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Making heat stress assessment relevant again.

Bernard TE
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-003983, 2005 Jan; :1-48
Occupational heat stress occurs in hot work environments, during heavy work, while wearing protective clothing, or from any combination of these. The exposures may be routine for those in hot industries like metal and glass manufacturing to intermittent like emergency response. Workers frequently experience heat exhaustion and are at risk for exertional heat stroke. They are also more likely to make mistakes and suffer injuries. The usual method to assess heat stress is to report the environmental conditions in an index called wet bulb globe temperature (WBGT). An occupational exposure limit can be set as an upper limit on WBGT, where the limit is modified for different work demands. The limit assumes continuous exposures over the day. This research addressed two shortcomings of this approach. First, the occupational exposure limits were developed with work clothes in mind. This means that non-woven protective clothing is not accounted for; leaving it to the professional judgment of the health professional. Second, many heat stress exposures are not continuous and an open question is how long should a person work above the occupational exposure limit. A third aim of the research was to examine the resistance of the clothing to cooling by the evaporation of sweat. The evaluation of clothing for use in the standard heat stress evaluation method was very productive. Factors were recommended for four common clothing ensembles that could be added to the ambient WBGT to reflect the added heat stress of the clothing. The adjustment applies to a wide range of work demands and environments. The recommended adjustment factor for cloth coveralls is 0 (this same as work clothes); for a polyethylene-based particle-barrier coveralls, 0.5 C; for a water-barrier, vapor-permeable film laminate coveralls, 2 C; and for a limited-use vapor-barrier coverall, 10 C. Because a vapor-barrier fabric behaves much differently, the recommendation was made for a value that would be protective over a wide range of humidities. Based on trials with time-limited heat stress exposures, a curve relating WBGT to safe exposure time was generated that is protective of 95% of the people. The clothing adjustment factors account for the added heat stress in the time-limited exposures as well. In addition, the research allowed for an adjustment to the equivalent WBGT for different work demands. The curve is most applicable to safe exposure times between 20 and 90 minutes. Above 90 minutes, the occupational exposure limit for continuous work would apply. There is a tendency to be overly protective above 45 minutes and further investigations into the data from this research combined with other research is likely to compensate for this. Quantifying the resistance to cooling by sweat evaporation can distinguish among different fabrics and clothing construction. This is useful for the comparison of clothing systems. An unexpected new finding was that the resistance increases with the decrease in ambient humidity. This means that the predicted cooling is less the observed cooling. The implications for this require further study.
Heat; Heat-stress; Work-environment; Protective-clothing; Metal-industry-workers; Glass-workers; Emergency-responders; Heat-exhaustion; Heat-stroke; Occupational-exposure; Occupational-health; Risk-analysis; Risk-factors; Environmental-factors; Exposure-limits; Exposure-levels
Thomas E. Bernard, University of South Florida, College of Public Health, 13201 Bruce B. Downs Blvd., Tampa, FL 33612-3805
Publication Date
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Final Grant Report
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Priority Area
Research Tools and Approaches: Exposure Assessment Methods
Source Name
National Institute for Occupational Safety and Health
Performing Organization
University of South Florida, Tampa, Florida
Page last reviewed: September 2, 2020
Content source: National Institute for Occupational Safety and Health Education and Information Division