OSHA Method ID142 and employ- ing a sample train of a size-separating cyclone followed by a preweighed PVC filter. In all cases, both respirable dust concentration and silica content of that dust should be analyzed. If exposures are detected that
approach or exceed the OSHA PEL, it is advisable to proceed directly to real-time engineering sampling methods. Additional TWA exposure sampling may improve confidence in the compliance status, but it will prob- ably not advance the critically needed understanding of the factors that cause the exposure to be elevated. Step 2. Establish bound-
ary conditions for root cause sampling. Many air contaminant
sources can potentially contribute to the air con- taminant concentrations present at workstations in metalcasting facilities. Depending on the charac- teristics of the ventilation system, the intermixing of fugitive airborne dust from multiple sources can create a condition called cross- contamination, where the background dust levels at a workstation can be due to fugitive dust contributions from multiple surrounding dust sources. Cross-contam- ination produces a masking effect that can confound efforts to identify root causes of exposure present at workstations. Techniques to address potential issues associated with cross-contamination include: • Respirable dust mapping of the entire facility or of the general area of the workstation in question.
• Air mass balance analysis and ventilation pat- tern analysis to detect ventilation defects such as concentration buildup, cross-contamination and stagnation.
• Evaluation of the effectiveness of measures to control air contami- nants at the source. A technique has been employed
successfully to minimize the impact of cross-contamination of a worksta- tion during root cause analysis of silica exposure sources existing at that workstation. It involves evaluating the workstation during a non-production time, with no other silica-producing processes operating. Step 3. Gather real-time samples. Prior to real-time monitoring, it is
important to assure the target pro- cesses as well as their current exposure control methods are operating at baseline conditions. Conditions qualify as baseline when processes, equipment, ventilation controls, work practices and housekeeping methods are operating as they were designed to operate. Root cause analysis is capable of detecting causes of exposure occurring for any reason. However, the method should be employed principally to identify opportunities to improve exposure control, not merely to define the need to restore malfunctioning processes, equipment and work practices. To perform real-time
personal monitoring, the real-time monitoring device should be positioned as close as practicable to the breathing zone of the employee (Fig. 2). As this device will measure both the source and fugi- tive emissions present at the employee, often it is advisable to use a second monitor, placed near the workstation in a location that is representative of the background air at the work- station. As work begins, the observer needs to continu- ously make written notes of tasks and conditions sampled, noting the starting and finishing times associ- ated with each of these tasks and conditions. Once a representative
Fig. 3. The graphs illustrate the plotted real-time results of personal (up- per) and area (lower) sampling at a casting finishing workstation.
duration of monitoring has been performed, the collected data has been downloaded and calibra- tion of the data has been completed, data analysis can commence. Because the downloaded data has been time synchronized to written observations noting tasks, worker positions, tools and other conditions that can affect personal exposure, the exposures associated with each of these items can be isolated and quantified.
December 2014 MODERN CASTING | 35
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