College of Public Health, University of South Florida, 13201 Bruce B Downs Boulevard, Tampa, FL 33612, USA.
Ann Work Expo Health. 2017 Jul 1;61(6):611-620. doi: 10.1093/annweh/wxx034.
Heat stress exposure limits based on wet-bulb globe temperature (WBGT) were designed to limit exposures to those that could be sustained for an 8-h day using limited data from Lind in the 1960s. In general, Sustainable exposures are heat stress levels at which thermal equilibrium can be achieved, and Unsustainable exposures occur when there is a steady increase in core temperature. This paper addresses the ability of the ACGIH® Threshold Limit Value (TLV®) to differentiate between Sustainable and Unsustainable heat exposures, to propose alternative occupational exposure limits, and ask whether an adjustment for body surface area improves the exposure decision.
Two progressive heat stress studies provided data on 176 trials with 352 pairs of Sustainable and Unsustainable exposures over a range of relative humidities and metabolic rates using 29 participants wearing woven cotton clothing. To assess the discrimination ability of the TLV, the exposure metric was the difference between the observed WBGT and the TLV adjusted for metabolic rate. Conditional logistic regression models and receiver operating characteristic curves (ROC) along with ROC's area under the curve (AUC) were used. Four alternative models for an occupational exposure limit were also developed and compared to the TLV.
For the TLV, the odds ratio (OR) for Unsustainable was 2.5 per 1°C-WBGT [confidence interval (CI) 2.12-2.88]. The AUC for the TLV was 0.85 (CI 0.81-0.89). For the alternative models, the ORs were also about 2.5/°C-WBGT, with AUCs between 0.84 and 0.88, which were significantly different from the TLV's AUC but have little practical difference.
This study (1) confirmed that the TLV is appropriate for heat stress screening; (2) demonstrated the TLV's discrimination accuracy with an ROC AUC of 0.85; and (3) established the OR of 2.5/°C-WBGT for unsustainable exposures. The TLV has high sensitivity, but its specificity is very low, which is protective. There were no important improvements with alternative exposure limits, and there was weak evidence to support metabolic rate normalized to body surface area. In sum, the TLV is protective with an appropriate margin of safety for relatively constant occupational exposures to heat stress.
基于湿球黑球温度 (WBGT) 的热应激暴露限值旨在限制在 60 年代林德 (Lind) 有限数据基础上能够持续 8 小时的暴露。一般来说，可持续暴露是指可以达到热平衡的热应激水平，而不可持续暴露是指核心温度持续升高。本文探讨了 ACGIH®阈限值 (TLV®) 区分可持续和不可持续热暴露的能力，提出了替代职业暴露限值，并询问是否需要对体表面积进行调整以改善暴露决策。
两项渐进性热应激研究提供了数据，涉及 29 名穿着梭织棉服的参与者在不同相对湿度和代谢率下进行的 176 次试验，其中有 352 对可持续和不可持续暴露。为了评估 TLV 的区分能力，暴露指标是观察到的 WBGT 与代谢率调整后的 TLV 之间的差值。使用条件逻辑回归模型和接收者操作特征曲线 (ROC) 及其曲线下面积 (AUC) 进行分析。还开发了四种替代职业暴露限值模型并与 TLV 进行了比较。
对于 TLV，不可持续暴露的优势比 (OR) 为每 1°C-WBGT 2.5（置信区间 [CI] 2.12-2.88）。TLV 的 AUC 为 0.85（CI 0.81-0.89）。对于替代模型，OR 也约为 2.5/°C-WBGT，AUC 在 0.84 到 0.88 之间，与 TLV 的 AUC 显著不同，但实际差异很小。
本研究（1）证实 TLV 适用于热应激筛查；（2）用 ROC AUC 为 0.85 证明了 TLV 的区分准确性；（3）确定了不可持续暴露的 2.5/°C-WBGT OR。TLV 具有高灵敏度，但特异性非常低，具有保护作用。替代暴露限值没有重要改进，并且有微弱的证据支持代谢率与体表面积归一化。总之，TLV 对于相对稳定的职业热应激暴露具有保护作用，且安全裕度适当。
