Exposure to particles and nitrogen dioxide among taxi, bus and lorry drivers

ABSTRACT
 

Aim: The aims of this study have been to investigate the occurrence of systematic differences in the personal exposure to motor exhaust between different groups of taxi, bus and lorry drivers, and to study if these are influenced by the choice of exposure indicator. 

Methods: We used one indicator of the gaseous phase, nitrogen dioxide (NO2), and one of the particle phase (measured by DataRAM), of the exhausts. A total of 121 drivers were included in the study: 39 taxi drivers, 42 bus drivers and 40 lorry drivers. Personal measurements were performed during one working day. Nitrogen dioxide was measured with passive diffusive samplers and particles with Data-RAM, a logging instrument using nephelometric monitoring. The instrument measures particles between 0.1 and 10 lm in size.

Results: The average exposure to NO2 for lorry drivers was 68 lg/m3; for bus drivers 60 lg/m3 and for taxi drivers 48 lg/m3. For particles the exposure was 57 lg/m3 for lorry drivers, 44 lg/m3 for bus drivers and 26 lg/m3 for taxi drivers. The result remained unchanged when exposures were adjusted for variation in urban back- ground levels of NO2 and particulate matter with an aerodynamic diameter <10 lm (PM10).

Conclusion: Lorry drivers experienced the highest exposure and taxi drivers the lowest with bus drivers in an intermediate position, regardless of whether NO2 or particles were used as exposure indicator. The levels of both NO2 and particles were higher for bus drivers in the city than for them driving in the suburbs. Using diesel or petrol as a fuel for taxis had no influence on the exposure for the drivers, indicating that the taxi drivers’ exposure mainly depends on exhaust from surrounding traffic.

METHODS

All measurements were performed by personal monitoring during one working day. The sampling time corresponded to the working hours of the driver. The samplers were placed at the shoulder of the driver, close to the breathing zone. All drivers were told to work as usual during the measuring day, that is, they were free to use the fan and open the windows etc.The measurements were carried out between September 1997 and October 1999. The sampling was balanced between the groups with regard to season. Nitrogen dioxide was measured with diffusive samplers developed and analyzed by IVL Swedish Environmental Research Institute. The detection limit was approximately 4.5lg/m3 for 8 h of sampling (Ferm and Svanberg 1998; Ferm and Rodhe 1997). The lunch breaks and coffee breaks were included in the sampling time, except for the 14 bus drivers who worked in two separate shifts during the day.

Particles were measured with a logging instrument,Data-RAM (MIE pDR1000), using nephelometric monitoring. The instrument recorded the mean value foreach minute. The data for lunchtime, but not for coffee breaks, were excluded before calculation of the arithmetic mean for the day. The Data-RAM measures particles between 0.1 and 10 lm in size and has been optimized for the measurement of the respirable fraction (MIE Instruction manual). The reading range of the instrument is 1lg/m3–4x105lg/m3. Before each sampling day, the Data-RAM was set to zero against particle-free air. The instrument is calibrated by the manufacturer against a standard dust (Arizona road dust) and the conversion from the nephelometric registration of the particles to concentration, in microgram per meter cube, is dependent on the particle size and density of the dust. This method of particle registration is not directly comparable with gravimetric sampling. Neither is it fully comparable with the tapered element oscillating microbalance (TEOM)-methods used for environmental measurements of PM10, described below. Anyway the method can be used to describe differences between groups of workers exposed to similar types of dust.

The measurements were performed over a relatively long time period. To investigate if any differences between the driver groups were influenced by variations in background air pollution, we calculated the drivers’ exposure both with and without adjustment for background air pollution during the day of the measurement.

The Environment and Health Protection Administration in Stockholm supplied data of nitrogen dioxide, particulate matter with an aerodynamic diameter <10lm (PM10), relative humidity, and temperature at an urban background site (a roof-top-site in the City of Stockholm) during the sampling days. Nitrogen dioxide was measured with a chemiluminiscence instrument (Environnement, AC31M-LCD) and particles (PM10) with a TEOM ambient particulate monitor. The mean values of urban background levels were calculated for the time period between 6:00 a.m. and 6:00 p.m. To be sure that differences between the groups of drivers were not only an effect of differences in the urban background levels, the average urban background values for the day (nitrogen dioxide and PM10respectively) were subtracted from the measured average value for each individual. The mean values for each group of drivers were then recalculated and the results compared with the unadjusted group mean values.

Statistical methods

Kolmogorov–Smirnov test was used for check of normal distribution (SPSS 11.5). All group values were normally distributed. Student’s t-test was therefore used for calculation of differences of means.

RESULTS

Nitrogen dioxide

Lorry drivers were exposed to the highest level of nitrogen dioxide and taxi drivers to the lowest (Table 2 and Fig. 1). The arithmetic mean differed statistically significant between taxi drivers and bus drivers (P<0.01), as well as between taxi and lorry drivers (P<0.01), but not between bus and lorry drivers. There was a larger variation in exposure among lorry and bus drivers than among taxi drivers. After individual day-to-day adjustment for the urban background levels of nitrogen dioxide, the order between the groups was the same, and the same differences were statistically significant.

The bus drivers and lorry drivers were subdivided according to areas of work, within or outside the City of Stockholm. Five of the lorry drivers were excluded in this analysis because they did not fulfil the criteria for driving within only one area. Bus and lorry drivers driving in the city had statistically significantly higher exposure to nitrogen dioxide than drivers of the corresponding type of vehicle in the suburbs (Table 3). There were no differences in nitrogen dioxide levels between bus and lorry drivers driving within Stockholm City. When driving in the suburbs, significantly higher mean values were found for lorry drivers than for bus drivers (P=0.02). There were no significant differences in nitrogen dioxide level between the lorry drivers from the six companies (result not shown).

When taxi drivers were subdivided according to the fuel used, i.e. petrol or diesel, no difference in nitrogen dioxide levels between the groups was found (Table 4).

Particles 

As for nitrogen dioxide, the lorry drivers were exposed to the highest levels of particles and the taxi drivers to the lowest (Table 2 and Fig. 2). The mean particle exposure differed statistically significant between taxi drivers and bus drivers (P<0.01), taxi drivers and lorry drivers (P<0.01) and also between bus drivers and lorry drivers (P=0.02). The variation in exposure was highest for lorry drivers, followed by bus drivers and taxi drivers. After individual day-to-day adjustment for the urban background levels of PM 10, the order between the groups was the same and the same differences were statistical significant.

When comparing bus drivers in Stockholm City with bus drivers in the suburbs, there were no differences. For lorry drivers, exposure to particles was, quite unexpected, higher when driving in suburban areas (Table 3), but numbers were small and the difference was not statistically significant. 

When bus drivers and lorry drivers in the city area were compared, there were no differences in exposure to particles. In the suburbs, the levels were significantly higher for lorry drivers than for bus drivers (P=0.02). There was no difference in particle exposure between taxi drivers using petrol or diesel (Table 4). The statistical test (Kolmogorov–Smirnov test) indicated a normal distribution of the readings for both NO2 and particles, but the box-plots graphs indicated that the
readings also could be log-normally distributed. However, the results and the significance tests between the groups were unchanged after a logarithmic transformation.
