Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands. For cooling, the heart begins to pump more blood, blood vessels dilate to accomodate the increased flow, and the bundles of tiny capillaries threading through the upper layers of skin are put into operation. The blood is circulated closer to the skin’s surface, and excess heat drains off into the cooler atmosphere. At the same time, water diffuses through the skin as perspiration. The skin handles about 90 percent of the body’s heat dissipating function.
Sweating, by itself, does nothing to cool the body, unless the water is removed by evaporation, and high humidity retards evaporation. Under conditions of high temperature and high relative humidity, the body wants to maintain the 37°C inside. The heart is pumping a torrent of blood through dilated circulatory vessels; the sweat glands are pouring liquid, including essential dissolved chemicals, like sodium and chloride onto the surface of the skin. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body’s inner core begins to rise and heat-related illness may develop. Death rates can increase markedly as a result of heat waves, and the peaks correlate with maximum daily temperature 1-2 days before death; that is, there is a 1-2 day lag between the hottest temperatures and the peak in death rate. Illness (heat stroke, heat exhaustion, etc.) may occur in healthy people who are overexposed to, or overactive in, the heat. However, the majority of excess deaths that occur during heat waves are primarily from other illnesses in which heat stress accelerates death. Infants, the elderly, and people already ill, in particular those with circulatory problems, are most at risk during excessive heat.