Dust sampling strategy on coalfaces in relation to modern coalmining methods. Final report on CEC research contract 7260/03/62/08

Work carried out by the National Coal Board’s Pneumoconiosis Field Research (PFR) enabled gravimetric respirable dust standards to be introduced in the British coal industry in 1970. The original standard at a point 70m from the coalface in the return roadway was set at 8 mgm-3 and corresponded to a mean concentration over all longwall coalfaces of 4.3 mgm-3 for all coalface workers in collieries where the dustiest coalface just met the standard. The difference between the two concentrations was accounted for by two factors. One factor (1.4) took account of the difference between the dust concentrations at the 70m point and the coalface, and the second factor (1.32) made allowance for the variation in dust concentrations on different coalfaces within a colliery. The standard was reduced to 7 mgm-3 in 1977 corresponding to a mean respirable dust concentration of 3.8 mgm-3 averaged over all coalfaces.Further work by the PFR in the 1970s confirmed the relationships on which the standard was based and the protection it gave to mineworkers. In the 1980s, however, the industry underwent considerable changes in mining methods, using more powerful and efficient coal-getting machinery. The emphasis on high technology and greater production has led to developments in face end working methods, a greater dependency on retreat mining and a reliance on fewer working coalfaces at each colliery. All of these changes have made a potential contribution to altering the pattern of airborne dust distribution between the coalface area and the statutory dust control point. Taken together with changes to mineworkers’ working schedules it was considered that the exposure of coalface workers may not now be described by reference to the above ratios applied to dust concentration as measured at the 70m point.This research, therefore, set out to examine the distribution of respirable dust concentrations around modern coalfaces with the aim of assessing whether there was a need to modify the present dust sampling strategy. An additional objective was to re-evaluate the location and method of use of dust measuring instruments for the purpose of control monitoring.Measurements were carried out in seven collieries on a total of thirteen coalfaces (three advancing and ten retreat, to reflect British Coal’s objective of further increasing retreat mining). Both advancing and retreat coalfaces had a variety of face end configurations. Airborne respirable dust concentrations (including compositional analysis for respirablequartz) were taken over a period of up to ten production shifts at PFR sampling sites on and around each coalface including the 70m statutory sampling site in the return roadway. During the surveys observations were made on coalface parameters and the principal activities carried out by coalface workers to help explain any variations in dust concentrations. The analysis of the data for each measurement site made allowance for reject samples missing from the data set and enabled comparison of sampling sites within and between coalfaces to be of like with like.The main findings of the study showed that the pattern of dust distribution around longwall coalfaces appears to have undergone some changes in the years since it was last investigated.Intake pollution was shown to have a significant effect on dust concentrations in the intake roadway and at the intake end of the coalface at a number of the coalfaces studied. It is probable that intake pollution also has an effect on coalface dust concentrations, however respirable dust concentrations measured on the coalface and in the return roadway were clearly related to coalface production and varied by daily shift output.The control point to coalface ratios determined from the measurements made in this study were approximately 1.0 for the thirteen coalfaces. These measurements refer only to time spent at the coalface. If allowance is made for the full underground shift period, then the control point to coalface exposure concentration is now likely to be between 1.1 and 1.2 compared with the value of 1.4 calculated from the PFR.Sampling on two or more coalfaces at five of the collieries enabled some observations to be made on the relative dustiness of coalfaces in the same colliery. However, it is concluded that making allowance in the respirable dust standard for variations in dust concentrations at different coalfaces within a colliery is no longer valid, given the reduced number of operational coalfaces at each colliery.The implications of the study are that the control point to coalface exposure ratio has reduced and the dustiest face ratio no longer applies, so that the protection afforded to coalface workers would also be reduced if the control point concentration is close to the standard. Specifically, if control point measurements just meet the standard of 7 mgm-3 then on average coalface workers are exposed to concentrations of about 6 mgm-3 over a full working shift (portal to portal). Statutory respirable dust concentrations measured by British Coal, however, indicate that mean return control point concentrations are under half the standard and that consequently exposure of coalface workers is still on average controlled to levels intended by the current standard.The pattern of distribution of respirable quartz concentrations around coalfaces was different from that of mixed respirable dust as measurements at the 70m point may underestimate exposure to quartz at the return end of the coalface.Measurements of mixed respirable dust at the 70m point continue to allow estimation of coalface workers’ exposure and we recommend that the 70m point continues to be used for statutory dust measurements. We recommend, that the frequency of measurements is increased to twice per month and that sampling should also take place during shifts where no less than a specified minimum production output is achieved. This will assist dust control engineers to ensure the efficiency of coalface dust suppression measures.