Te exposure attribut- able to each item is calculated by averaging the digital data during the noted time period of that item. The federal OSHA
PEL for respirable crystalline silica cur- rently regulates the amount of silica-bear- ing respirable dust. In real-time silica exposure monitoring, a real-time respirable dust monitor can- not directly measure silica. Because of this, respirable dust is used as a surrogate to determine real-time silica exposures, and a means to estimate the silica content of respirable dust must be devised. Silica content can be determined in two ways: 1. The filter used to calibrate the respirable dust monitor could be analyzed for crystalline silica, provided the real-time sampling run is long enough to produce a silica sample above the silica detection limit.
2. Independent exposure monitoring at the workstation may be used to provide an estimate of the silica content of the dust being sampled by the real-time monitor.
Example of Isolation of Root Causes of Silica Exposure
One typical worksta-
tion in which a real- time root cause evalua- tion has been found to be particularly useful is during chipping and grinding ferrous castings using portable, powered (usually pneumatic) tools. Casting finishing workers typically use several different types of portable tools to perform chipping and grinding on castings
Fig. 4. This graph illustrates the view of individual task exposure with area under curve highlighted.
Te impact on overall exposure of spikes or any elevation in con- centration depends on both the magnitude of the elevated level as well as the duration. Some of the spikes in Fig. 4 are of such short dura- tion that they do not contribute meaningfully to the overall exposure TWA. On the other hand, sustained periods of elevated dust expo- sure, such as the one highlighted in Fig. 4, are of particular interest in a root cause evalu- ation. Te important parameter in evaluating elevated exposures is the area under the curve.
of varying geometry: cup, cone and wheel grinders, and various chisels. Te real-time monitor logs a digital measurement at a preset short interval. Figure 3 presents the real-time display of the digital respirable particulate concentration data, both personal (upper) and area (lower). It should be noted that in the upper graph, a series of spikes occurred in the personal sampling data. Tese spikes are associated with elevated concentrations of exposure.
Te larger the area under the curve, the larger the contribution to the worker’s exposure. Te real-time data presented in Fig.
3 were gathered at a time when this casting cleaning was, by design, con- ducted without other silica dust produc- ing processes operating in the vicinity of the workstation. Te worker carried out his tasks on a ventilated workbench. In this situation, the area sampling results (lower graph, Fig. 3) indicate fugitive dust emissions were occurring from the casting finishing process, which caused a back- ground dust exposure for the worker. To identify which tasks
Fig. 5. The pie graph shows the percentage of time per task during moni- tored chipping/grinding work.
36 | MODERN CASTING December 2014
contribute the most to employee exposures over the work shift, real-time exposure data can be grouped according to task. An overall time-weighted average then can be determined for each task, based on a composite of measurements from all of the periods in which that repetitive task was performed. In the case of the chipping/grinding operator, the chart in Fig. 5 demonstrates the per- centage of time dedicated
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