Controlling Exposures to Air Contaminants
Tom Slavin and Robert Scholz
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Protecting worker health from potentially harmful airborne exposures requires recognition, evaluation and control of hazards. The traditional industrial hygiene hierarchy of control specifies hazard elimination first, followed by engineering controls, administrative controls and personal protective equipment (PPE). This provides an overly simplified approach to an often complex problem of defining and implementing effective and reliable measures to control air contaminant exposures in metalcasting facilities. Protecting workers from exposure to air contaminants requires a performance-based approach to decision making, which adds additional considerations to the long-standing industrial hygiene decision model.
Because each control method has advantages and disadvantages, a performance-based model does not automatically rank approaches but rather focuses on the application of each technique to the situation at hand. In some cases, combinations of controls or redundant controls may be appropriate. An exposure control scheme is sought to achieve the highest level of exposure control performance, reliability and effectiveness. This article discusses a step by step method in which each exposure situation is first approached by developing an appropriate base of information concerning the hazards to be controlled and the circumstances under which workers are exposed. The sources of air contamination affecting the exposures are then identified, followed by sectioning candidate exposure control measures that address these sources. Finally, the expected performance of each potential control measure can be assessed using performance factors applied to each potential control method. What results is a proposed control strategy for that situation which then can be further assessed with regards to technical and economic feasibility.
The health and well-being of the workers who produce castings requires that the air they breathe while performing their jobs is not harmful to their health, safety and well-being. A key goal for air quality management is the indoor air environment satisfies metalcasting ventilation needs and air quality regulations in a safe, effective and cost-efficient manner.
The starting point for an indoor air management program should be the undertaking of an exposure hazard assessment that is plant-wide. A needed outcome of an exposure hazard assessment is to identify jobs where measures to control exposure of workers to air contaminants need improvement. Once the need has been established, a variety of guidance can be applied to plan and implement improved exposure control measures.
A guidance model for selecting exposure control measures which has been used for many years by the industrial hygiene profession is the “Hierarchy of Exposure Controls.” The term “hierarchy” connotes a series of systematic groupings in graded order. A typical example is the following ranking published by the National Institute for Occupational Safety and Health (NIOSH):
Elimination is seen as the most effective control. Elimination includes design of a process to eliminate hazards. For example, a shut off valve that can only be accessed by climbing a ladder can be moved to provide access at ground level in order to eliminate a fall hazard. Elimination tends to be the most difficult to implement in an existing process. In the design or development stage, elimination of hazards may be simple and inexpensive to implement. However, for an existing process, major process or equipment changes may be required to eliminate a hazard.
Substitution can be used to replace the hazard with alternative methods or materials, thereby reducing the risk of harm from hazards. For example, substituting less hazardous materials or materials with a lower vapor pressure to limit evaporation may reduce hazardous inhalation exposure. For existing processes, substitution may require equipment or process modification.
Engineering controls can be used to isolate people from the hazard. Barriers or guards can be used for mechanical hazards. Local exhaust ventilation can be used to remove airborne contaminants from workers’ breathing zones. Engineering controls are preferred over administrative and personal protective equipment (PPE) because they are designed to control the hazard at the source, before it comes in contact with the worker, and are typically independent of worker intervention to achieve protection. The initial cost of engineering controls can be higher than the cost of administrative controls or PPE, but over the longer term, operating costs are frequently lower.
Administrative controls are used to change the way people work and include a broad range of control measures such as training, job rotation, work procedures, warning signs, temporary barricades and inspections. Administrative controls can be relatively inexpensive to establish, but require a high level of effort to sustain.
PPE such as safety eyewear, respirators, hard hats, aluminized clothing, gloves and safety toed shoes can protect the worker but require the greatest amount of worker intervention to be effective. PPE is also often subject to fit problems and maintenance issues. From the standpoint of the hierarchy, PPE is considered the least desirable control. Nevertheless it is one of the most common control techniques employed and there are good reasons for that.
All versions of the hierarchy of controls are based on assumptions and generalizations about effectiveness and reliability. The bases of the hierarchies are that control methods at the top or preferred end of the hierarchy are potentially more effective and protective than those at the bottom. Measures that require human intervention, such as warnings, training, or PPE, are seen as less effective than measures that do not require human intervention. Measures that can be defeated or circumvented are less preferable to those that are integral. Selecting risk reduction measures in the order of preference suggested by the hierarchy is expected to lead to more effective controls and implementation of inherently safer systems, where the risk of illness or injury is minimized.
OSHA requires industry to work through the development and implementation of exposure control measures to achieve effective and reliable performance from them “to the extent feasible.” If feasibility turns out to be limited, PPE is necessitated going forward. In a dynamic foundry production environment, an exposure control development and implementation program must be carefully managed and monitored. Until that is established, PPE stands as the primary assurance of exposure control, not as a recourse measure at the bottom of the hierarchy rankings.
Although these generalizations can form the basis of a useful control strategy, they are not infallible and must be applied in context. To begin with, the exposure and risk must be understood. Does the risk warrant control? In some cases, the risk may not require control and in other cases properly addressing the risk may call for redundant controls, especially where the likelihood and potential consequence of control failure are significant. In addition, the sources of exposure must be identified and understood if control measures are to be effective and reliable. Controls will not be effective if applied to the wrong exposure source or causal condition. Existing control measures should be evaluated to confirm that they are capable and performing at their design parameters.
Selecting exposure control measures for metalcasting applications based on a simple ranking provides insufficient guidance to address the complexities of the foundry work environment:
In metalcasting operations, several sources of air contamination may impact the exposure levels of particular workers, resulting in the potential need for a strategy involving the interworking of multiple control measure types.
The processes, equipment and materials employed in casting production are not amenable to extensive changes to effectively accomplish exposure reduction to a significant extent through elimination and substitution.
For many current metalcasting operations which extend across wide areas of the plant, may be mobile in nature or require extensive worker involvement, effective engineering control measures such as ventilation have not been developed to the extent that they can address many current process operations.
Ventilation control of air contaminants at or near their sources is potentially the most effective form of ventilation. However, this exposure control method would not apply if its application caused the creation of a new safety or health hazard, interference with production, or unacceptability to workers.
A stepwise performance-based approach has been formulated for use in directing efforts to reduce worker exposures to air contaminants in foundries, with guidance starting at initial exposure hazard assessment through evaluation of the performance of installed exposure control measures.
ACTION ITEM 1: CONDUCT
AN EXPOSURE HAZARD ASSESSMENT
Determine compliance status with standards for worker exposures to air contaminants.
Identify jobs where exposure controls need improvement.
An assessment should be done that considers the risk to workers (both severity of potential harm and likelihood of exposure.)
Poorly understood single exposure measurements are not a sound basis for risk or controls decisions. The exposure hazard assessment should take into account the expected variability in exposures measured in metalcasting environments. NIOSH has recommended a statistical method for analyzing exposure results in industrial environments which takes variability of exposure sampling results into account. Variability in exposure sampling results can occur from the following sources:
Normal metalcasting operations that occur regularly and have expected outcomes.
Situations occurring in metalcasting operations that deviate from the norm.
Differences among workers.
Differences in the work environment.
ACTION ITEM 2: DETERMINE THE SOURCES OF AIR CONTAMINATION
Document the work tasks associated with each job identified as needing exposure control improvements, noting any special circumstances surrounding the work tasks and work environment which could affect worker exposure.
Determine the potential sources of air contamination associated with each work task which can result in worker exposure while performing the job.
Table 1 can be used to summarize the source identification process for specific worker exposure situations. In a source assessment, you need to know the work task being conducted, portion of the work shift involved with the task, potential sources of exposure, and circumstances surrounding the exposure.
Effective and reliable exposure reduction requires the control measures to directly address the sources of air contamination. Some of these potential sources will be apparent during exposure sampling activities in Action Item 1. However, in many cases where exposure sampling results are quantified in terms of time-weighted-average (TWA) exposures over the work shift, the key contributors to the measured average concentrations may not be apparent.
The manner in which air contaminants from multiple sources are dispersed can complicate pinpointing the sources with the greatest impact on worker exposures. Because the concentrations of air contaminants are highest near the points of generation, workers directly involved with those processes stand the highest risk of being exposed. As air contaminants migrate away from their sources, their concentrations decrease. The contributions to worker exposures from background air contamination could ultimately comprise a significant portion of the worker’s total exposure.
Several different air sampling techniques can be employed as needed to locate air contaminant sources and assess their impacts on worker exposure levels as well as to provide a broad-based view of air quality in the facility:
1. Gathering area samples concurrently with exposure samples.
2. Mapping air contaminant concentrations throughout an area.
3. Determining sources of air contamination of exposures associated with job tasks.
ACTION ITEM 3: IDENTIFY CANDIDATE EXPOSURE CONTROL MEASURES FOR SPECIFIC JOB TASKS.
Identify candidate exposure control measures for each work task with identified sources of air contamination.
Evaluate the potential use of any control measure option in light of the capabilities and limitations of the control method when applied to the situation at hand.
Candidate exposure control measures should be listed in Table 2 for each potential exposure source associated with a work task. Work practices associated with operational and maintenance tasks are critical to the performance of processes and their exposure controls and should be defined as exposure controls where they apply.
It is unlikely at the outset that one could predict with any degree of accuracy which exposure control measure or combination of measures could achieve compliance with exposure limits in a feasible manner for a specific work situation. Thus, it is advisable the initial process of identifying control options not be limited or prejudged. All of the control measure categories in the hierarchy have general capabilities and limitations which should be considered.
ACTION ITEM 4: SELECT EXPOSURE CONTROL MEASURES FOR IMPLEMENTATION
Rate the expected performance of each candidate exposure control measure based on a set of performance factors as applied to each specific case.
Select exposure control measures for implementation on the basis of the performance ratings.
A Performance Factors Checklist is presented in Table 3 as a tool to assist in the evaluation of expected exposure control performance. This set of factors relate specifically to the potential for a candidate control measure to perform effectively and reliably in controlling worker exposures to air contaminants. An implementation plan for exposure control measures should include the step of demonstrating the effectiveness and reliability of the exposure measures chosen as part of the commissioning process.
Protecting worker health by controlling air contaminants is an important objective for metalcasters. Achieving that goal requires an understanding of exposures, sources and control options. Each of these elements requires careful attention to and evaluation of data in order to make effective decisions that produce the intended result. Important opportunities to protect workers may be lost if assumptions are made about exposures based on misinterpretation of scant data, if exposure sources are not correctly identified, or if suboptimal control strategies are used based on incomplete evaluation of performance. The performance based approach to selecting exposure control methods presented here is a stepwise process designed to avoid these errors and achieve optimal worker protection.
This article is based on a paper (16-062) published in the 2016 AFS Transactions, “Controlling Exposures to Air Contaminants in Metalcasting‑A Performance Based Approach.”