyclic or monocyclic aromatic hydrocarbons. The data collected from these measurements is available in real time on our website.

Furthermore, we are currently doing research with the Paris environmental health department (SPSE) and the central laboratory at police headquarters (LCPP). The goal is to compare exposure during the home-work commute, on the RATP network, in a private car, on paths for pedestrians or in bicycle lanes. 

Which air quality regulations do you have to obey?

There are national and European criteria for outdoor air quality and European standards for road-going rolling stock. There are no specific regulatory values in France regarding the air in underground rail networks. Only the French labour code provides a regulatory basis to assess the exposure of workers.  What are the RATP’s solutions to improve air quality in its underground rail network?

The air is renewed thanks to the ventilation system: more than 400 ventilators are installed in the tunnels in the Ile-de-France regional network. They work permanently to renew the air and to improve thermal comfort. If necessary, they can also be used for smoke extraction.

We are working on the reduction of mechanical brakes with electric brakes in an effort to reduce particle emissions. We are also looking into the installation of filtration equipment, like the innovative  IP’Air solution that we have developed in partnership with SUEZ and with the support of the Ile-de-France region.

What is this IP’Air solution?

We chose this technology in response to a call for projects by the Ile-de-France region. This pilot filtration project uses positive ionisation. Trials started in June 2019, with the installation of two IP’Air units in Alexandre Dumas station on line 2. The system uses the charge of the particles in the incoming air, which are collected on panels that work like a magnet.

A number of measurements are being made to assess the efficiency of the solution. SUEZ measures the quality of the air at the inlet and the outlet of the filter, while the RATP and Airparif measure the levels of type PM10 and PM2.5 particles in the station. The trial and data collection will continue until December 2019.

If the results are conclusive and we observe an improvement of the air quality, then we will be able to test IP’Air in stations with different configurations.

There is nothing inevitable about the air quality in the Paris region’s underground network. On the contrary! We are fully committed to improve the everyday conditions on the network for travellers and for our employees.