Tracking Down Respirable Silica Exposure

Rebecca Ferrell and Robert Scholz
Click here to see this story as it appears in the February 2018 issue of Modern Casting

The reduced OSHA exposure limits for silica have elevated the need for sampling tools such as real-time personal and area sampling.  Real-time sampling can better pinpoint respirable silica sources that will need to be controlled to meet the revised standard and to help determine the feasibility and potential effectiveness of candidate exposure control options. It is non-productive to keep guessing on actions to be taken based primarily on time-weighted average (TWA) data that determine whether you are over or under an OSHA exposure limit for a full shift. The industry needs more guidance that can identify specific tasks or activities as root causes that can be assessed to develop control options. Real-time respirable particulate matter monitors are a potential solution to this problem. Although there is currently no real-time monitor specific to silica available, industrial hygiene consultants have already employed the respirable particulate matter instruments at some foundries to assist in identifying root causes of exposure and to provide guidance for exposure control in certain job categories. The personal air sampling method which has been used was described in the January 2018 issue of Modern Casting magazine.

Responding to the need for guidance and better assessment tools, the AFS Research Board initiated a project in 2017 to develop and demonstrate a real-time monitoring sampling method. The goal of this project is to define the real-time personal sampling method sufficiently so individual foundries can utilize it as part of their silica exposure control programs. TRC Environmental Corp. was engaged to conduct the development and demonstration project in four foundries, who volunteered to host the demonstrations. The project is ongoing with an expected completion date in the middle of this year.

Foundries can use a systematic approach to address worker exposures to a potential exposure hazard such as respirable silica. The approach revolves around a holistic assessment that will ultimately require inputs and involvement of all key manufacturing functions in the foundry, including process, manufacturing and facility engineering; maintenance; environmental, safety and health management; human resources; supervisors and workers.

Identification as a Component of Silica Exposure Control
The flow chart in Figure 1 traces the steps of silica exposure control in foundries. Identification of potential silica exposure hazards begins in earnest with the first step—TWA sampling. TWA sampling needs to go beyond defining exposure risks solely by quantifying average silica exposure concentrations associated with the tasks conducted by a worker over the course of a shift.  Analysis of the risk of silica exposure also needs to focus strongly on the observations made during the sampled shift and during foundry production operations in general as to what may be causing these exposures. While respirable silica is invisible under most lighting conditions at the factory floor, most often it is dispersed along with larger, visible particles in the foundry air environment.

TWA silica exposure sampling should be conducted with existing silica exposure control measures operating as intended. This is called baseline air sampling. Exposure sampling results gathered under baseline exposure control conditions define the current exposure control capability and facilitate decision making concerning the need for improvement. Malfunctioning exposure controls mask the need for actual improvements and can lead to unnecessary changes. 

To obtain better understanding of their TWA results, metalcasting facilities should be sampling employee positions more than one time. The workplace is variable from day to day and that variability can impact sampling results. Foundries should sample work situations to the extent that the amount of, and reasons for, sampling variability are understood.

Use of real-time, engineering sampling methods expands on the identification process begun during TWA sampling. Keen observations during TWA sampling can raise questions in the mind of the observer about what the causes of the exposure could possibly be. When stated as hypotheses, (e.g., exterior door openings lower the exposures of nearby workers), the real-time sampling conditions can be set to prove or disprove that hypothesis. When hypothetical assumptions are made as part of the real-time sampling plan, the potential exists for making important determinations in an efficient manner. Real-time monitoring is deployed on specific tasks throughout the shift and rarely is done by sampling the entire shift. 

Description of the Real-Time, Personal Sampling Method
Typical real-time air sampling instruments in use by industrial hygiene consultants employ an optical sampling technique used for decades in industrial hygiene sampling. The technique is called scattered light photometry. Recently, some manufacturers of these instruments have developed new designs that have miniaturized the sampler to the extent that the instrument can be mounted to the worker’s chest, within the breathing zone. This includes the inlet cyclone or impactor (for size selection), the optical chamber, and the filter cassette (employed to calibrate instrument signal to the actual respirable particulate matter in the foundry environment).  In some instances, the battery powered air pump of these systems is either included on the chest or at the worker’s waist. Sampling instruments of this design are classified and marketed as personal, real-time, respirable particulate matter samplers. 

These new instruments hold strong potential for the continuing development of methods to evaluate worker exposures in real-time. The AFS project is employing two such commercially available instruments in the demonstrations of root causes analysis in foundries. The demonstration team is working closely with the manufacturers to assure the instruments are set up and used appropriately to obtain accurate and repeatable results.

The Importance of Concentrating on Accuracy
One of the most sought-after characteristics of a real-time sampler is the ability to provide accurate results. In particular, accuracy has significant implications when real-time instruments are employed both before and after changes to the foundry are made which can affect silica exposure concentrations. The accuracy of the before and after samples can significantly impact decisions on whether to move forward with a specific silica control measure.

Because scattered light photometers do not “see” all particle sizes the same, calibration of real-time scattered light photometers to the actual foundry environment being tested is essential for accuracy. Photometers are factory calibrated with a defined particle size distribution, referred to as “Arizona road dust.” Using a calibration filter, the conversion factor is developed using the recorded real-time respirable particulate matter concentration data, averaged over the same run time as the filter. Table 1 shows the particle types routinely present in the foundry. Foundry dust is quite similar to Arizona road dust; however, metal fume and smoke have many particles that are much smaller than dust particles and have a different calibration factor than dust. The AFS demonstration study is evaluating all five of the process areas in Table 1.

Accuracy was considered by the developers of the two new scattered light photometers being evaluated in the AFS project. These manufacturers mounted the photometer in the breathing zone, directly connected to the cyclone/impactors from which they receive their sample flow.

In this manner, they eliminated the need for a sampling tube to connect the cyclone/impactor to a photometer mounted at the waist. Studies have shown particulate matter can be retained on the walls of interconnecting tubing used between these two components.

The Issue of Correlation
To better establish the validity of the alternative real-time silica exposure assessment method, the correlation between respirable silica and respirable particulate matter concentration needs to be strengthened. Up until now, the average silica percentage associated with a worker’s shift-long exposure has been used in converting real-time, personal respirable particulate matter concentrations to real-time silica concentrations. The industrial hygienists employing the alternative real-time method in the past were well aware of the pitfalls of using a conversion factor based on a constant ratio between the percentage of silica in the inhaled air and the concentration of all of the remaining particulate matter in the air. For this reason, the typical use of the alternative method was on exposures to silica that were received by individuals who primarily performed manual tasks on processes that generated and dispersed silica dust. 

The manual process tasks assessed include processes from the categories listed in Table 1, as well as manual maintenance tasks such as furnace and ladle relining, return sand handling and clearing one’s work station of spilt sand and dust buildup. For these processes, the hypothesis that respirable particulate matter could be a surrogate for silica appeared to be appropriate for use in isolating the probable causes of silica exposure. The validity of this approach has been borne out more and more by empirical findings, which have demonstrated that implementing improved silica exposure controls to address specific root causes defined by the alternative real-time method either successfully reduced silica exposures or flagged root causes for which feasible control measures were not available. As an example, use of the alternative real-time sampling method has been able to show in multiple foundries that sufficient protection of workers who chip and grind ferrous castings with portable tools is not feasible when conducting finishing activities inside castings ventilated by sidedraft or downdraft exhaust hoods at their work benches. 

The challenge to protect workers properly against exposure to hazardous air contaminants deserves the best thought process we can give it. Achieving this mission relies on the quality of information we can acquire. As in any profession, the health and safety field needs tools to help assess risks caused by health and safety hazards and to provide direction for identifying root causes of exposure and implementing effective exposure controls.    

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