Please finish exiting and boarding, there is better air quality at the next station

The researchers chose the Munich metro system in Germany for their research. For comparison, they chose two types of trains - "old" and "new". The old trains have no automatic ventilation system except for windows and doors, and the interior is separated into individual wagons. The new trains, on the other hand, have ventilation, permanently closed windows and the interior of the three wagons is interconnected.

Jan Bendl developed a mobile measurement system for aerosol particulate matter (PM), Particle Number Concentration (PNC), their visualization, and metal analysis. He also devised a methodology to monitor the spatiotemporal variability of these particles, personal exposure of the passenger during the ride and movement in the metro. The system also allowed them to identify pollution “hot spots” and pollution sources. They focused both on the differences between stations and changes during the day, as well as on the second-by-second dynamics of particle concentrations on platforms.

Measuring air quality on one of Munich's subway platforms (left), a modular mobile air quality measurement system developed by Jan Bendel (right). Source: Jan Bendl.

The measurements showed that in terms of spatial variability, towards the city centre there was a gradual increase in pollutant concentrations from tens to peak values of PM₁₀ (particles with a diameter of 10 micrometres or less) of around 200–300 μg.m^-3 and in the respirable fraction PM_2.5 up to 80 μg.m^-3. The spatial differences were much more significant than the temporal ones, which were mainly related to train arrivals/departures and their frequency. During the ride, air exchange between the train interior and the tunnel was rapid and high, both in the old passively ventilated trains and, surprisingly, also in the new air-conditioned trains. When the composition of the individual aerosols was analysed, more than ninety percent of the mass of these particles was made up of iron abrasion and corrosion products from the rails and wheels of the subway trains. No local health hazardous exhaust emissions are generated in the underground metro, but in a few cases, the supply of polluted air by ventilation with a high number of nanoparticles from automobile traffic was observed, which shows the importance of appropriate placement of any surface stacks of ventilation shafts.

Compared to the World Health Organization (WHO) outdoor air quality recommendations, PM levels in metro areas were generally high, especially for long-term or repeated exposure, putting metro drivers and employees at a possible risk. For example, the WHO maximum guideline values for PM₁₀ are 45 µg.m^-3 on a 24-hour average, which was exceeded especially in areas identified as 'hot spots' and the air quality there was therefore poor. These were mainly busy transfer stations in the city centre.

Old or new - is there a difference?  Photo: Magda Křelinová.

Although the metro is perceived as an efficient and environmentally friendly means of transport, there is still room for improvement. There are places that have an excess of aerosol particles and long-term or repeated exposure to them may cause a significant adverse health effect. However, all is not lost. Using the innovative methodology for effective monitoring and mapping of PM in the metro presented in this study, it is possible to identify the locations with the highest concentrations of PM and apply effective targeted measures, for example by improving the ventilation system using adequate filters. For drivers, the authors recommend installing air purifiers in the driver’s cabin. This could significantly reduce the health risks of metro travel in the future, making it more pleasant and attractive.

Kateřina Fraindová

Bendl, J., Neukirchen, C., Mudan, A., Padoan, S., Zimmermann, R., Adam, T. (2023): Personal measurements and sampling of particulate matter in a subway – Identification of hot-spots, spatio-temporal variability and sources of pollutants. Atmospheric Environment 308, 119883. https://doi.org/10.1016/j.atmosenv.2023.119883