Hot Days & Hot Heads
Understanding heat illness
Across the thundering
heat of an African savanna, a million years ago, give or take a few hundred
centuries, runs something you would recognize instantly as almost human.
It runs to live. Not fast, and not quick, it will run, with short breaks,
for nearly 24 hours. In the end it will catch and kill a prehistoric antelope,
using its bare hands as weapons.
In order to survive as a species, humans adapted in their own special
ways, with virtually hairless skin filled with abundant sweat glands and
powered by a cardiovascular system of marvelous endurance. Those same
additions allow you spring and summer. You are not however, a foolproof
design. Oveheating can ruin your day, and your life.
To understand heat illness, let's start with the fundamentals of human
thermal dynamics, body heat production vs. body heat loss. There are two
major internal sources of heat. Even as you sit quietly reading these
words, you are making heat via basal metabolism, the energy necessary
to sustain your life at complete rest. When you leave the house and start
exercising, a second heat source kicks in, exercise metabolism. Strenuous
exercise metabolism may produce 15 to 18 times the amount of heat of basal
metabolism, depending on your level of fitness. From external sources,
the human body can absorb heat, from things such as the sun, a fire, and
the ingestion of hot drinks. You end up with far more heat than you need
and, if you couldn't shed the excess, you would literally cook in your
Human heat is lost in four ways: conduction, radiation, convection, and
evaporation. Conduction, heat loss through direct contact with something
cooler than you, does not help much on a hot day. On a desert-like afternoon,
you may actually take in heat from a hot environment. Radiation is energy
lost directly from your skin's surface and, as the air warms up around
you, it may effectively stop on a sunny summer day. Convection is heat
loss through the movement of air around your body. Without wind, convective
heat loss practically stops when you're not moving, and it stops when
the air temperature reaches approximately 92 degrees F.
That leaves evaporation, the vaporization of sweat from your skin, the
primary source of heat loss for the human. As your skin heats up, pores
dilate and sweat floods out. Evaporation of the sweat cools your skin,
heat is drawn from your blood near the surface of your body, and the cooler
blood circulates to keep your insides maintained at an acceptable temperature.
In other words, if you don't sweat, you ain't gonna make it.
Sweat comes from your circulatory system, and it's not uncommon to sweat
out a liter of water in an hour during periods of exercise in a hot environment.
This water loss may reach two-and-a-half liters per hour with prolonged
exercise. And sweat contains salt, a critical component of normal body
function. It is this combined water and electrolyte depletion that forms
the basis of a spectrum of problems with one general name: heat illness.
Although you are unique in the animal world in your ability to shed excess
heat, if body heat production gets a jump on body heat loss, you are heading
toward heat illness. On the minor end of the problem are heat cramps,
a painful spasm of major muscles that are being exercised. Those most
often cramped are people unacclimatized to heat who are sweating profusely.
Heat cramps are poorly understood, but probably result not only from the
water lost in sweat, but also the salt lost in sweat. Gentle massage and
stretching of the affected muscles usually provides relief. Drinking water,
preferably with a pinch of salt per liter added, is advisable. Heat cramps
do not often occur in someone who is adequately hydrated. Once the pain
is gone, exercise may be continued if necessary, but a day of rest is
Prolonged sweating may move you along the spectrum to heat exhaustion,
characterized by headache, dizziness, nausea, rapid breathing, and, of
course, exhaustion. Sufferers are so sweaty they often feel cool, grow
goose bumps and complain of chills. Treatment should include moving the
exhausted person to a shady spot and oral rehydration with cool, very
slightly salty water. Some experts prefer using an electrolyte-balanced
drink such as Gatorade, but the drink should be watered-down three or
four times for more rapid absorption in a resting person. Maximum absorption
ranges from 150 to 250 ml per 15 minutes, so it takes about an hour to
get a liter back into circulation. Heat exhaustion is not physiologically
damaging, but it should be treated aggressively before it progresses to
a more serious condition.
On the serious end of the spectrum lies heat stroke, a problem that kills
approximately 4000 people in the United States every year. There are two
varieties of heat stroke. In classic heat stroke, the patient is usually
elderly or sick, or both. Temperature and humidity have been high for
several days, and the patient has dehydrated to the point where his or
her heat loss mechanisms are overwhelmed. You might say they simply run
out of sweat. Skin gets hot, red, and dry. They lapse into a coma and,
if untreated, die.
But more and more people are being killed of the second variety, exertional
heat stroke. The victim is usually young, fit, and unaccustomed to heat,
sweating but producing heat faster than it can be shed. Signs include,
primarily, a sudden and very noticeable alteration in normal mental function:
disorientation, irritability, combativeness, bizarre delusions, incoherent
speech. Skin is hot and red, but wet with sweat. Rapid breathing and rapid
heart rates are almost universal. Collapse is imminent.
Quick cooling may be required to saver this victim's life, and the best
method includes removal of clothing, covering with wet cotton, and vigorously
fanning, all of which increases evaporative heat loss. Massaging of arms
and legs and ice packs at the neck, groin, and armpits increase heat loss.
Throwing patients into cold water is less effective and often dangerous
since they are difficult to manage and may drown. Heat stroke victims
should be seen by a physician as soon as possible, even if they seem to
have recovered. Too much internal heat can cause breakdowns in some body
systems that show up later.
ACCLIMATIZATION TO HEAT
Acclimatization usually takes 2 to 3 weeks.
Exercise in the early morning and late evening for the first week.
Work your way gradually toward midday exercise.
On spectacularly hot days, only exercise early and late.
Drink one-fourth liter of cold water every 15 minutes during intense exercise.
Drink at least three liters of water every day.