Abstract The problem of assessing the thermal impact of an industrial situation is complex because of the multiplicity of other stresses which may be present in the environment. To define the problem requires, as a minimum, data on the climatic environment of the work, the demands of the job, the daily work-rest regimen, the heat exposure history, the health and nutritional status, the state of body hydration, and the non-working physical environment and activities of the individuals. A study of industrial heat stress conducted by this Division incorporated a simple standard laboratory type heat-work test along with an exhaustive study of the men at the work site. The physiological responses of the men to the standard tests were significantly correlated with the responses on the job and reflected the magnitude of the on-the-job environmental stress.