Benchmarking Heat Index as an occupational exposure limit for heat stress

Abstract

While wet bulb globe temperature (WBGT) is the long-accepted index to represent the environmental contributions to heat stress, Heat Index (HI) is a commonly reported index and is used for heat stress guidance. The purpose of this article was to propose an HI-based heat stress exposure limit. The data came from previous progressive heat stress studies that identified the critical conditions between sustainable and unsustainable exposures. The experimental trials included five clothing ensembles at three levels each of relative humidity (rh) and metabolic rate (M). The critical Heat Index (HI_crit) was used to characterize the trial exposure. An analysis of variance (ANOVA) assessed the effects of M, clothing, and rh on HI_crit. After proposing a relationship between HI_crit and M to represent a benchmark exposure limit based on HI (called HI_bel), the ability of the proposed relationship to discriminate between Sustainable and Unsustainable conditions was assessed using receiver operating characteristics curves (ROC curves). Based on the ANOVA results, the main effects of M, rh, and clothing on HI_crit were significant; the interaction between rh and clothing was not significant. There were differences in mean HI_crit among all the ensembles. For effects of relative humidity on HI_crit, the mean HI_crit at rh at 20% was 3 °C lower than the mean values for 50% and 70%. The benchmark exposure limit from the woven clothing data was HI_bel [°C] = 49–0.026 M [W]. In terms of the ability of HI_bel to discriminate, area under the ROC curve was 0.86, which was similar to WBGT-based exposure limits. Similar in purpose for WBGT-based exposure assessment, HI clothing adjustment values (HI_cav) of 1.5 °C (particle barrier coveralls), 6 °C (water barrier coveralls), and 18.5 °C (vapor barrier coveralls) were supported. It should also be noted that the effects of the sun and lack of acclimatization were not included in this analysis; where the sun might reasonably increase the effects of the ambient HI by an additional 3.5 °C and being unacclimatized by 5.5 °C.

Keywords:

• Clothing
• exposure limit
• Heat Index
• heat stress

Acknowledgments

The data used in this study were collected under CDC/NIOSH R01-OH03983. The authors recognize and thank the many laboratory assistants and trial participants who made this study possible. We also wish to acknowledge the support of Jeffrey Shire of CDC/NIOSH.

Conflicts of Interest

One of the authors (Bernard) has acted as an expert witness for both private companies and OSHA in litigation concerning heat stress exposures and may in the future serve as an expert witness in court proceedings related to heat stress. The authors declare no conflict of interest relating to the material presented in this article. Its contents, including any opinions and/or conclusions expressed, are solely those of the authors.

Additional information

Funding

Dr. Garzón was supported by the Republic of Ecuador through the Ecuadorian Institute of Human Talent Development. Funding for this project was provided by CDC/NIOSH (R01-OH03983 and a services contract).