Heat stress causes mental fatigue and physical exhaustion OSHA fact sheet on working outdoors safely Many studies have shown that temperature exceeding a person’s normal skin temperature (90º to 95º at the surface) will cause mental fatigue and physical exhaustion. A NASA study concluded that when the temperature reaches 95º for an extended period of time, people could make 60 mistakes per hour without realizing it. In addition to external heat sources, protective clothing, heavy gear, and heavy outdoor work can cause heat stress. Outdoor workers face many physical challenges in the summer. An OSHA fact sheet reminds workers to be safe in hot weather, avoiding heat stress, sunburn, ticks and other insects, dehydration, and other dangers.