A70 Air pollution and city travel: choices in commuter exposure to inhalable particulates

Abstract

Abstract Background Commuters usually have choices regarding the mode of public transport they use in the city, but in the context of particulate pollution it is not an especially well-informed choice. We demonstrate from an innovative study in Barcelona that not only the concentration and number but also the chemistry of airborne particles inhaled during the commute vary greatly depending on the chosen travel mode. Methods Portable equipment for measuring black carbon, PM2.5 (mass and chemistry), and number of ultrafine particles (UFP) were carried in a backpack on 78 8-10 km round trip suburb-to-city-centre morning commutes by either tram, subway, bus, or exclusively on foot, with the same start and finish points. All equipment were duplicated, allowing two people (carrying a GPS) to make a simultaneous synchronised journey using different transport modes. Results The study revealed notable differences between the chosen morning commutes. In terms of average number of UFP concentration, the poorest air quality was associated with walking in the city centre (5.4-5.9 × 104 particles/cm3, depending on route chosen). The highest UFP concentrations using public transport occurred during bus travel (mean concentrations of 4.7 × 104 particles/cm3), the lowest in the subway (2.3 × 104 particles/cm3), with the tram registering values in between the other two (3.0 × 104 particles/cm3). Average UFP size was highest in the subway (90nm) and lowest when walking in traffic (54nm), with black carbon levels correspondingly highest when walking (7.3 µg/m3), decreasing to half this value progressively through metro>bus>tram. PM2.5 concentrations were lower when walking and/or in the tram and higher in the subway and bus. Chemical compositions also reveal interesting differences, with peaks in metallic tracers (such as Cu, Sb, Fe, Mn, Al and Ti) depending on transport mode. High transient peaks for all contaminants were related mostly to traffic (UFP numbers, black carbon), roadworks (PM concentrations) or fellow commuters (e.g. smoking). Conclusions Individual exposure to air pollutants while travelling across the city varies greatly depending on the type of transport selected and route taken. To access Barcelona city centre, in terms of both UFP number and PM2.5 concentrations the subway and tram systems are cleaner than the bus. Although walking is to be encouraged for personal exercise and lower average concentrations of PM2.5, the penalty to be paid by pedestrians in the city centre is the exposure to a concentration of well over double the one when travelling by subway.