HT30-EU-EN V3.4 5/13
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Heat Index / Heat Disorders
Heat Index / Heat Disorders
Heat
Index
Possible heat disorders for
people in higher risk groups
130°F or
higher
Heatstroke/sunstroke
highly likely with continued exposure.
105° -
130°F
Sunstroke, heat cramps or heat exhaustion likely, and heatstroke
possible with prolonged exposure and/or physical activity.
90° -
105°F
Sunstroke, heat cramps and heat exhaustion possible with prolonged
exposure and/or physical activity.
80° - 90°F Fatigue possible with prolonged exposure and/or physical activity.
The "Heat Index/Heat Disorders" table relates ranges of HI with specific disorders, particularly for people in the higher
risk groups.
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.
