There is a daily loss on the order of approximately 500 ml of water through the skin. However, when the body is exposed to a heat stress and behavioral and vasomotor mechanisms are insufficient to prevent a rise in temperature, the physiological responses generally include an increase in sweat production in an attempt to prevent hyperthermia; the high latent heat of vaporization of water makes the evaporation of sweat an effective heat loss mechanism (evaporation of 11 of water from the skin surface will remove 2.4 MJ (580kcal) of heat from the body). The heat stress may be of external origin (i.e., from the environmental conditions), of internal origin due to muscular work or fever, or from a combination of these factors.
In many individuals sweat rates can be in excess of 2 l per hour, especially in situations of exercise undertaken in a warm, humid environment, and these high sweat rates can be maintained for a number of hours. For example, body mass losses in marathon runners have been reported to range from about 1-6% (0.7-4.2 kg) at low (10 °C) ambient temperatures to more than 8% (5.6 kg) in warmer conditions. However, when sweat rates are high, a significant fraction of the sweat secreted onto the skin may drip from the body and is therefore ineffective at removing heat.
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