Tolerance to high temperatures was studied. Twelve heat-acclimatized males were each exposed to heat stress for eight hours a day in 24 conditions. The four environmental temperatures were 33 ET* (30°C, 80% rh), 35 ET* (35°C, 50% rh), 36 ET* (40°C, 30% rh), and 38 ET* (50°C, 10% rh). The six work-rest schedules were 90% work-10% rest, 78% work-22% rest, and 67% work-33% rest, all with both hot and cool rests. Four subjects wore clothing with .57 clo, four with .83 clo, and four with .93 clo.

Rise in rectal temperature of more than 1.1°C was the least important reason for time lost (2.2% of scheduled time). More important were miscel­laneous (mostly high blood pressure), 3.6%; headache/nausea/vomiting, 3%; and heart rate over 160 beats a minute with 2.5%. If the environmental temperature was over 35 CET*, productive time was higher for 22% scheduled rest than for 10% or 33% scheduled rest. At no temperature was 33% scheduled rest best. Cool rest areas had no appreciable benefit on work time, unless the work environment temperature was over 35 CET*.

In general, clothing did not affect physiological responses, although the jumpsuit (.84 clo) was preferred over the two shirt and trousers combinations (.57 and .93 clo).