Introduction

2In Europe alone, commuters in more than 60 cities use rail subways. Internationally, more than 120 million people commute by subway every day. We count around 4.8 million users per day in London, 5.3 million in Paris, 6.8 million in Tokyo, 9.7 million in Moscow and 10 million in Beijing.

3The use of public rather than private transport to abate urban atmospheric emissions is to be encouraged, and, in this context, subway systems are especially desirable. Subways are vital for commuting in crowded cities, something that will become more and more important over time – according to a United Nations 2014 report, half of the world’s population is now urban. They can play a part in reducing outdoor air pollution in large metropolises by helping to reduce motor-vehicle use. Large amounts of breathable particles (particulate matter, or PM) and nitrogen dioxide (NO₂), produced in part by road traffic, residential heating and industrial emissions, are responsible for shortening the lifespans of city dwellers. Public transportation systems such as subways have thus seemed like a solution to reduce air pollution in the urban environment. However, in response to increasing scientific and public awareness of the importance of clean air to human health, a number of studies have revealed unacceptably high levels of inhalable particulate matter (PM) in some subway systems.

4As such, we may wonder what the air that we breathe is like underground, on rail platforms and inside trains.

Mixed air quality

5Over the last decade, several pioneering studies have monitored subway air quality across a range of cities in Europe, Asia and the Americas. The database is incomplete but is growing and is already valuable.

7Such differences have been attributed to different wheel materials and braking mechanisms, as well as to variations in ventilation and air conditioning systems, but may also relate to differences in measurement campaign protocols and choice of sampling sites.

The effects on health

Of wheels and brakes

11Much subway particulate matter is sourced from moving train parts such as wheels and brake pads, as well as from the steel rails and power-supply materials, making the particles dominantly iron-containing. The dominantly ferrous particles are mixed with particles from a range of other sources, including rock ballast from the track, biological aerosols (such as bacteria and viruses), and air from the outdoors, and driven through the tunnel system on turbulent air currents generated by the trains themselves and ventilation systems.

12Key factors influencing subway air pollution include types of brakes (electromagnetic or conventional brake pads) and wheels (rubber or steel) used on the trains but also station depth, date of construction, type of ventilation (natural/air conditioning), , train frequency and more recently the presence or absence of platform screen-door systems.

Comparing platforms

14During this project, we sampled widely from a range of subway lines and station types. Conclusions can be reached concerning which kind of subway stations are likely to have the best and worst air quality in any given system:

• The subway stations likely to have the worst air quality will be those with limited air volume (such as single tube lines with one narrow platform), weak or inappropriately designed ventilation systems (especially in deeper stations), a lack of platform screen doors protecting the commuter from the free ingress of contaminated tunnel air, a topography that involves elevation changes and therefore requires harder braking, and that are old enough to have generated years of particulate pollutants available for repeated resuspension throughout the system.

• In contrast, subway stations with the best air quality are likely to be larger and/or newer, with good air interchange with outdoor street air (although not sourcing from traffic hotspots in the city), with full length screen doors fitted to all platforms, and with a straight, horizontal trajectory that minimises brake and wheel wear.

15The stations with just a single tunnel with one rail track separated from the platform by glass barrier systems showed on average half the concentration of such particles in comparison with conventional stations, which have no barrier between the platform and tracks. The use of air-conditioning has been shown to produce lower particle-matter concentrations inside carriages. Moreover, subway platform air quality is markedly influenced by the power setting of tunnel ventilation fans and whether or not the platform air is being introduced by impulsion or removed by extraction. Switching from platform impulsion to extraction with higher fan power in the tunnel results immediately in a marked increase in ambient inhalable PM, especially in the number of finest particles (submicron), which are presumably being drawn into the platform from the tunnel.

16In trains where it is possible to open the windows, such as in Athens, concentrations can be shown to generally increase inside the train when passing through tunnels and more specifically when the train enters the tunnel at high speed.

Monitoring stations and other recommendations

17Although there are no existing legal controls on air quality in the subway environment, research should be moving towards realistic methods of mitigating particle pollution. Our experience in the Barcelona subway system, with its considerable range of different station designs and operating ventilation systems, is that each platform has its own specific atmospheric micro environment.

18To design solutions, one will need to take into account the local conditions of each station. Only then researchers can assess the influences of pollution generated from moving train parts. Such research is still growing and will increase as subway operating companies are now more aware of how cleaner air leads directly to better health for city commuters.

19These are some important points to consider in order to improve air quality in the subway environment:

• Trace metal components of moving train parts can be recognized in subway air and this prompts the question: are these materials as least toxic as possible? Some of the identified metals, such as manganese, copper, antimony and chromium, are known to produce toxic effects in humans, and so we would urge further research into the toxicity of inhalable friction- generated polymetallic particles, particularly brakes and copper-bearing catenary systems.

• At night, when neither train nor platform ventilation fans are operational, platform air quality improves when tunnel fans are working at lower power, whether or not they are operating on impulsion or extraction. The resulting reduction in air movement from tunnel to platform, due to subdued fan power and no train piston effect, presumably allows particles to settle out of suspension. Slowing down the speed of trains in places on lines where there are sharp curves and high gradients should reduce the emissions of iron-rich particles.

• Controlling the exchange between the outdoor and underground air masses using intelligent ventilation systems, avoiding sourcing from traffic hotspots in the city by careful selection and design of outdoor ventilation grill locations: impulsion of outdoor air at platforms during metro hours; Ventilation on platforms at frequencies higher than 25 Hz; Forced extraction of outdoor air at tunnel during operating hours; Air conditioning systems inside trains.

• The use of air purifiers: their effect is dependent on the distance to the passenger and the flow rate.

• Platform screen doors: modern subway lines are being fitted with platform screen doors, primarily for passenger safety reasons. The additional benefit to passenger health is their efficiency in reducing the ingress of contaminated tunnel air into the platform, especially of relatively coarse inhalable particles.

• Night maintenance: some good practices must be taken into account to ensure dust emission reduction such as conducting the cleaning as early in the night as possible and using dust suppressant (water and/or antiresupension polymer) when laying ballast.