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Heat

excerpt from, Heat Production(, Heat Illnesses)

Heat Production, Heat Illnesses, and First Aid (see link above)

A body at work generates heat faster than at rest, often more than needed. Roughly three-fourths of the stored energy the body draws on during activity converts to heat rather than motion, and more strenuous activity naturally generates more heat. Elevation of core body temperature disturbs functioning, so the body protects itself by dissipating excess heat. The mechanisms of vasodilation and sweating are critical to moving heat from a human body to the environment.

When the body’s core temperature exceeds its norm (generally 98.6 degrees), veins and capillaries expand, the heart beats faster, and blood flow increases to outer layers of skin, from which heat is radiated to the cooler exterior environment. If, however, the body cannot cool fast enough through this means, or when surrounding air is warmer than the skin, the brain signals sweat glands to release sweat to the skin surface. Evaporation of the sweat carries additional heat from the body. Because high humidity decreases the sweat evaporation rate, it slows cooling. After becoming “acclimatized” to a hot environment over time, people sweat more readily and thus cool more efficiently.

These cooling mechanisms, however, can impair strength and comfort. Increasing blood flow to the body surface reduces the volume carrying oxygen to muscles, brain, and other internal organs, which in turn accelerates fatigue and diminishes mental alertness. The loss of water volume through sweating also contributes to fatigue by increasing blood viscosity, making it more difficult for the heart to pump and reducing the body’s capacity for subsequent cooling. Finally, because prolonged sweating depletes not only heat but also electrolytes that are needed for muscle functioning, it can cause cramping.

To maintain comfort and health when working in a hot environment, it is critical for people to replace both the water and electrolytes they lose through sweating. If body fluid is not replenished at the same rate as it is lost, or if replacement lacks electrolytes, the cooling mechanisms lose effectiveness and exposure to heat stress rises.