Apparent evaporative resistance at critical conditions for five clothing ensembles
A limiting factor for clothing ensembles inherent during heat stress exposures is the evaporative resistance, which can be used to compare candidate ensembles and in rational models of heat exchange. In this study, the apparent total evaporative resistance of five clothing ensembles (cotton work clothes, cotton coveralls, and coveralls made of Tyvek® 1424 and 1427, NexGen® and Tychem QC®) was estimated empirically from wear trials using a progressive heat stress protocol and from clothing insulation adjustments based on ISO 9920 (2007) and wetness. The metabolic rate was moderate at 165 W m−2 and relative humidity was held at 50%. Twenty-nine heat-acclimated participants (20 men and 9 women) completed trials for all clothing ensembles. A general linear mixed effects model (ensemble and participants as a random effect) was used to analyze the data. Significant differences (p < 0.0001) among ensembles were observed for apparent total evaporative resistance. As expected, Tychem QC had the highest apparent total evaporative resistance at 0.033 kPa m2 W−1. NexGen was next at 0.017 kPa m2 W−1. These were followed by Tyvek 1424 at 0.015 kPa m2 W−1, and Tyvek 1427, Cotton Coveralls and Work Clothes all at 0.013 kPa m2 W−1. This wear test method improves on past methods using the progressive protocol to determine evaporative resistance by including the effects of movement, air motion and wetness on the estimate of clothing insulation. The pattern of evaporative resistance is the same as that for critical WBGTs and a linear relationship between apparent total evaporative resistance and WBGT clothing adjustment factor is suggested. With the large sample size, a good estimate of sample variance associated with progressive method can be made, where the standard error is 0.0044 kPa m2 W−1 with a 95% confidence interval of 0.0040–0.0050 kPa m2 W−1.
KeywordsHeat stress Protective clothing Evaporative resistance
We would like to thank all of the participants and study staff. This research was supported by a grant from the National Institute for Occupational Safety and Health (1R01 OH03983); and its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIOSH, CDC, the United States Air Force or US Department of Defense. Clothing was provided by DuPont, Kappler and Red Kap Industries, and beverages were provided by Gatorade.
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