Short-Term Heat Stress Exposure Limits Based on Wet Bulb Globe Temperature Adjusted for Clothing and Metabolic Rate

Abstract

Most heat stress exposure assessments based on wet bulb globe temperature (WBGT) consider the environmental conditions, metabolic demands, and clothing requirements, and the exposure limit is for extended work periods (e.g., a typical workday). The U.S. Navy physiological heat exposure limit (PHEL) curves and rational models of heat stress also consider time as a job risk factor so that there is a limiting time for exposures above a conventional WBGT exposure limit. The PHEL charts have not been examined for different clothing and the rational models require personal computers. The current study examined the role of clothing in short-term (time limited) exposures and proposed a relationship between a Safe Exposure Time and WBGT adjusted for clothing and metabolic rate. Twelve participants worked at a metabolic rate of 380 W in three clothing ensembles [clothing adjustment factors]: (1) work clothes (0°C-WBGT), (2) NexGen microporous coveralls (2.5°C-WBGT), and (2) vapor-barrier coveralls (6.5°C-WBGT) at five levels of heat stress (approximately at the clothing adjusted TLV plus 7.0, 8.0, 9.5, 11.5 and 15.0°C-WBGT). The combinations of metabolic rate, clothing, and environment were selected in anticipation that the participants would reach a physiological limit in less than 120 min. WBGT-based clothing adjustment factors were used to account for different clothing ensembles, and no differences were found for ensemble, which meant that the clothing adjustment factor can be used in WBGT-based time limited exposures. An equation was proposed to recommend a Safe Exposure Time for exposures under 120 min. The recommended times were longer than the PHEL times or times from a rational model of heat stress.

Keywords:

• heat stress
• time limit
• WBGT

ACKNOWLEDGMENTS

This research was supported by a grant from the U.S. National Institute for Occupational Safety and Health (1R01 OH03983). We wish to thank the participants and laboratory staff for their cooperation in a very demanding study.

Notes

^A Least square means exclude data from NexGen at Heat Stress Level L2 (shaded cell). Least square means account for missing data and represent the basis for multiple comparison testing.