Heat stress conditions are prevalent in the working environment around the world. Often they are not readily engineered out. Administrative controls and, in extreme/toxic environments, personal protective gear are the means available to protect workers. For every combination of metabolic work rate, clothing ensemble and environmental WBGT, there is a time of exposure threshold, beyond which the worker can no longer compensate for the heat stress, and signs and symptoms of heat strain appear. Increasingly, worker environments require specialty clothing either for worker protection or to maintain a clean/sanitary environment. Prior to the publication of the USF safe exposure time equation, no simple method was available for determining safe worker exposure time based on a clothing adjustment factor. To demonstrate the validity of the USF SET equation, both direct and indirect data from different environments, metabolic rates, and clothing ensembles were collected to compare observed tolerance times to the predicted safe exposure time. Statistical analysis was performed using the Kolmogorov-Smirnov test. The USF SET equation predicted an acceptable safe exposure time, 19 % of the trials. Based upon this data, the USF safe exposure time heat stress equation over estimates safe exposure time for workers in hot environments, in various clothing ensembles at various metabolic work rates.