An advanced method helps metalcasters determine the causes of exposure.

Eric Pylkas, Stantec Consulting, Mequon, Wisconsin, and Robert Scholz, TRC Environmental Corp., Brookfield, Wisconsin

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Exposures to respirable crystalline silica have been regulated by the Occupational Safety and Health Administration (OSHA) since the introduction of the OSHA Act of 1970. Since that time, metalcasting facilities have worked to lower silica exposure levels to meet the OSHA permissible exposure limit (PEL), but improvements in engineering and work practice controls have not been successful in all cases. In particular, manual operations where dust is dispersed with substantial energy produced by the process itself, such as chipping and grinding castings with portable tools, have produced substantial challenges to providing exposure protection using engineering means. OSHA recently issued a proposed rule to lower the silica PEL, which would increase the challenge.

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. This method 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.

A method employing real-time exposure sampling enables metalcasters to quantify and prioritize root causes of silica exposure. It can be used to provide a basis for establishing engineering parameters for effective ventilation control of silica sources and for optimizing work practices.

Modes of Silica Exposure in Sand Casting

In a sand casting facility, silica dust sources may be distributed throughout operations, creating a situation where it is ubiquitous in the workplace environment. Potential sources of exposure include:

• Transport and handling new and recycled silica sand.
• Molding and coremaking
• processes.
• Shakeout and handling castings following shakeout.
• Sand reclamation.
• Transport, staging and remelting scrap.
• Cleaning and finishing castings.
• Furnace and ladle relining.
• Housekeeping.
• Dust collection system
• maintenance.

Each employee’s overall exposure to indoor air contaminants is a result of the sum of exposures received directly from air contaminant sources and those received from breathing the background air surrounding the workstation. Figure 1 graphically depicts these two different exposure modes. Source emissions can be directly controlled by local exhaust hoods, and the worker may or may not breathe these emissions prior to capture occurring.

The remaining fugitive emissions, which are not controlled at the source by close capture exhaust hoods, disperse into the background air, where they remain available to inhalation and are captured by local exhaust hoods (termed secondary capture) and evacuated from the building through general exhaust fans.

In the majority of root cause analysis cases, it is recommended that two real-time dust concentration measurements be made simultaneously: worker exposure measurements and measurements of background air concentration in the vicinity of the workstation being evaluated.

Real-Time Monitoring of Respirable Particulate Matter

A number of optical measurement devices are available for use in real-time root cause evaluations. One of these instruments is shown in Fig. 2, mounted on the worker’s body for exposure monitoring.
When selecting an optical device for root cause evaluation, one should put added importance on the following qualities:

Portability—the device is mounted easily on the worker’s body within his or her
breathing zone.

Active Sampling—pollutant-laden air is drawn through the unit using an external pump. An active sampling method results in quicker responses to changing particulate concentrations than a passive instrument can provide.

Logging—logging functionality is essential to time synchronization with written field notes and for field calibration of the instrument. Logging frequencies down to one second are useful.

Field Calibration—the calibration of optical particle monitors is affected by particle size distribution in the air being sampled. Optical devices often are factory calibrated to a standardized particle size distribution, such as Arizona road dust having a median particulate sizing of 5 µm. Unless calibration is done based on the actual aerosols tested, larger particles such as in grinding, or smaller particles such as in welding, are likely to result in skewed particle concentration results. The sample train of the monitor includes a PVC preweighed filter, which allows for calibration of real-time sample data.

Stepwise Approach to Root Cause Analysis

Step 1. Industrial hygiene exposure assessment.

Efforts to control worker exposures to air contaminants, including the assessment of root causes of exposure, should be preceded by a thorough industrial hygiene hazard assessment of the facility. Hazard assessment is aimed at defining workers who are at risk of being overexposed to chemical and physical hazards, including potential exposure to respirable crystalline silica. Measurement of exposures is an integral component of hazard 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.

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 probably not advance the critically needed understanding of the factors that cause the exposure to be elevated.

Step 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. Depending on the characteristics 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-contamination 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 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.

A technique has been employed successfully to minimize the impact of cross-contamination of a workstation 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 processes 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 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 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. The exposure attributable 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 currently regulates the amount of silica-bearing respirable dust. In real-time silica exposure monitoring, a real-time respirable dust monitor cannot 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 workstation in which a real-time root cause evaluation 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 of varying geometry: cup, cone and wheel grinders, and various chisels.

The 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. 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 in Fig. 4 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, such as the one highlighted in Fig. 4, 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.

The real-time data presented in Fig. 3 were gathered at a time when this casting cleaning was, by design, conducted without other silica dust producing processes operating in the vicinity of the workstation. The 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 background dust exposure for the worker.

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. In the case of the chipping/grinding operator, the chart in Fig. 5 demonstrates the percentage of time dedicated to each activity. A separate chart in Fig. 6 demonstrates the overall exposure during the same activities. In these figures, use of the cup grinder was noted to provide the highest exposure percentage relative to time, followed closely by the cone grinder.

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 (Fig. 7).

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.

While the collection of eight-hour TWA compliance samples is well-suited for determining if work exposures are in compliance with OSHA standards, real-time exposure monitoring allows for a comprehensive root cause analysis that can lead to effective and cost-efficient actions to lower worker exposures to air contaminants, such as respirable crystalline silica.

Analysis of real-time exposure sampling data, as in the example of manually chipping and grinding castings with portable tools, can assist in defining the inherent limitations of ventilation methods to fully protect workers from exposure. These findings also could provide direction for research and development efforts to improve the effectiveness of engineering measures to control exposures to air contaminants.Real-time exposure data also is used effectively as part of worker training programs. Workers can see from the real-time graphical data the impact specific work practices have on preventing spikes of elevated exposure. 

This article is based on paper 14-035 published in the 2014 AFS Transactions and presented at the 2014 AFS Environmental, Health and Safety Conference in Atlanta. The AFS Transactions paper is an original work whose genesis was in the collaboration over many years between Eric Pylkas and Bob Scholz working together in TRC Environmental Corp.’s (formerly RMT, Inc.’s) industrial consultative effort in the field of health and safety.