OSHA’s proposed heat rule, and the Cal/OSHA standard already in force, put the same control at the high-heat trigger: somebody watches. A buddy, a supervisor covering twenty or fewer workers, or a radio check every two hours. A study published online on Aug. 20 in the American Journal of Industrial Medicine tested whether the person being watched can even tell, and the answer isn’t reassuring.
What the study did
A University of Ottawa team put 97 healthy adults on treadmills in a climate chamber and walked them to what the literature calls initial stay time, the point where core temperature reaches 38.0 degrees Celsius, or 100.4 Fahrenheit, or rises 1.0 degree above resting. Forty-nine did moderate-intensity work at 200 watts per square meter and 48 did heavy work at 260. Conditions were wet-bulb globe temperatures of 26, 29 and 32 Celsius, which is 78.8, 84.2 and 89.6 Fahrenheit. Groups were split by age and sex.
The researchers compared two physiological measures, the Physiological Strain Index and an adaptive version of it, against the Perceptual Strain Index, which is built from thermal sensation and rating of perceived exertion. How hot you feel, and how hard the work feels.
What they found
Perceived strain did track measured strain in the aggregate. Both work intensities showed the perceptual index rising with the physiological one at p below 0.01. Agreement on any individual reading was another matter. Bland-Altman limits of agreement ran from minus 2.3 to 4.2 index units at moderate intensity and minus 3.0 to 4.0 at heavy intensity, spreads of roughly 6.5 and 7.0 points on scales conventionally running 0 to 10.
Nothing rescued it. The relationship “was not modified by group, environmental condition, body surface area, or aerobic fitness.” Age, sex, how fit the subject was and how hot the chamber got made no difference. The authors’ conclusion: “although perceptual responses increase with physiological heat strain, they do not reliably reflect physiological burden at IST, when decisions regarding the implementation of heat mitigation strategies are required.”
Why it lands on the standards
Cal/OSHA Title 8 section 3395 names construction as one of five industries subject to high-heat provisions, triggers at 95 degrees, and requires “observing employees for alertness and signs or symptoms of heat illness” through supervisor observation of 20 or fewer workers, a mandatory buddy system, or regular communication with a lone worker. OSHA’s proposed federal rule uses nearly identical architecture at its own high-heat trigger.
Every one of those controls depends on a human noticing something at the moment a decision is needed. This study says that at exactly that moment the instrument is imprecise by six or seven points on a ten-point scale, and that the worker is no better calibrated than the supervisor looking at him. The practical read is that an observation program needs a backstop that isn’t a judgment call: scheduled work-rest cycles, a fixed WBGT trigger, or physiological monitoring. That applies hardest to exterior envelope work in a southern summer, the kind of scope facing crews on the tower at the 100 North Main redevelopment in Memphis.
The limits are real and worth stating plainly. This is one study, on healthy adults, on treadmills, in a chamber. The 200 and 260 watt intensities are the authors’ own labels for moderate and heavy work, and mapping them onto a roofer in August is an inference rather than a finding. The federal heat rule is still a proposal, not a standard. The work was funded by Ontario’s Workplace Safety and Insurance Board and NSERC, which is to say a workers’ compensation insurer rather than an industry group.
If this is a subject you’re dealing with personally on a crew, the underlying paper is at doi.org/10.1002/ajim.70127 and the Cal/OSHA text is at dir.ca.gov.