Vehicle interior air quality conditions when travelling by taxi

Abstract

Vehicle interior air quality (VIAQ) was investigated inside 14 diesel/non-diesel taxi pairs operating simultaneously and under normal working conditions over six weekday hours (10.00–16.00) in the city of Barcelona, Spain. Parameters measured included PM10 mass and inorganic chemistry, ultrafine particle number (N) and size, lung surface deposited area (LDSA), black carbon (BC), CO2, CO, and a range of volatile organic compounds (VOCs). Most taxi drivers elected to drive with windows open, thus keeping levels of CO2 and internally-generated VOCs low but exposing them to high levels of traffic-related air pollutants entering from outside and confirming that air exchange rates are the dominant influence on VIAQ. Median values of N and LDSA (both sensitive markers of VIAQ fluctuations and likely health effects) were reduced to around 104 #/cm3 and < 20 µm2/cm3 respectively under closed conditions, but more than doubled with windows open and sometimes approached 105 #/cm3 and 240 µm2/cm3. In exceptional traffic conditions, transient pollution peaks caused by outside infiltration exceeded N = 106 #/cm3 and LDSA= 1000 µm2/cm3. Indications of self-pollution were implicated by higher BC and CO levels, and larger UFP sizes, measured inside diesel taxis as compared to their non-diesel pair, and the highest concentrations of CO (>2 ppm) were commonly associated with older, high-km diesel taxis. Median PM10 concentrations (67 µg/m3) were treble those of urban background, mainly due to increased levels of organic and elemental carbon, with source apportionment calculations identifying the main pollutants as vehicle exhaust and non-exhaust particles. Enhancements in PM10 concentrations of Cr, Cu, Sn, Sb, and a “High Field Strength Element” zircon-related group characterised by Zr, Hf, Nb, Y and U, are attributed mainly to the presence of brake-derived PM. Volatile organic compounds display a mixture which reflects the complexity of traffic-related organic carbon emissions infiltrating the taxi interior, with 2-methylbutane and n-pentane being the most abundant VOCs, followed by toluene, m-xylene, o-xylene, 1,2,4-trimethylbenzene, ethylbenzene, p-xylene, benzene, and 1,3,5-trimethylbenzene. Internally sourced VOCs included high monoterpene concentrations from an air freshener, and interior off-gassing may explain why the youngest taxi registered the highest content of alkanes and aromatic compounds. Carbon dioxide concentrations quickly climbed to undesirable levels (>2500 ppm) under closed ventilation conditions and could stay high for much of the working day. Taxi drivers face daily occupational exposure to traffic-related air pollutants and would benefit from a greater awareness of VIAQ issues, notably the use of ventilation, to encourage them to minimise possible health effects caused by their working environment.