Occupational exposure to respirable dust, respirable crystalline silica and diesel engine exhaust emissions in the London tunnelling environment

Crossrail is a 118-kilometre (73-mile) railway line under construction in England. Work began in 2009 and it is Europe’s largest railway and infrastructure construction projects. During tunnelling projects exposures to potentially hazardous agents may occur due to the work being carried out in a confined space. Exposure measurements reported in the peer-reviewed literature suggest that all exposures are higher in underground work compared to outdoor construction workers. Relatively few studies have been conducted specifically looking at occupational exposures in tunnelling, and results from personal exposure measurements are especially lacking.A series of measurement campaigns were undertaken to investigate exposure to respirable dust (RD), respirable crystalline silica (RCS) and diesel engine exhaust emissions (DEEE) of the workers involved in tunnel construction. Workers monitored included those working on TBMs (tunnel boring machines) and SCL (sprayed concrete lining) activities. Personal exposure measurements were obtained using cyclone sampling heads, with temporal variability and levels of black carbon (BC) (as a marker for DEEE) and RD being assessed using a MicroAeth AE51 and TSI SidePak fitted with a cyclone, respectively.During SCL work job titles with the highest mean personal RD exposures were the back-up sprayer (Arithmetic Mean (AM) 2.40 mg/m3) and sprayer (AM 1.55 mg/m3). Both undertook the concrete spray activities and were observed to be in the restricted zone during excavation and spray activities. The inspector and pump men experienced the lowest mean RD exposures. All RD concentrations were below 4 mg/m3, although there were several instances where 8-hr concentrations were in excess of 1 mg/m3. The highest RCS exposure was observed for the back-up sprayer, with a maximum measured RCS concentration of 0.24 mg/m3. Concentrations in excess of 0.1 mg/m3, the 8-hr TWA WEL, for RCS were also reported for an engineer, lead miner and two plant operators. With respect to the SCL static samples, the highest mean (1.28 mg/m3) and maximum RD concentration (1.4 mg/m3) were measured within the restricted zone. The maximum RCS concentration of 0.04 mg/m3 was also observed within the restricted zone.All the REC concentrations measured with personal samples in the TBM environment were below the 100 µg/m3 reference limit. All RD measurements collected in the TBM environment were below 4 mg/m3, although for the job titles segment lift, grout pump, ring building, rail extension and conveyor extension there were a number of instances where the RD exposures were in excess of 1 mg/m3. The job titles with mean RD concentrations in excess of 1 mg/m3 were the conveyor extension (AM 1.65 mg/m3) and rail extension operators (AM 1.10 mg/m3). The highest observed REC concentration was for a segment lifter (37 µg/m3) and the conveyor extension and rail extension operators were reported to have the highest mean REC concentrations (AM 30 and 26 µg/m3 respectively). For the static samples, the highest mean RD and REC concentrations were observed at the transfer hopper location (RD AM 1.38 mg/m3 and REC AM33 µg/m3) and segment lift area (RD 1.38 and REC 40 µg/m3 although this was based on only one sample). The maximum RD concentration was alsoreported in the segment lift area and the TBM bridge area (2.2 mg/m3) withthe maximum REC concentration also being observed at the TBM bridge area (52 µg/m3).Temporal direct reading measurements using the MicroAeth revealed the impact of excavation, vehicular and locomotive movements with peaks in BC concentrations usually coinciding with these activities. During the TBM monitoring period the supply air was turned down during surveying activities. The impact of this on BC concentrations was evident in the temporal data and concentrations were reduced when the supply air was returned back to the usual settings. BC concentrations were higher in the SCL environment in comparison to the TBM environment (mean SCL concentrations over measurement period ranged from 17-54 µg/m3 in SCL environment compared to 3-8 µg/m3 in TBM environment).SidePaks were used to log the temporal trends in RD over the monitoring period. Similar trends as for the BC temporal and summary data were observed, with excavation, spray and vehicular activities leading to relatively higher concentrations in the SCL environment. The impact of the locomotive movement and the reduction in supply air was again demonstrated in the TBM SidePak data.The results in our monitoring campaigns suggest that DEEE exposures are higher in the SCL environments than the TBM environment and it is considered that future monitoring campaigns should focus on assessing DEEE exposures in the SCL environment and necessary measures should be implemented to control exposure. In the SCL environment, exposure to RD and RCS could be significant. Engineering controls such as de-dusters should be appropriately located and used during tunnelling operations. Respiratory protection equipment is provided and refresher operator training should be given on how to use and wear these correctly. It is recommended that face-fit testing be undertaken to ensure that suitable and sufficient RPE is provided to the SCL operators.Given the levels, further engineering control measures should be taken to reduce the RD exposure throughout the TBM but particularly in the conveyor extension area. The REC concentrations were lower than the SCL environment. However, measures should be taken to reduce exposures further, for example by replacing older, less efficient locomotives (‘yellow’ engines), and avoiding unnecessary running or idling of diesel engines. Efficient ventilation is needed to assist in the removal of RD and DEEE. The supply air to the TBM was turned down on two of the monitoring days and the impact of this on RD and REC was clearly observed. Further investigation is needed to ascertain why this practice takes place so that it can be eliminated or reduced.