Occupational Exposure to Respirable Dust, Respirable Crystalline Silica and Diesel Engine Exhaust Emissions in the London Tunnelling Environment

ABSTRACT

Personal 8-h shift exposure to respirable dust, diesel engine exhaust emissions (DEEE) (as respirable elemental carbon), and respirable crystalline silica of workers involved in constructing an underground metro railway tunnel was assessed. Black carbon (BC) concentrations were also assessed using a MicroAeth AE51. During sprayed concrete lining (SCL) activities in the tunnel, the geometric mean (GM) respirable dust exposure level was 0.91mg m−3, with the highest exposure measured on a back-up sprayer (3.20mg m−3). The GM respirable crystalline silica concentration for SCL workers was 0.03mg m−3, with the highest measurement also for the back-up sprayer (0.24mg m−3). During tunnel boring machine (TBM) activities, the GM respirable dust concentration was 0.54mg m−3. The GM respirable elemental carbon concentration for all the TBM operators was 18 µg m−3; with the highest concentration measured on a segment lifter. The BC concentrations were higher in the SCL environment in comparison to the TBM environment (daily GM 18–54 µg m−3 versus 3–6 µg m−3). This small-scale monitoring campaign provides additional personal data on exposures experienced by underground tunnel construction workers.

METHODOLOGY

Four 3-consecutive day measurement surveys were completed, two during TBM activities and two during SCL activities. It was agreed a priori, following discussion with Crossrail and their contractors’ health and safety and occupational hygiene personnel that RD and RCS were to be sampled during SCL activities, whereas RD and REC were to be sampled during TBM activities.

Personal inhalation exposure measurements were obtained using Higgins Dewell cyclones in accordance with MDHS 14/4 (HSE, 2014). Those operators a priori expected as potentially being more highly exposed due to their work activities were primarily targeted for sampling, although convenience sampling was also employed. 

Cyclones were loaded with either a polyvinyl chloride (PVC) or heat-treated filter for RCS and REC measurements, respectively. A MicroAeth AE51 aethalometer equipped with a microCyclone (time base 60 s, flow rate 50 ml min−1) measured BC concentrations in a fixed location. In the first SCL survey, the MicroAeth was located on a raised access way outside the restricted working zone (where only designated SCL personnel are permitted to work); during the second survey it was located within the restricted zone. In the TBM, the MicroAeth was positioned at the grout pump operator station, which had been expected a priori as potentially experiencing the highest DEEE exposure.

The PVC filters were analysed to determine the RD and RCS concentrations (HSE, 2014, 2005). The heat-treated quartz filters were analysed for RD and REC (HSE, 2014; NIOSH, 2003). The analytical limits of detection (LOD) were 0.05 mg for RD, 0.02 mg for RCS and 1 µg for EC. Results were field blank corrected and reported as 8-h TWA.

A random imputation method was used to substitute those RCS values below the LOD [26/49 values (53%)] (Helsel, 2005). Since the data were not normally distributed, measurements were log(e)- transformed prior to analysis. For each value below the LOD, a number between 0 and the LOD was randomly generated from the log-normal distribution. The results were summarized in terms for geometric mean (GM) and geometric standard deviation (GSD).

RESULTS

Engineers’ shift work reports were made available; summaries of the activities and control measures in place during each survey are provided in detail in Galea et al. (2015). SCL activities involved the formation of expansions for station platforms and also cross passages, linking platforms, and running tunnels. Wet concrete was applied using a remote controlled nozzle application. Concrete was delivered to the pump via a small diesel powered mixer. Forced air was supplied to the workface.

In summary, the activities involved tunnelling through areas of clay; 25 SCL operators participated with 49 personal measurements being collected and 44% of SCL operators provided one sample, with 16 and 40% providing two or three samples respectively. In the TBM environment, 20 operators participated, with 36 personal measurements being obtained; 45% of the TBM operators provided one sample, with 40, 5, and 10% of operators providing two, three, or four samples respectively.

The results of the individual personal samples are presented by job group in Fig. 1 for the SCL exposure measurements and Fig. 2 for the TBM measurements. Figure 3 presents the GM exposure measurement results by job group for both environments.

In the SCL environment the GM RD exposure was 0.91 mg m−3 (GSD 1.98; range 0.30–3.20 mg m−3) compared to 0.54 mg m−3 (GSD 2.08; range 0.07–1.79 mg m−3) in the TBM environment. A two-sample t-test (assuming unequal variances) shows that these are statistically different (P > 0.005). Operators that experienced the highest RD exposures were the (back- up) sprayers (SCL) and conveyor extenders (TBM).

GM average RCS exposure during all SCL activities was 0.03 mg m−3 (GSD 2.59; range <LOD–0.24 mg m−3), with the back-up sprayer experiencing the highest exposure. Five RCS measurements (10% of all measurements) exceeded 0.1 mg m−3, the 8-h WEL for RCS (HSE, 2011). The silica content of the RD samples was on average 5% (range 0.65–18%).

During the TBM activities, GM REC exposure was 18 µg m−3 (GSD 1.0; range 11–37 µg m−3), with a segment lifter experiencing the highest exposure.

Table 1 summarizes the BC concentrations logged during each measurement period. The GM concentrations during SCL activities ranged from 18 to 54 µg m−3, compared to 3–6 µg m−3 during TBM activities. The peak 1-min average logged was 206 µg m−3 during SCL activities and 41 µg m−3 during TBM activities. Peaks in BC concentrations were evident when the locomotive entered the TBM in most instances (Fig. 4).

