WBGT and Work/Rest Schedules – Two Useful Tools in Heat Injury Prevention
With OSHA’s National Emphasis Program (NEP) on heat stress prevention, the Advance Notice of Proposed Rulemaking (ANPR) to initiate the rulemaking process towards a federal heat standard, some state OSHA plans already having heat injury prevention standards, and the need to protect workers from heat stress and heat injuries in both indoor and outdoor work environments, the OSH Professional will need to develop a toolbox from which to protect the workers. Two such tools are the wet bulb globe temperature (WBGT) and the work/rest schedule. Using these two tools along with other tools such as acclimatization, rest/shade, and hydration, will help the OSH Professional guide the employer in preventing heat injuries.
Since OSHA began to focus efforts on preventing heat injuries, OSHA has used the National Weather Service (NWS) Heat Index that only works in outdoor environments and cannot be modified to use indoors. An OSH Review Commission Administrative Law Judge, in July of 2020, ruled in one case that the NWS Heat Index has no scientific basis of the heat risk categories and could not be used (in the specific case) as evidence of a violation.
The wet bulb globe temperature (WBGT) can be used indoors and outdoors. It is used by the US military and college athletic programs to determine the workloads the workers or athletes are placed under and the guidelines under which to place them. By using the WBGT and developing company-specific, geographic area-specific work-rest schedules, along with other such tools, OSH professionals will be able to prevent heat injuries.
The WBGT and the use of work/rest schedules is discussed on the OSHA Heat Illness Prevention web pages and on OSHA’s Safety and Health Topics page for Heat. Between OSHA’s information, the information provided by the ACGIH in the 2022 TLV and BEI documents, NIOSH, and the US military, there is a significant amount of information for the OSH Professional to use and examples on which to base their own programs. However, many OSHA Professionals have not used the WBGT or work/rest schedules as part of their heat injury prevention program.
OSHA is moving more to using the NWS Heat Index as a screening tool that would indicate when the employer should start applying their heat injury prevention policy which includes the conducting of a heat hazard assessment. The heat hazard assessment involves the use of the Effective WBGT (from ACGIH) and the company-specific, geographic area-specific work-rest schedules. The Effective WBGT is the temperature from the WBGT and then adding a Clothing Adjustment Value (CAV). This adjusted value considers that the clothing a worker is wearing, and additional PPE add to the potential heat stress. Geographic area-specific work-rest schedules consider if the work is being done indoors or outdoors, in the north where there are not as many hot days or in the south where there are many more hot days.
OSH Professionals need to develop their workload categories and associated task lists based upon ACGIH/OSHA guidance and examples, then determine the Effective WBGT ranges per their geographic area and the associated risk levels which are provided in the ACGIH/OSHA guidance that include the associated work/rest ratios. The work/rest schedule is based upon the workload categories that could be Light, Moderate, and Heavy, and Very Heavy per ACGIH and OSHA guidance or Easy, Moderate, and Hard as used in the US military work/rest schedules or otherwise be developed by the employer. These workload classifications in ACGIH and OSHA guidance are based upon the metabolic rate given in watts for each of their workload categories. ACGIH gives some basic descriptions of tasks performed in each of their four work categories. OSHA guidance further lists examples of specific tasks further expanding on the ACGIH basic description. Using these lists, OSH Professionals can develop the list of tasks that best fit into each of the workload categories, so they are representative of the tasks their workers perform.
With the development of the workload categories and using the Effective WBGT, OSH Professionals then needs to determine if the workers are acclimatized or unacclimatized. Using the Effective WBGT, find the range in which their Effective WBGT temperature is in, look at the associated workload category, and the correlating work/rest ratio or schedule to determine based upon all of this information how long a worker can work and how long a worker should rest in the shade along with hydration to protect a worker from heat stress and reduce the potential of a heat injury.
Learning about the WBGT and how to develop then apply work/rest schedules as part of a heat injury prevention program is important for an OSH Professional. We need to better understand and develop new tools using technology such as waterless WBGT monitors, well-established examples and guidance on work/rest schedules, and understanding the differences in the determination of the WBGT in an indoor environment versus an outdoor environment. OSH Professional with practical knowledge of the WBGT, determining the Effective WBGT, establishing work categories, and the development and application of the work/rest schedule following ACGIH and OSHA guidance, OSH Professionals will be more able to prevent heat injuries.
For more information and/or assistance, contact:
Wayne Vanderhoof CSP, CIT
RJR Safety Inc.