What We Breathe When Riding The Subway Train

By now, health scientists and many people are well aware that breathing poor quality air will likely shorten your life. Living in the city has many benefits, but inhaling clean air is rarely one of them. The biggest problem is road traffic, which delivers not only toxic gases but also millions of tiny particles to your daily dose of inhaled pollutants. So the message is: don’t commute by car, and if you can’t walk or cycle, then use public transport.

The obvious alternative, when available, is the subway, which offers the city a commuting lifeline that is not just cheap and efficient but also gets people off the street and away from roadside proximity to exhaust pipes and, especially, diesel emissions. An incredible number of people use the subway worldwide, with over 40 billion journeys being made every year, which is more than five times the current world human population. However, nothing is perfect; a potential disadvantage of any underground transport system is that it operates in a confined space in which a fine dust of inhalable particles may accumulate.

The EU-funded IMPROVE LIFE project has recently completed an exhaustive study of subway air, focusing in particular on the city of Barcelona and involving close and willing collaboration between research scientists and the local rail operator Transports Metropolitans de Barcelona (TMB). This co-operation in itself has made the IMPROVE project stand out as unusual and innovative: very few subway operators worldwide want to open what they fear may be a Pandora’s box of alarmist media attention on air quality underground. However, a preference for ignorance is unlikely to produce enlightened environmental improvement. As cities begin to improve air quality above ground, we can be confident that interest will increasingly turn to what we breathe when traveling underground.

So, how is it down there? The answer to this is “variable.” Whereas the air breathed on some subway platforms is downright poor due to high levels of inhalable dust (or “particulate matter: PM), on other stations (such as along the new Barcelona airport line), it is really quite clean: better, in fact, than outside city air in some cases. It all depends on a number of factors, but especially the ventilation system on platforms and inside trains, station design, train frequency, line topography, and whether or not full-length platform screen doors (PSD) have been installed. The PSD systems were introduced primarily to help with passenger safety, but the IMPROVE project has demonstrated that such doors greatly improve platform air quality because they limit the ingress of contaminated tunnel air.

And this is the problem: particles produced by the train itself as it moves through the tunnel. Most subway PM sources from friction between the moving parts of the train such as brakes and wheels. This is obvious if you look at the chemistry of subway dust, which is completely different from anything breathed above ground. This is perhaps the most interesting thing about subway air, which has a peculiarly metallic character, with iron being the classic “subway metal”, followed by carbon (from pantographs and electric brushes) and accompanied by a heady cocktail of trace metals such as chromium, nickel, and cobalt (from the wheels), and barium, strontium, zirconium, titanium, and antimony (from the brakes).

In a quite remarkable part of the study, the IMPROVE results demonstrated how we can identify the type of brakes used by the train in the dust breathed on the platform. In Line 3 of the Barcelona subway system, which uses barium-rich brakes and has a strong mountain-to-sea gradient, for example, barium concentrations in inhalable platform PM can rise to 1500 nanograms per cubic meter, which is 300 times higher than above ground. Fortunately, toxicological studies have failed to demonstrate conclusively that this metallic mix is any worse for your health than the chemistry of air breathed in the city above but, as with any dust loading, less inhaled PM is always better for your health.

In this context, a technical guide produced by IMPROVE (and freely available on their website) focuses on strategies involving practical steps that can be taken to promote cleaner subway air, the first being to create an informed awareness of the problem where it exists. Following an initial air quality audit to assess the current situation on station platforms and inside trains, protocols need to be established to minimise fugitive dusts (released by night workers, for example), clean tunnels and platforms to avoid PM build-up, replace outdated ventilation systems by more intelligent designs linked to air quality monitors above and below ground, install platform screen doors, and consider the use of air purifiers on platforms and in trains. Some of the most advanced subway systems, such as that of Seoul, are already using air purifiers both on platforms and in trains.

Finally, the IMPROVE project has proposed the introduction of a simple color-coded scheme designed to encourage transport authorities to aim for progressive PM reductions on platforms. The scheme defines a series of thresholds that cascade down through red, orange, yellow, green and blue bands of decreasing dust concentrations (measured in concentrations of PM_2.5: particles less than 2.5 microns in size) towards the ideal World Health Organization’s Annual Mean Air Quality Guideline of just 10 micrograms per cubic meter, which is the lowest level of long-term exposure at which total cardiopulmonary and lung cancer mortality has been shown to increase.

The fact that some subways already enjoy “Blue Zone” air quality that approaches the holy grail of the WHO guideline demonstrates that clean subway air is possible. A commitment by subway authorities to lower dust levels underground where necessary would be a welcome addition to our drive for cleaner, greener cities.

These findings are described in the article entitled Improving air quality in subway systems: An overview, recently published in the journal Environmental Pollution. This work was conducted by Teresa Moreno from the Institute for Environmental Assessment and Water Studies and Eladio de Miguel from Transports Metropolitans de Barcelona.