Real-Time Monitoring for Silica Exposure Analysis

Eric Pylkas, Robert Scholz
Click here to see this story as it appears in the January 2018 issue of Modern Casting

The compliance date for OSHA’s new respirable crystalline silica standard is June 23. Part of ensuring compliance will include measuring employee exposures, investigating dust sources and implementing engineering controls to keep the exposure level under the permissible exposure limit (PEL).

One method for measurement and analysis of exposure is real-time monitoring. While the principal measure to assess silica exposure levels and provide a basis for reduction actions has been shift-long time-weighted average (TWA) exposure sampling, measuring real-time exposure enables metalcasters to quantify and prioritize root causes of silica exposure.

TWA sampling averages air contaminant concentration in the breathing zone over the work shift. Measuring an average exposure is useful in establishing compliance status but not in defining and prioritizing root causes that contributed to that exposure average, which is an essential step in an effective exposure reduction program.

Real-time monitoring can be used to provide a basis for establishing engineering parameters for effective ventilation control of silica sources and for optimizing work practices.

Real-time monitoring also gives statistical confidence that the controls put in place are effective. As each theory for engineering controls are tested, metalcasters collect hundreds of data points that help ensure the changes are working.

The data helps metalcasters create a credible timeline correlating  each correction made and the resulting reduction in exposure. Eventually, after several iterations of engineering controls have been implemented, the real-time monitoring data may show the exposure level has stopped coming down. If the work area is still above the PEL, the hundreds of data points from monitoring can support an argument for infeasibility.

OSHA’s standard outlines a hierarchy of controls which must be implemented in order:
- Elimination.
- Substitution.
- Engineering controls.
- Administrative controls.
- Personal protective equipment (PPE).

Each level of control must be the primary method of control unless proven infeasible. Metalcasters should note that even if engineering controls cannot reduce exposure levels to below the PEL, they must be used to the extent they can lower exposures and before PPE may be used to reduce silice exposures.

Air silica is present in work areas as a combination of source and fugitive concentrations.

Potential sources of exposure in a sand casting facility include:
- Transporting and handling new and recycled silica sand.
- Molding and coremaking processes.
- Shakeout and handling castings following shakeout.
- Sand reclamation.
- Transporting, staging and remelting scrap.
- Cleaning and finishing castings.
- Furnace and ladle relining.
- Housekeeping.
- Dust collection system maintenance.
- Fugitive concentrations are uncaptured pollutants in the background air surrounding the work station.

Full-shift TWA sampling typically takes one data point per the person being sampled during an 8-hour timeframe and is good for determining whether an area is compliant with the PEL. However, it does not differentiate source from fugitive concentrations. Real-time sampling collects potentially thousands of data points per worker and is useful in determining root causes of exposure.

Steps to Root Cause Assessment Using Real-Time Monitoring
1. Industrial hygiene exposure assessment.
The first step in any root causes evaluation for silica exposure is the collection of exposure compliance samples. These samples are full-shift TWA samples collected using OSHA Method ID142 and employing 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. Table 1 gives an example of results from an industrial hygiene assessment.

If exposures are detected that approach or exceed the OSHA PEL, metalcasters are advised to proceed directly to real-time engineering sampling methods.

2. Establish boundary conditions for root cause sampling.
Many air contaminant sources can potentially contribute to the air contaminant concentrations present at workstations in metalcasting facilities. Cross-contamination produces a masking effect that can confound efforts to identify root causes of exposure present at workstations.

Respirable dust mapping of the entire facility or of the general area of the workstation in question can detect defects and deficiencies such as inefficient source control, malfunctioning equipment and cross-contamination.
Dust mapping starts with the plant layout (Fig. 2). Column locations should be targeted, if possible, while source concentrations near workers and intermittent activities should be avoided. 

Recording measurements should have short measurement intervals. The amount of time will depend on the meter being used.

Once the measurements are recorded, contouring the results will give a snapshot of concentrations (over the course of multiple iterations) and identify fugitive sources.

Other methods for establishing boundary conditions include:
- Air mass balance analysis and ventilation pattern analysis to detect ventilation defects such as concentration buildup, cross-contamination and stagnation.
- Evaluation of the effectiveness of measures to control air contaminants at the source to identify inefficient source control or malfunctioning equipment.

3. Gather real-time samples.
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. As this device will measure both the source and fugitive 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 workstation. As work begins, the observer needs to continuously make written notes of tasks and conditions sampled, noting the starting and finishing times associated with each of these tasks and conditions.

Once a representative duration of monitoring has been performed, the collected data has been downloaded and calibration of the data has been completed, data analysis can begin. 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. The exposure attributable to each item is calculated by averaging the digital data during the noted time period of that item.

CASE STUDY: Isolating Root Cause of Exposure in Chipping and Grinding
Ferrous casting finishing workers typically use several different types of portable tools to perform chipping and grinding on castings of varying geometry: cup, cone and wheel grinders, and various chisels.

In a case study, two real-time monitors were used—one placed on the employee and the other located in the area representative of the background air. The real-time monitor logged a digital measurement at a preset short interval. It should be noted that in the upper graph, a series of spikes occurred in the personal sampling data. These spikes are associated with elevated concentrations of exposure. The impact on overall exposure of spikes or any elevation in concentration depends on both the magnitude of the elevated level as well as the duration. Some of the spikes are of such short duration that they do not contribute meaningfully to the overall exposure TWA. On the other hand, sustained periods of elevated dust exposure are of particular interest in a root cause evaluation. The important parameter in evaluating elevated exposures is the area under the curve. The larger the area under the curve, the larger the contribution to the worker’s exposure.

To identify which tasks 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.

During a root cause evaluation, detailed notes are collected from multiple variables, including the specific positioning of the tools. Because of this, the data collected during cup and cone grinding can be broken down further, yielding information, for example, on specific cup grinding activities.

This investigation shows that cup grinding on the top of the casting contributed a substantial portion of the employee’s exposure during use of the cup grinder. It was visually observed during the assessment that, when the cup grinder was used on the upper portions of the casting (the top and the upper edge), the grinding swarf produced often was not directed in an effective manner at the exhaust hood. This was due to the grinding surface of the cup grinder rotating on the casting surface being cleaned, dispersing the grinding swarf in all directions. Additionally, the exhaust of this tool was observed to disperse grinding dust in all directions.

Subsequent engineering and work practice controls can be implemented with a defined goal of reducing exposure during specific tasks that have been shown to contribute significantly to overall exposure. Additionally, the real-time data can serve effectively as a baseline exposure to establish a metric by which to compare the effectiveness of implemented controls.  

Note: This information in this article is updated from an article that ran in the December 2014 issue of Modern Casting and a presentation (17-109) given at the 2017 Metalcasting Congress.

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