THE ENVIRONMENT AND PHYSIOLOGY
Most of us know
from personal experience that environmental factors can affect how you feel
and perform your daily tasks. For example, when you step outside on a hot
and humid day, you may immediately notice an elevation in temperature, the
“heavy” quality of the air, and stickiness against your skin. But did you
know that factors like the weather can have a physiological affect on your
body?
Environmental
factors like extreme heat or cold, high altitude, and air pollution can
elevate heart rates, make it harder to breathe, and impair the ability to
exercise. A general understanding of how environmental factors can influence
your overall wellbeing and daily task performance is important for everyone;
young or old, fit or unfit. This segment discusses recent physiological
research and findings that delve into how the body adapts to short-term
(acute) and long-term (chronic) exposure to these environmental factors,
specifically temperature, altitude and air pollution.
TEMPERATURE
Humans are
homeotherms (homeo, “same,” and therm “temperature”),
which means our body temperature is regulated to remain close to a set
point—in our case, 98.6°F or 37°C. If body temperature falls too far below
or rises too far above this “normal” temperature, serious bodily injury can
result, so the body must maintain precise control over its temperature to
avoid a life threatening situation.
Extreme Heat
When your body is subjected to prolonged extreme heat, you can suffer
from heat sickness or stroke, exhaustion or dehydration. Extra caution
should be taken in high heat situations, especially when exercising or
training, because the body’s core temperature can rise when the mercury
soars.
Scientists have found that in
some cases, although it may be more uncomfortable to exercise in hot
weather, heat may not necessarily lead to a decrease in power.
“The
effects of heat stress on neuromuscular activity during endurance exercise.”
Pflugers Arch 444(6):738-43, 2002.
However, if the heat load is
high enough, performance can be negatively affected.
“Passive
hyperthermia reduces voluntary activation and isometric force production.”
Eur J Appl Physiol 91(5-6):729-36, 2004.
Extreme heat may cause a
decrease in performance due to the body’s heat production and the heat
gained by the environment. Adaptation and fluid replacement are imperative
for optimal performance under these circumstances.
“Exercise in the heat: challenges and opportunities.” J Sports Sci
22(10):917-27, 2004.
“Effect
of hydration status on thirst, drinking, and related hormonal responses
during low-intensity exercise in the heat.” J Appl Physiol 97:
39-44, 2004.
Humidity is also a factor. It can be just as disabling to those
at sea level as decreased oxygen is for people at higher elevations. The
highly water-vapor-saturated air can hinder the amount of internal heat that
you release into the environment, thereby affecting the body’s ability to
maintain its temperature. Humid air also shuts down the body’s main cooling
mechanism, the evaporation of sweat on the skin.
Taking extra steps to cool the
skin’s surface can increase comfort in hot and humid situations. For
example, new lighter materials that wick sweat away from skin’s surface can
help with cooling by facilitating better sweat evaporation. Physiologists
continue to explore new methods of artificial cooling methods and how they
can bolster performance in high temperature conditions.
“Cooling
vest worn during active warm-up improves 5-km run
performance
in the heat.” J Appl Physiol 96: 1867-1874, 2004.
Artificial cooling technologies may also have a great benefit for
spinally-injured athletes, who have a higher risk of heat strain and
illness. “Effects
of two cooling strategies on thermoregulatory responses of tetraplegic
athletes during repeated intermittent exercise in the heat.”
J Appl Physiol 98: 2101-2107, 2005.
Speculation that women experience thermoregulatory differences at various
stages of the menstrual cycle has existed for some time. Some evidence
suggests that female hormones may help to start the cooling process early
during heat exposure, resulting in greater heat loss. Only recently have
studies begun to examine such questions.
“Heat acclimation and physical training adaptations of young women using
different contraceptive hormones”
Am J Physiol Endocrinol Metab 288: E868-E875, 2005.
Extreme Cold
Very cold environments can also cause physiological stress to the body.
The main priority in chilly climates is maintaining body heat to avoid
conditions like hypothermia and frostbite. When coupled with low
temperatures, factors like wind and moisture can make cold weather even more
treacherous. Physiologists study how the body reacts to extremely low
temperatures, how people can best function in the cold, and what we can
learn from animals that function well in these environments (like
hibernators).
Like extreme heat, extreme cold has the potential to decrease human
performance. A drop in core temperature can lead to extensive bodily
damage.
Lowering of skin temperature decreases isokinetic maximal force production
independent of core temperature. Eur J Appl Physiol.
91(5-6):723-8, 2004.
By generating heat during exercise, the body may be able to produce
enough heat to delay hypothermia and survive in extreme cold.
“Wet-cold
exposure and hypothermia: thermal and metabolic responses to prolonged
exercise in rain.” J Appl Physiol 81: 1128-1137,
1996
Additionally, hormones play a
key role in helping to regulate the body’s response to extreme cold.
“Temperature
regulation during rest and exercise in the
cold in premenarcheal and menarcheal girls” J
Appl Physiol 96: 1393-1398, 2004.
And unlike extremely hot conditions, cold weather can sometimes be a
preserving influence, slowing reactions down enough to pause extensive
bodily damage for a period of time. For example, sudden exposure to extreme
cold (like falling through the ice into freezing water) may cause “metabolic
arrest”. This slowing of the metabolism could act to preserve life until a
drowning victim is resuscitated.
“Cold
stress, near drowning and accidental hypothermia: a review.” Aviat
Space Environ Med. 71(7):733-52, 2000.
Contrary to some beliefs, acute exposure to cold may have a stimulating
effect on the immune system.
“Immune
changes in humans during
cold
exposure: effects of prior heating and exercise.”
J Appl Physiol 87: 699-710, 1999.
Hibernating animals adjust their physiology to increase the
tolerance of their organs to very low temperatures. This behavior could
lead to advances in various biomedical treatments, including human organ
preservation.
“Could
Hibernators Hold The Key To Improving Organ Preservation?”
“Comparative,
Ecological and Evolutionary Physiology: Nature’s Solutions to Biomedical
Problems”
“Mammalian hibernation: cellular and molecular responses to depressed
metabolism and low temperature”.
Physiological Reviews
83: 1153-1181, 2003
ALTITUDE
Each year, more and more people go to high
altitudes to participate in recreational activities such as skiing, hiking,
and camping. The primary concern with activities at high altitude (5000
feet - the approximate elevation of Denver, Colo. - and above) is that lower
pressure in the atmosphere limits the amount of oxygen transported in the
blood. This decrease in oxygen results in a reduction in oxygen transport to
the tissues (hypoxia), and therefore both mental and physical activities can
be affected.
Anyone who spends time at high elevations can experience the symptoms of
mountain sickness, which can include a continuous dry cough, shortness of
breath, dizziness, headache, fatigue, a rapid pulse, etc. But altitude has
the greatest effect on individuals who are physically active. Generally, at
altitudes above 5000 feet, higher elevation leads to a reduction in exercise
tolerance.
Reduced oxygen at altitude causes about one half of highly conditioned
ski athletes to develop airway constriction after a race and most do not
realize the problem.
“Undiagnosed
Exercise-Induced Bronchoconstriction in Ski-Mountaineers.” Int J
Sports Med. 26(3):233-7, 2005.
More commonly, rapidly ascending, non-acclimatized individuals experience
fluid collection in the lungs, a serious consequence of going to high
altitudes. It is difficult to predict who will develop this condition
(pulmonary edema) on a trek to extreme altitude. But studies suggest that
blood pressure in the pulmonary artery may be a good predictor.
“Identification
of individuals susceptible to high-altitude pulmonary oedema at low
altitude.” Eur Respir J. 25(3):545-51, 2005.
“Physiological
aspects of high-altitude pulmonary edema.” J Appl Physiol 98:
1101-1110, 2005
And recent evidence suggests that a naturally occurring substance, Ginkgo
Biloba, reduces the severity of acute mountain sickness in humans and also
reduces high altitude pulmonary edema in rats.
“Ginkgo
biloba extract prevents high altitude pulmonary edema in rats.” High
Alt Med Biol. 2004 5(4):429-34, 2004.
The hypoxia associated with extreme altitude also has dire consequences
for cerebral circulation. This reduces oxygen supply to the central nervous
system and may cause a variety of neuropsychological impairments.
“Effects
of high altitude exposure on cerebral hemodynamics in normal subjects.”
Stroke. 36(3):557-60, 2005.
“Cognitive
and emotional processing at high altitude.” Aviat Space Environ Med.
76(1):28-33, 2005.
Though high altitude has a number of negative effects on the body,
physiologists have observed that chronic hypoxia can provide a long-term
benefit to the cardiovascular system, including long-lasting
cardioprotection.
“Effect
of sustained hypobaric hypoxia during maturation and aging on rat
myocardium. I. Mechanical activity.” J Appl Physiol 98:
2363-2369, 2005.
AIR POLLUTION
Air pollution is a growing problem in many
parts of the world. In fact, according to the World Health Organization,
air pollution and smog will cause an estimated 8 million avoidable deaths by
the year 2020. In an effort to protect citizens from air pollution, many
cities monitor air quality and issue health alerts when it is poor. Stage 1
health alerts are issued when ozone reaches 0.2 ppm (parts per million), and
stage 2 alerts are issued at 0.35 ppm. These alerts suggest that anyone with
lung problems, such as asthma, should not exercise outdoors. Many large
metropolitan areas now have stage 1 alerts on more than 100 days out of the
year. Although the long-term effects of ozone exposure are not clear, recent
research suggests that chronic exposure to ozone results in diminished lung
function. The articles listed below address the physiological response to
acute and chronic exposure to pollutants.
Air pollution is also a known trigger for asthma and the type of
particulate matter commonly found in urban air can sometimes lead to more
severe asthma attacks, hospitalization or death.
“Airway
responsiveness after acute exposure to urban particulate matter 1648 in a
DO11.10 murine model.” Am J Physiol Lung Cell Mol Physiol 286:
L337-L343, 2004.
“Road-traffic
pollution and asthma – using modelled exposure assessment for routine public
health surveillance.” Int J Health Geogr. 3: 24, 2004.
Additionally, research shows that increased levels of particulate matter
in the air affect not only lungs, but the skin and the cardiovascular system
as well. “Cardiovascular
pathophysiology of environmental pollutants.” Am J Physiol Heart
Circ Physiol 286: H479-H485, 2004.
It is important for all individuals to understand how the body reacts to
the environment. This knowledge will lead to smarter, safer activity
outdoors and increased awareness of physiological stress incurred by one’s
surroundings. Physiologists will continue to study how external factors
affect us internally and how we can best adapt to the world around us.
To speak with a physiologist about comparative physiology or any of the
research discussed above, please contact Donna Krupa (301) 634-7209).
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