Air quality in public transport: no longer an option to restore confidence

Air quality is a structural problem, which did not appear with Covid-19. On the other hand, the health crisis has put it back at the centre of users’ concerns. They have turned away from public transport in favour of individual transport. There is a strong desire on the part of operators and transport authorities to provide solutions and restore confidence, which is essential if passengers are to use public transport again. What are the solutions to improve air quality? How can the most appropriate one be chosen for each situation? Our experts provide some answers. 

Air quality is one of the most pressing issues of the 21st century. Throughout the world, people suffer from serious diseases as a result of poor air quality in cities due to the development of ground transport and the increasing impact of residential heating and industrial activities. Air pollution contributes to around 7 million premature deaths annually worldwide (11.6% of all global deaths) including 630,000 in Europe. Big cities are the worst affected by the high level of air pollution and the annual costs of premature mortality[1].

Nowhere are people effectively protected from pollution and public transport is not an exception. Pollution from outside (the city) and produced by circulation of the rolling stock itself, fills metro stations, tunnels and trains. With ground transport, travellers and operators’ employees are constantly exposed to traffic pollution.

Recent air quality measurements undertaken in metro stations around the world have shown that pollution considerably exceeds WHO (World Health Organization) recommended levels, especially in the case of the concentration of particle matter PM2.5. Considering that many passengers spend 60 to 90 minutes a day on public transport during their working lives, the impact of air pollution here can no longer be ignored. In the case of high speed and regional trains, passengers spend 2 to 6 hours on board per trip so ensuring good air quality remains an of paramount importance.

The current global health crisis related to Covid-19 has changed the transportation priorities of populations, with a decline in trust for public transport and a shift towards individual transport modes. Regular disinfection and social distancing measures are essential if the population is to feel safe again2. But these measures are insufficient.

A systemic approach, not limited to rolling stock

Air quality and cleanliness of surfaces are major issues in restoring confidence in public transport and speeding up a return to normal operations. The question of air quality should be approached with all aspects of the transport system in mind, including the rolling stock and the infrastructures.

  • In urban and suburban areas, passengers spend relatively short periods in transport, usually crowded, that increase the risk of contact with potentially infected surfaces (handrails and seats) and proximity to other passengers. The air circulated by the ventilation and/or conditioning devices contributes to spreading pollution and viruses, but the risk is low because of distance from the air source. On the other hand, for drivers and other personnel, who spend hours on board, air conditioning systems must aim at significantly reducing the mixing of bacteria and viruses.
  • In suburban and interurban areas, the health risks are relatively high because of air recirculation by the air conditioning system and long-lasting contacts with potentially infected surfaces (seats, tables) and toilets.
  • For infrastructure, all locations and access controls, including payment terminals, should be considered too.

How can the most effective air treatment solutions be chosen?

In order to propose the most suitable solution for each situation, we propose a three-step process.

Step 1: acquire extended knowledge and understanding of the current situation

Establishing the current situation on-board, with the rolling stock, or within the stations goes through a reverse engineering analysis: How has it been built or manufactured? Why have certain constructive and air treatment solutions been chosen? What could be modified at low cost? What cannot be modified?

  • Transport system environment

Concerning metro infrastructure, for each case, SYSTRA will build an air circulation profile of the station using  CFD (Computational Fluid Dynamics) modelling software to define how the air flows from/to the tunnel, through the exhaust shafts and through the station accesses, at platform level.

The analysis of the current on-board and rolling stock situation  involves understanding of the air treatment solution implemented at the HVAC (Heating, Ventilation, Air Conditioning) (or simply ventilation) level, the quality of the doors and windows sealing, the frequency of the stops and doors opening, the train’s interior design and the air circulation ensured by the HVAC or ventilation.

  • Nature, source and quantity of air pollutants

The data collected by the operator are then classified in terms of occurrence, nature, quantity and toxicity. This then enables the appropriate measurement campaigns to be specified and the figures analysed after collection.

SYSTRA has an overall view of the risks prevailing in the environment of the different modes of transport and these are summarised in Table 1 and Table 2 below.

Comparison of air quality in different modes of transport.
Sources: Pollution atmosphérique n°215 – July-September 2012, Assessment of the exposure of city dwellers to air pollutants during their journeys in the Paris conurbation ; EASA, 2014, “Preliminary cabin air quality measurement campaign”.

PollutantPollutant nature
PM10 PM2.5Primary particles: mixture of different chemical compounds from road traffic, residential sector, construction sites, agriculture. Secondary particles: NO2, SO2, NH3, VOC react with each other, suspension of soil dust.
NO2Incomplete combustion (road traffic and residential heating). Forms naturally in the atmosphere from NO and O3.
CO2Source linked to human activity: combustion of fossil fuels. Natural source: respiration and decomposition
COIncomplete combustion (road traffic and residential heating)
VOC (volatile organic compounds)Wide range of products from: petrol refining, combustion, use of paint and cleaning products, coating.
Principal sources of the air pollution in public transport. Source: https://www.airparif.asso.fr/

Step 2: simulate various air treatment solutions

Improving the air quality in a metro station or in a train can be achieved thanks to the combination of various devices, equipment and structural changes.

In addition to our thorough understanding of ventilation policy on metro stations linked to international standards (NFPA 130) and regulations in ‘ERP’ (establishments receiving the public) in France, for example, SYSTRA maintains a regular benchmark on industrial air treatment solutions and will offer a customised set of recommendations in each case.

These recommendations will concern infrastructure design that can have an impact on the air quality (platforms, access, equipment rooms, tunnels etc.). The impact of platform screen doors on the air flow, temperature management and smoke control are very well understood at SYSTRA and will feed the feasibility studies.

Thanks to a comprehensive understanding of the origins of the various pollutants present in public transport areas and prior to any technical treatment of the air, SYSTRA will recommend limiting the emissions at their source, as far as possible.

Finally, as far as possible, it is preferable to limit emissions at source. Transport systems produce them, especially through mechanical braking, tyre wear on the rails for pneumatic metros, friction on the rails, infrastructure degradation (ballast, concrete track), diesel vehicle emissions, etc. To reduce them at the outset, solutions exist, and industrialists and equipment manufacturers are currently developing innovative ones.  All sources of pollutants that cannot be avoided should be treated afterwards.

There are several air treatment solutions on the market today, and others are under development. The challenge lies in selecting the most appropriate air filtration equipment according to the assessed risks. A combination of several solutions may be necessary, as shown in the figure below.

Step 3: evaluate the technical solution performance

After having selected solutions involving architectural and structural changes if necessary, and the industrial air treatment devices, SYSTRA will validate the proposed solutions by means of simulation.

SYSTRA offers its clients a portfolio of services to assist them in creating solutions, from design to implementation:

  • Supporting the operator in planning the implementation phases with respect to the Operational and Maintenance constraints of the transport system.
  • Preparing the technical requirements of the structural changes and of the equipment to acquire and install with a focus on RAM+S (Reliability, Availability, Maintainability+ Safety) factors of the new equipment and on the overall performance of the modified station or train.
  • Consolidating a preliminary cost estimate of the air quality improvement project and assist the operator in the selection of the proposed solutions.
  • After a building and testing phase of the project, follow up of the air quality improvement in the short and mid-term.

 In conclusion

We are involved in creating more sustainable, safer and more accessible mobility. We offer a system approach aimed at solving the air quality problem in a public transport network and restoring the confidence of passengers.

Restoring this confidence in transport networks is a major challenge for public transport if it is to remain the backbone of tomorrow’s sustainable cities.

We are ready to meet the specific challenges of its clients and create with them the clean transport of tomorrow, from the original design or the assessment stage, until the testing & commissioning of a transport system considered at its full extent as a healthy environment.

Even if we all wish that the Covid-19 issue will soon be over, the awareness that it has generated should start a strong and sustainable momentum towards improving air quality in transport networks.


[1] European Public Health Alliance: “How much is air pollution costing our health?”, August 2020.

[2] McKinsey study: “Transportation during the pandemic: what is considered safe?”, September 2020.

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