The National Weather Service considers heat waves a national problem since heat kills by taxing the human body beyond its capacity to properly maintiain its optimal core body temperature of 98.6°F (37°C). Studies of heat waves in urban areas have shown an association between increases in mortality and increases in heat. These risks increase when the humidity is above 70% and the temperature is higher than 70°F (21°C). According to the National Weather Service, In a normal year about 175 Americans succomb to the demands of summer heat. In the 40 year period from 1936 through 1975, nearly 20,000 people were killed in the US by the effects of heat and solar radiation. In the heat wave of 1980, more than 1250 people died. After a 5-day heat wave in 1995 in which maximum temperatures in chicago ranged from 93° to 104°F (34° to 40°C), the number of deaths increased 85% over the number recording during the same period the year before.
The National Weather Service devised a Heat Index, in order to be able to alert more effectively the general public and appropriate autorities to the dangers of heat waves - periods with prolonged excessive heat and humidity.
The Heat Index, sometimes referred to as the "Apparent Temperature", is an accurate measure of how hot it really feels when the effects of the relative humidity (RH) is added to the actual air temperature.
Here are excerpts from the National Weather Service document Heat Wave Links:
Heat Index/Heat Disorders
||Possible heat disorders for people in high risk groups
|130°F or higher
(54°C or higher)
|Heat stroke or sunstroke likely.
||105 - 129°F
(41 - 54°C)
|Sunstroke, muscle cramps, and/or heat exhaustion likely. Heatstroke possible with
prolonged exposure and/or physical activity.
|90 - 105°F
(32 - 41°C)
|Sunstroke, muscle cramps, and/or heat exhaustion possible with prolonged exposure and/or
||80 - 90°F
(27 - 32°C)
|Fatigue possible with prolonged exposure and/or physical activity.
Summary of NWS's Alert Procedures
The NWS will initiate alert procedures when the HI is expected to exceed 105 degrees to 110 degrees F
(depending on local climate) for at least two consecutive days. The procedures are:
- Include HI values in zone and city forecasts.
- Issue Special Weather Statements and/or Public Information Statements presenting a detailed discussion of
- the extent of the hazard including HI values,
- who is most at risk,
- safety rules for reducing the risk.
- Assist state/local health officials in preparing Civil Emergency Messages in severe heat waves.
Meteorological information from Special Weather Statements will be included as well as more detailed medical
information, advice, and names and telephone numbers of health officials.
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 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 concentrations, 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 1980 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.
Preventing Heat-Related Illness
Elderly persons, small children, chronic invalids, those on certain medications or drugs (especially tranquilizers and anticholinergics), and persons with weight and alcohol problems are particularly susceptible to heat reactions, especially during heat waves in areas where a moderate climate usually prevails.
Know These Heat Disorder Symptoms
- SUNBURN: Redness and pain. In severe cases swelling of skin, blisters, fever, headaches. Ointments for mild cases
if blisters appear and do not break. If breaking occurs, apply dry sterile dressing. Serious, extensive cases should be seen by physician.
- HEAT CRAMPS: Painful spasms usually in muscles of legs and abdomen possible. Heavy sweating. Firm pressure on cramping muscles, or gentle massage to relieve spasm. Give sips of water. If nausea occurs, discontinue use.
- HEAT EXHAUSTION: Heavy sweating, weakness, skin cold, pale, and clammy. Pulse thready. Normal temperature
possible. Fainting and vomiting. Get victim out of sun. Lay down and loosen clothing. Apply cool, wet cloths. Fan or move victim to air conditioned room. Sips of water. If nausea occurs. discontinue use. If vomiting continues, seek immediate medical attention.
- HEAT STROKE or SUN STROKE: High body temperature (106 degrees F or higher). Hot dry skin. Rapid and strong pulse. Possible unconsciousness. HEAT STROKE IS A SEVERE MEDICAL EMERGENCY SUMMON EMERGENCY MEDICAL ASSISTANCE OR GET THE VICTIM TO A HOSPITAL IMMEDIATELY. DELAY CAN BE FATAL. Move the victim to a cooler environment. Reduce body temperature with cold bath or sponging. Use extreme caution. Remove clothing, use fans and air conditioners. If temperature rises again, repeat process. Do not give fluids.
You can use this calculator for temperature conversion, windchill, relative humidity, and the Heat Index.
Heat Safety Rules
About 175 people die in this country every year due to excessive heat. When the Heat Index gets above 90 then it is time to take some precautions:
- Slow down. Strenuous activities should be reduced, eliminated, or rescheduled to the coolest time
of the day. Individuals at risk should stay in the coolest available place, not necessarily
- Dress for summer. Lightweight, light-colored clothing reflects heat and sunlight, and helps your
body maintain normal temperatures.
- Put less fuel on your inner fires. Foods (like proteins) that increase metabolic heat production also
increase water loss.
- Drink plenty of water or other non-alcohol fluids. Your body needs water to keep cool. Drink
plenty of fluids even if you don't feel thirsty. Persons who
- have epilepsy or heart, kidney, or liver disease,
- are on fluid restrictive diets, or
- have a problem with fluid retention should consult a physician before increasing their
consumption of fluids.
- Do not drink alcoholic beverages.
- Do not take salt tablets unless specified by a physician. Persons on salt restrictive diets should
consult a physician before increasing their salt intake.
- Spend more time in air-conditioned places. Air conditioning in homes and other buildings markedly
reduces danger from the heat. If you cannot afford an air conditioner, spending some time each day
(during hot weather) in an air conditioned environment affords some protection.
- Don't get too much sun. Sunburn makes the job of heat dissipation that much more difficult.
Know these heat disorder symptoms
- Sunburn - Redness and pain. In severe cases swelling of skin, blisters, fever and headaches.
- Heat Cramps - Painful spasms usually in muscles of legs and abdomen possible. Heavy sweating.
- Heat Exhaustion - Heavy sweating, weakness, skin cold, pale and clammy. Fainting and vomiting.
Normal temperature possible.
- Heat or Sun Stroke - High body temperature (106 or higher). Hot, dry skin. Rapid and strong pulse.
Task performance in hot weather
Of interest is a report by Ramsey J.D., Ergonomics 1995 Jan; 38(1):154-65, who summarized approximately 160 individual studies of perceptual motor performance in the heat and concluded taht "for perceptual morot tasks other than very simple or mental tasks, an onset of performance decrement was noted in the 86° to 91° F(30° to 33° C) range of temperature."
- Heat Wave Links National Weather Service, Southern Region Headquarters
- Heat Safety Rules National Weather Service, Southern Region Headquarters
- Killing Heat The New England Journal of Medicine, July 11, 1996
- Trouble in Paradise
- The Potential Health Impacts of Climate Variability and Change for the United States: Executive Summary of the Report of the Health Sector of the U.S. National Assessment
- How does hot weather affect the heart? American Heart Association
- Heat-Related Illness: What You Can Do To Prevent It
- When Summertime Gets Too Hot to Handle U.S. Food and Drug Administration
- Outdoor Action Guide to Heat-Related Illnesses & Fluid Balance Princeton University Outdoor Action
- Hyperthermia, A Hot Weather Hazard for Older People
- Battling the Elements Safely University of Maine Cooperative Extension
- Climate change and the human health in Europe. British Medical Journal, June 19, 1999
- Stay Cool and Beat the Heat Safely American College of Emergency Physicians
- What about..? Do I need to take extra salt on hot days when I sweat a lot? No.. Mayo Clinic Health Oasis
- Heatstroke - Deadly threat in hot weather May Clinic Health Oasis, July 20 1999.
- Sports safety for summer Health Answers
- "Heat -- related illnesses and deaths - United States, 1994-1995.". MMWR Morb. Mortal. Wkly Rep. 1995 Jun 30; 44(25):465-8.
- "Biometorological classification of daily weather types for the humid tropics." Int. J. Biometeorology 1998 Dec; 42(2):77-83.
- "Task performance in heat: a review.", Ergonomics 1995 Jan; 38(1):145-65.