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recording the sampling time — the average silica concentration in the area where the sample was collected can be calculated. This process can be done in just a few min- utes at the end of a shift, rather than the few weeks required to complete the traditional practices. Why a New Monitoring Solution is Needed The importance of measuring silica at the end of shift is driven by the fact that just knowing dust levels is not enough — since the amount of airborne silica in the dust can vary significantly in both space and time, exposure to dust is not always indicative of silica exposure. As an example, measure- ments of silica made in the same area of a mine over multiple days show considerable variation (see Figure 1). Figure 1 shows the results, in terms of s i l i c a c o n t e n t ( % ) , i n r e s p i r a b l e d u s t samples collected for eight consecutive days in the same continuous miner sec- tion of an underground coal mine. Four samples were collected each day in dif- ferent locations of the same section. The minimum (green) and maximum (red) percent of silica found in the samples each day are shown. While the daily sili- ca content in the samples was on average quite consistent — around 10% — a large spatial variability can be noticed, espe- cially after day four. In this case, did the mine operator know there would be a relatively high silica content in the respirable dust? Probably. For exam- ple, miners know that silica levels will be higher when they need to mine more roof rock to accommodate equipment clearance. What the miners generally don't know is how the silica content in the dust will vary, both in space and over time. Figure 1 is a good exam- ple of the high temporal and spatial variabili- ty of the silica content in the respirable dust generally present in a coal mine. Figure 2, which depicts the minimum (green) and maximum (red) silica content in respirable dust samples collected in six mines, demonstrates clearly a mine-to- mine variability. For each mine, samples (10 or more) were collected over one or two days. For some mines — for example, Mine 4 — the silica content in the samples is quite constant. For other mines — for example, Mine 6a — the silica content can vary from 12% to 30%. Importantly, note that one of the mines was sampled twice in two differ- ent years (Mine 6 and Mine 6a), and the data show significant difference in airborne sili- ca for the two visits. Knowing that the silica content in the dust can be variable — even in the same section of a coal mine — and that it can also change over time, it is clear why the tradi- tional silica monitoring approach is not able to provide a timely picture of worker expo- sure or dust levels in an area. Put simply, the silica concentration, expressed as micro- r e s p i r a b l e d u s t c o n t i n u e d 32 www.coalage.com January 2016 Figure 2 — Silica content in respirable dust samples collected in six different coal mines. Minimum (green) and maximum (red) silica content is displayed. Figure 3 — Three steps for the new monitoring solution: (left) open the sampling cassette; (center) insert the cartridge in a holder; and (right) place the holder in the portable instrument. Figure 4 — Comparison of OMSHR direct-on-filter estimation of silica in dust samples collected in an underground coal mine with the results of the P7 standard analysis.