This article is from the Aerobics Fitness FAQ, by firstname.lastname@example.org (Robles) with numerous contributions by others.
Extremes of temperature and humidity pose special problems
for the exerciser. In hot weather, care must be taken to wear
clothing that is light, breathes well, and allows for the
evaporation of sweat.
"Sauna suits", "tummy wraps", and other products designed
to encourage quick weight loss through sweat are particularly
dangerous - the body can reach dangerous (or even fatal) core
temperatures in very short periods of time. Weight lost by
these methods will be regained as soon as water is ingested
again, and so the risk outweighs any "benefit".
On extremely humid days care must be taken to exercise at
an appropriately lowered intensity, out of the high heat/humidity,
or even to postpone exercise until the heat/humidity diminish.
As exercise intensity increases and more heat must be dissipated,
evaporation of sweat becomes the principal means by which cooling
occurs. In a high-humidity environment, evaporation
becomes less effective at cooling, and the risk of heat-related
injury is greater.
Adequate hydration is also key to safe exercise in the heat,
as the body will produce large quantities of sweat. 1-2 cups
of water before exercise and 1/2-1 cup of water during exercise
are recommended, though more can be ingested.
It is important to remember that the thirst mechanism lags
behind the body's need for fluid - by the time one is thirsty
one is already substantially dehydrated. Even small amounts of
dehydration can affect performance, and severe dehydration can be
Contrary to popular belief, water consumed during exercise will
not contribute to cramping, so "swish and spit" should be avoided in
favor of consuming small amounts of water steadily during the
exercise session, especially during periods of prolonged exercise.
In the cold, care must be taken as well. It is best to dress in
layers that will wick sweat away from the body - many of the
"high-tech" fabrics that are now available will do this admirably.
Outer layers can be used to keep the body warm during warm-up, and
removed as exercise progresses to allow the body to cool itself,
and then be replaced during the cool-down to avoid an excessive
Garments made of fabrics like wool, which will insulate even
when wet, are superior to garments made of materials like cotton,
which will contain sweat and can contribute to heat lost by
evaporation and conduction as the activity level decreases.