Human bodies loose heat by changing the rate and depth at which the blood is circulated and by water loss through the skin and sweat glands. To cool the body, the heart begins to pump more blood, blood vessels dilate (expand) 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 is responsible for about 90 percent of the body’s heat loss function. Sweating alone does nothing to cool the body. For cooling to occur, the sweat must be lost by evaporation from the surface of the skin. High humidity conditions retard this process of evaporation.
Under conditions of high temperature and high relative humidity, the body aims to maintain an internal temperature of 37°C. To do this, the heart pumps more blood through dilated blood vessels and sweat glands pour liquid, which includes essential dissolved chemicals like sodium and chloride, onto the surface of the skin.
When more heat enters the body than 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 illnesses 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. Illnesses such as heat stroke and heat exhaustion can occur in healthy people who are overexposed to, or are overactive in the heat. However, the majority of excess deaths that occur during heat waves are actually the result of other illnesses, which are exacerbated by heat stress. Children, the elderly and people who are already ill, particularly those with circulatory problems, are most at risk during excessive heat.