If the relative humidity rises to 80 percent at the same temperature, that same person may be incapacitated within 30 minutes. The body will undergo certain physiological changes to counteract heat stress. Blood flow to other organs, such as the kidneys and liver, is reduced, and the heart rate is increased so that the body can maintain an adequate blood pressure. As the heat builds up, receptors in the skin, brain, and neuromuscular system are stimulated to increase sweat production. If a person does not replace this sweat loss by drinking liquids, the body rapidly dehydrates, the rate of sweat production drops, and the body temperature increases, causing further heat injury.
Individuals vary in their response to heat stress.
Some serious reactions are heat cramps, heat exhaustion, and heatstroke. Factors that influence the physiological responses to heat stress include the amount of work that individuals perform and their physical condition as well as their ability to adapt to the environment. Old age, excessive alcohol ingestion, lack of sleep, obesity, or previous heatstroke can also diminish tolerance to heat stress. A previous episode of heatstroke can predispose an individual to repeated episodes. Heat stress not only causes general physiological changes but also results in performance impairment.