Relationships between wet bulb globe temperature and evaporative cooling responses under heat stress

Monica Rodriguez, Purdue University

Abstract

The aim of the present study was to compare the evaporative losses response in a sample of thirty unacclimatized subjects (sixteen males and fourteen females), ages 19 to 44. Most of the participants considered themselves physically active, while six of them perceived themselves as sedentary. Participants performed three, ten-minute exercise trials of moderate, intermittent exercise on a wooden step in an environmentally controlled chamber at an average temperature of 84.31 ± 1.8 °F, 42 ± 10% of relative humidity, and 73.46 ± 1.5 °F of wet bulb globe temperature (WBGT). Repeated measurements of transepidermal water loss (TEWL) were performed on the subjects' forearm prior to and post exercise. The routine procedures of this study included: a brief interview with the subject which included the reading and signing of the consent form, a baseline TEWL measurement in the neutral environment, a second TEWL measurement inside the controlled room environment prior to exercise, and a third TEWL measurement inside the controlled room environment post exercise. Each subject performed three consecutive exercise trials, meaning that pre- and post- exercise TEWL measurements were taken a second and third time as well. TEWL readings differed significantly between subjects and showed significant trial-to-trial variation. Total TEWL levels prior to exercise (overall mean for the 3 exercise trials = 26.73 g/m2h) were significantly higher than total TEWL levels post exercise (overall mean for the 3 exercise trials =11.67 g/m2h) under thermal stressed conditions inside the environmentally controlled chamber. It was not until the last of three ten-minute time frames that the post exercise TEWL level (27.09 g/m 2h) varied, indicating that lesser cooling actually took place via evaporation over time. In comparison, post exercise TEWL levels for the first and second exercise trial time frames measured 16.08 g/m2h and 28.56 g/m 2h respectively. There was evidence that there is a gender difference in TEWL levels for unacclimatized participants when exercising under heat stress controlled conditions. At similar heat stress levels, men experienced a greater evaporative cooling response evidenced by greater TEWL levels (overall mean for the 3 exercise trials = 46.33 g/m2h without baseline = 52.83 g/m2h), compared to women (overall mean for the 3 exercise trials = 17.43 g/m2h without baseline = 19.20 g/m2h). However, there was not enough evidence to conclude that there were any significant differences in TEWL levels when observing age and activity levels for unacclimatized participants. The VapoMeter™ (Delfin Technologies Ltd, Kuopio, Finland) measured TEWL with consistency during the exercise trials and yielded values similar to those previously reported in the literature. It was easy to use and yielded information that appeared consistent with less user-friendly devices and invasive techniques. Therefore, the potential of TEWL measurements to be used as an additional and practical tool for routine monitoring may be helpful as part of an established heat stress prevention program in sports and occupational settings. However, for use in clinical studies, the significant variations across subjects would make changes induced by predisposing conditions difficult to reliably detect. Evaporative losses were affected by the exercise intensity, heat acclimation, and individual characteristics of the participants. Sweat rate varied greatly between participants across the exercise trials. Even though sweat rate depends upon a variety of factors including wind speed, ambient, temperature, and relative humidity, there was not a clear relationship between the environmental conditions and TEWL, which strongly indicates that subjects’ evaporative cooling responses depended on possible interference factors stemming from inter-individual variability. These findings along with other studies raise the question as to whether or not it is appropriate to rely solely on the use of the WBGT thermal risk marker for the prevention of the occurrence of heat-related illness under different combinations of temperature and relative humidity.

Degree

Ph.D.

Advisors

Jackson, Purdue University.

Subject Area

Environmental Health|Public health

COinS