HT30 V3.0 08/07

9

How Heat Affects the Body

Human bodies dissipate heat by varying the rate and depth of blood circulation, by losing water through the

skin and sweat glands, and -- as the last extremity is reached -- by panting, when blood is heated above

98.6 degrees. The heart begins to pump more blood, blood vessels dilate to accommodate the increased

flow, and the bundles of tiny capillaries threading through the upper layers of skin are put into operation.

The body's 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

relative humidity retards evaporation. The evaporation process itself works this way: the heat energy

required to evaporate the sweat is extracted from the body, thereby cooling it. Under conditions of high

temperature (above 90 degrees) and high relative humidity, the body is doing everything it can to maintain

98.6 degrees 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.

Too Much Heat

Heat disorders generally have to do with a reduction or collapse of the body's ability to shed heat by

circulatory changes and sweating, or a chemical (salt) imbalance caused by too much sweating. 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.

Ranging in severity, heat disorders share one common feature: the individual has overexposed or

overexercised for his/her age and physical condition in the existing thermal environment.

Sunburn, with its ultraviolet radiation burns, can significantly retard the skin's ability to shed excess heat.

Studies indicate that, other things being equal, the severity of heat disorders tend to increase with age --

heat cramps in a 17-year-old may be heat exhaustion in someone 40, and heat stroke in a person over 60.

Acclimatization has to do with adjusting sweat-salt concentration, among other things. The idea is to lose

enough water to regulate body temperature, with the least possible chemical disturbance.

Cities Pose Special Hazards

The stagnant atmospheric conditions of the heat wave trap pollutants in urban areas and add the stresses

of severe pollution to the already dangerous stresses of hot weather, creating a health problem of

undiscovered dimensions. A map of heat-related deaths in St. Louis during 1966, for example, shows a

heavier concentration in the crowded alleys and towers of the inner city, where air quality would also be

poor during a heat wave.

The high inner-city death rates also can be read as poor access to air-conditioned rooms. While air-

conditioning may be a luxury in normal times, it can be a lifesaver during heat wave conditions.

The cost of cool air moves steadily higher, adding what appears to be a cruel economic side to heat wave

fatalities. Indications from the 1978 Texas heat wave suggest that some elderly people on fixed incomes,

many of them in buildings that could not be ventilated without air conditioning, found the cost too high,

turned off their units, and ultimately succumbed to the stresses of heat.