The metro system in the city of Barcelona (Transports Metropolitans de Barcelona: TMB) is one of the older underground transport systems in Europe with its first line beginning operation in 1924. By the present decade, the TMB comprises 11 lines (3 of them driverless lines since 2009, Lines 9-11) with a total length of 102.6 km and including 140 train stations. Trains run from 5 h in the morning until midnight during weekdays and Sundays, and for 24 h on Saturdays, with a frequency between 2 and 15 min, depending on the day (weekend or weekday) and time of day. Trains from all lines are equipped with an efficient air conditioning system that works continuously throughout the year, but with higher intensity in the summer period. Although commuting in Barcelona is mainly done by car (40%, Baldasano et al., 2007), the Barcelona metro absorbs a very important part of the urban commuting load, transporting around 1.25 million passengers on workdays (around 50% of the city’s public transport). The most frequent average journey time is 35 min (approx 10.2 km round trip).
The stations in the new lines differ radically from older lines in having single platforms in different tunnels with the platform being separated from the rail track by a wall with mechanical doors that are opened simultaneously with the train doors (known as platform screen door systems). The system is automatic, with a computer controlled driving system that optimises speeds, braking and stopping processes. The platforms also have a specific ventilation system that channels the convective dynamics caused by the train approach to renew the air throughout lateral ventilation outlets across the closed platform. This system allows air renovation in the platform, but also produces convective dynamics in the tunnel that may cause the resuspension of tunnel dust and the subsequent arrival of this dust to the platforms. Although during the approach to the platform braking is electric, all trains of both old and new systems use pneumatic braking (asbestos-free) after deceleration to a certain velocity to finally stop in the platform. The different composition of brake pads of these braking systems are potentially responsible for large differences in the concentrations of specific metals such as Ba, As, Sr, Mo, Cu released into the atmosphere as airborne particles.
Although WHO has implemented guidelines for indoor air quality in selected pollutants (WHO 2010), no indoor air quality legislation is available yet in Europe. The European Commission Scientific Committee on Health and Environmental Risks (SCHER) concludes in their web page that more data are needed, particularly on particles and microbes, volatile organic compounds from consumer products, building dampness, levels of exposure, and effects on vulnerable populations, as there are still gaps in knowledge that should be addressed by European-wide multidisciplinary research. Within this context IMPROVE will approach this environmental problem aiming to not only improve air quality in the subway system, improving the life of the users and workers of the subway, but also to make policy makers and the general public (subway travellers) aware of the possibilities for improvement.