Like any other hazard, heat stress has to be managed by a process that begins with hazard assessment and detection. OSHA doesn’t have a heat stress standard. However, an OSHA Directive (TED 01-00-015) includes instructions to help OSHA inspectors determine whether heat stress dangers are present at a worksite. Knowing how OSHA assesses heat stress hazards is—or should be—of obvious relevance to your own heat stress control efforts.
Click here for a Heat Stress Hazard Assessment Checklist Work-Load Assessment
The first phase in assessment is consideration of the work-load for particular jobs. Under conditions of high temperature and heavy workload, the inspector should determine the work-load category of each job by averaging metabolic rates for the tasks and then ranking them:
Cool Rest Area: Where heat conditions in the rest area are different from the work area, the metabolic rate (M) should be calculated using a time-weighted average, as follows:
Equation III: 4-1. Average Metabolic Rate
|t||=||time in minutes|
A videotape may be helpful in evaluating work practices and metabolic load, according to the Directive. The Directive includes a chart listing examples of activities and a calculation:
The following table shows an assessment of work:
The Directive next addresses 5 methods that can be used to measure heat stress conditions.
Body Temperature Measurements: Instruments used to estimate deep body temperature by measuring the temperature in the ear canal or on the skin aren’t “sufficiently reliable” to use in compliance evaluations, says the Directive.
Environmental Heat Measurements: According to the Directive, environmental heat measurements should be made at, or as close as possible to, the specific work area where the worker is exposed. When a worker isn’t continuously exposed in a single hot area but moves between 2 or more areas having different levels of environmental heat, or when the environmental heat varies substantially at a single hot area, environmental heat exposures should be measured for each area and for each level of environmental heat to which employees are exposed.
Wet Bulb Globe Temperature Index: Inspectors should calculate Wet Bulb Globe Temperature (WBGT) using the appropriate formula in Appendix III:4-2 of the Directive. The WBGT for continuous all-day or several hour exposures should be averaged over a 60-minute period. Intermittent exposures should be averaged over a 120-minute period. These averages should be calculated using the following formula:
Equation III:4-2. Average Web Bulb Globe Temperature (WBGT)
For indoor and outdoor conditions with no solar load, WBGT is calculated as:
|WBGT = 0.7NWB + 0.3GT|
For outdoors with a solar load, WBGT is calculated as
|WBGT = 0.7NWB + 0.2GT + 0.1DB
|where:||WBGT||=||Wet Bulb Globe Temperature Index|
|NWB||=||Nature Wet-Bulb Temperature|
Measurement of Heat Conditions: The Directive notes that portable heat stress meters or monitors can be used to measure heat conditions and that such instruments can calculate both the indoor and outdoor WBGT index according to established ACGIH Threshold Limit Value equations. With this information and information on the type of work being performed, heat stress meters can determine how long a person can safely work or remain in a particular hot environment. Appendix III:4-2 lists an alternate method of calculation.
The Directive includes the following table illustrating permissible heat exposure TLVs:
PERMISSIBLE HEAT EXPOSURE THRESHOLD LIMIT VALUE
Other Thermal Stress Indices: The Directive refers to 2 more indices that can be used to calculate thermal stress:
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Click here for a short version of a Model Heat Stress Plan