As much as 2 pints of water an hour can be lost through sweat, and, particularly if the person is not accustomed to the heat, significant quantities of salt can also be lost. One of the reasons that humidity is also important is that sweating is less effective when the air is very humid. Dehydration causes further increases in pulse and body temperature. Salt deficiency causes heat cramps, nausea and vomiting. Excessive or prolonged sweating can result in sweat pores becoming blocked and ineffective - a condition called prickly heat.
Heat exhaustion causes impaired ability, dizziness, headache, vomiting and clammy skin.
All individuals will respond differently to the heat. A fit young person who is used to heat will find it easier to cope. Someone with a pre-existing heart or respiratory illness, or who is overweight, or is being treated with drugs such as tranquilisers or steroids which can affect the balance of salt and water in the body is likely to suffer more. Sweating stops, skin becomes dry and hot, body temperature increases. Other control mechanisms in the body also break down causing even further increases in temperature.