The effect of ventilation protocols on airborne particulate matter in subway systems

As part of the European-funded IMPROVE LIFE project work programme experiments were performed in the Barcelona Metro system with the objective of better understanding the relationship between ventilation and air quality. The results demonstrate that tunnel ventilation plays an extremely important role in maintaining cleaner air and is capable of reducing both inhalable particulate matter (PM) mass and particle number concentration (>0.3μm) on platforms by over 50%, even in the presence of full-length platform screen doors. Another key influence on platform air quality is the chosen combination of fan power and forced air flow direction (impulsion of outdoor ambient air or extraction of subway indoor air): cleaner platform air was achieved using platform impulsion at higher power settings designed to ameliorate high summer temperatures underground. Reversing platform air flow from impulsion to extraction produced an immediate deterioration in PM air quality, most notably if the higher power setting was maintained, when an especially marked increase in numbers of very fine (submicron) particles was observed and attributed to tunnel air being drawn into the platform. At night, in the absence of trains and platform ventilation, platform air quality improves when tunnel fans are working at reduced power, whatever the flow direction (impulsion/extraction). Inside the air conditioned Barcelona Metro trains (where underground commuters spend most of their time) air quality is markedly better than on the platform, and unchanged A/C filters were observed capable of maintaining a similar reduction in inside train PM for at least three months.