**1.6 Thermal Exposure:**

Review of basic thermoregulatory physiology

Heat transfer mechanisms (conduction, convection, radiation,
evaporation, thermal balance equation)

Measurement of the thermal environment (including definition of Tdb,
Twb, Tbg, Dew Point, Relative Humidity, vapor pressure, WBGT, ET, wind
velocity, pumping factor, CLO value, water resistance, water
permeability, etc.)

Definition of heat and cold stress

Sources of heat and cold stress in aerospace operations

Symptoms and signs of exposure to heat and cold stress

Performance effects of exposure to heat and cold stress

Predisposing and contributing factors to heat and cold stress

Prevention and treatment of heat and cold stress

Heat-related illnesses (definitions, signs and symptoms, treatments)

Cold-related illnesses (definitions, signs and symptoms, treatments)

**Thermal Exposure**

**Section II, 1.6 BASIC THERMOREGULATORY PHYSIOLOGY.**

Humans exist as homeotherms. As such, we are able to live, work, and
play in a wide range of environmental conditions. However, unlike
poikilotherms, we must constantly adapt to the environmental conditions
so that deep core temperatures are maintained within a fairly narrow
band. When core temperature is allowed to vary significantly from the
normal band serious physiological consequences often result. The
variance from normal core temperature is small, or for only a brief
period the result is usually reversible and unlikely to be accompanied
by significant cognitive performance excursions. However, when the core
temperature variance is moderate to large or prolonged, one should
expect to encounter serious physiological changes of the irreversible
type *and* significant cognitive performance deficits. In the strictest
physiologic sense, core temperature is defined by the absolute
temperature of the blood perfusing the hypothalmus. For clinical
purposes however, the rectal temperature is used to approximate the deep
core temperature and is the source of the temperatures reported is the
medical literature concerned with thermal physiology.

Although there is abundant evidence to suggest humans evolved as
tropical beings (well developed sweating capability, very little
insulating hair), humans tolerate hyperthermia less well than
hypothermia. During hyperthermic conditions, organ injury is
recognizable when core tissue temperature reaches 41.7°C (107°F). On the
other hand, patients have been documented to survive hypothermia as low
as 9°C (48.2°F). When a significant departure from the normal core
temperature occurs, the event can usually be placed into one of three
categories:

Exposure to extreme environmental conditions overcomes the ability to
thermoregulate

Endogenous heat production overcomes ability to dissipate heat

Normal thermoregulation may be influenced by illness, drugs, or toxins.

**Human Physiologic Response to Hot Environments.** Core temperature
maintenance when working or exercising in a hot environment is dependent
upon the ability to shunt blood from internal organs to dilated
cutaneous vascular networks. Closely related is the adapted ability to
increase sweat production in proportion to the length and intensity of
physical work and heat exposure. The fundamental limitation to working
in hot environments becomes the ability to *dissipate* the induced
thermal load to maintain an acceptable core temperature. The ability to
directly dissipate heat through increases in cutaneous blood flow and
indirectly via increased sweating depends on cardiac output as
influenced by maximal heart rate, stroke volume, effective intravascular
volume, degree of renal and splanchnic vasoconstriction. This
intersection of competing demands to maintain perfusion to exercising
skeletal muscle, perfusion to dilated cutaneous vascular beds (heated
blood to radiator), increase sweating (evaporative cooling), and
maintain perfusion of cooled blood to critical organs represents a major
cardiovascular challenge. If insufficient, hyperthermia with its
associated pathologic risks will result (see below).

**Human Physiologic Response to Cold Environments.** Core temperature
maintenance when exposed to cold environments in several respects is a
reversal of what happens in response to heat exposure. There is a
shunting of blood away from the cutaneous beds and periphery. This
results in a diuresis and effective intravascular volume reduction.
Significant differences in blood temperature may develop between the
periphery and core spaces. This may become so severe that peripheral
cold induced tissue injury (see below) may occur in the absence of
central hypothermia. To a limited degree, hypothermia results in reduced
metabolic demands in cooled tissues; the metabolic rate declines about
6% for each degree centigrade decrease. This is manifest by a reduction
in tissue oxygen requirements and may be livesaving in critical tissues
such as brain, liver, and myocardium. In addition to peripheral
vasoconstriction and central shunting, the onset of cold exposure is
accompanied by violent shivering which increases skeletal muscle work
and heat production.

**HEAT TRANSFER MECHAHISMS.**

**Conduction:** The transfer of heat directly from one object to
another, from one body to another, or from the body to surrounding
fluid. Conduction is the major mechanism of heat loss encountered during
cold water immersion.

**Convection:** Heat transfer by means of a moving fluid or gas, may
occur between different parts of an organism, as in shunting from core
to periphery, or between an organism and its external environment where
heat is transferred to or from air molecules. The heat exchange between
the organism and its external environment may be amplified by fans,
pumps, wind, or body movement.

**Radiation:** Heat transfer via electromagnetic waves given off by all
bodies at all temperatures; this thermal exchange varies as the
difference to the fourth power of their absolute surface temperatures
and emissivity (matt black surfaces emit and absorb radiant heat in
greater quantities than shiny white surfaces).

**Evaporation:** Vaporization of water from the skin, the rate of this
vaporization depends on the difference in water vapor pressure between
the boundary layer of air molecules surrounding the skin and the
surrounding air in the environment. The evaporation of 1 g water
dissipates about 2.4 kJ of heat. When the water vapor pressure of the
surrounding air is high, i.e., high dew point and high relative
humidity, evaporation of water from the skin is impeded and the cooling
component contributed by sweat evaporation diminishes.

**Thermal balance equation (heat balance equation):** The exchange of
heat between the organism and its environment may be described by the
following equation: H [+]{.underline} K [+]{.underline} C
[+]{.underline} R -- E = O~H~ where

H = metabolic heat

K = conduction

C = convection

R = radiation

E = evaporation

O~H~ = organism heat load

**MEASUREMENT OF THE THERMAL ENVIRONMENT (Definitions).**

**T~db~:** Temperature, dry bulb; the temperature of a gas or mixture of
gases indicated by a thermometer shielded from radiation.

**T~wb~:** Temperature, wet bulb; the lowest temperature to which a
sample of air can be cooled by evaporating water, this term is usually
applied to the temperature recorded by a thermometer covered with a wet
sleeve and is shielded from radiation (to be an effective measure there
must be enough air movement across the wet sleeve to prevent relative
humidity saturation in the surrounding microclimate, but not enough to
cause convective heat exchange).

**T~bg~:** Temperature, black globe; temperature indicated by a
thermometer whose bulb is centered in an airtight blackened globe of
thin copper (approximately 6" diameter) exposed to the sun.

**T~co~:** Temperature, core temperature; core, or deep body,
temperature, not accompanied by any conscious sensation.

**T~sk~:** Temperature, skin; accompanied by conscious sensation which
is largely responsible for thermal comfort.

**Dew Point:** The temperature at which 100% relative humidity occurs,
at or near the dew point condensation will occur in the form of dew,
frost, fog, or clouds (or a combination).

**Relative Humidity:** The ratio (expressed in per cent) of the actual
moisture content of the air at any given time and place to the maximal
possible moisture content for the same temperature and barometric
pressure.

**Heat Index:** The heat index is the temperature the body feels when
heat and humidity are combined (see chart below).

**WBGT:** Wet bulb globe temperature; a widely used index value
calculated using measured information from three sources, the dry bulb,
wet bulb, and blackened globe; the WBGT predicts the rise of the rectal
(core temperature) in *healthy acclimatized* persons, *lightly clothed*,
during exercise or physically strenuous work (it is not a valid
predictive tool for persons wearing heavy protective clothing or
equipment)

WBGT = .7(T~wb~) + .1(T~db~) + .2(T~bg~) for outdoors work

WBGT = .7(T~wb~) + .3(T~db~) for indoors work

**ET:** Effective temperature, an index combining subjective thermal
sensation resulting from air temperature, humidity, air movement. This
index considers only light or sedentary levels of metabolic work, its
use is primarily limited to an index of comfort or as a correlate in the
analysis of productivity.

**Wind Velocity:** Air speed, important for calculating factors such as
wind chill, convective and evaporative heat exchange.

**Wind Chill Temperature:** The wind chill temperature is how cold
people and animals feel when outside. Wind chill is based on the rate of
heat loss from exposed skin caused by wind and cold. As the wind
increases, it draws heat from the body, driving down skin temperature
and eventually the internal body temperature. Therefore, the wind makes
it FEEL much colder (see chart below).

**CLO Value:** The CLO is a unit developed to express the relative
thermal insulation values of various types and combinations of clothing
or protective garments, a descriptor of a garment's resistance to heat
transfer; also used in heating and cooling ventilation engineering in
the determination of environmental conditions for human comfort; the
thermal insulation of clothing may be expressed in CLO units.

![](media/image1.png){width="4.180555555555555in"
height="1.8020833333333333in"}

**Internal heat load is dependent on multiple physiologic processes.**

**Heat retention is modified by numerous external factors.**

![](media/image2.jpeg){width="6.5625in" height="4.26875in"}

**Heat Index Chart** (Source: NOAA)

**II. HEAT STRESS SYNDROMES, TREATMENT, AND PREVENTION.**

**Heat Illness Categories:** Heat related illnesses are typically
described in terms of physiologic severity, and though not always true,
may be viewed as a continuum of manifestations. The commonly described
categories of heat related illnesses include *heat syncope, heat cramps,
heat exhaustion, and heat stroke.*

**Heat syncope.** This syncope typically occurs when unacclimatized
individuals must stand in the heat for long periods without appreciable
physical exercise. The lack of movement allows venous pooling to occur
in the lower extremities. Increased sweating and the venous pooling
causes a reduced effective intravascular volume which leads to
tachycardia. When the tachycardia is no longer able to maintain
sufficient cardiac output to support minimal essential brain perfusion,
syncope results. This event is not accompanied by a significant
elevation of core temperature. Heat syncope is treated conservatively by
removal from the hot environment along with oral replacement of lost
fluids (water).

**Heat exhaustion.** This form of heat related illness typically occurs
in unacclimatized or partially acclimatized individuals in association
with vigorous exertion in the heat, especially individuals with some
form of circulatory insufficiency. The physiologic consequences result
from the inability to shunt heat from the core and dissipation it
adequately at the surface. Usually several factors are necessary to
produce a physical collapse which halts further work in the heat. These
include sweating, failure to adequately replace fluid losses, cardiac
output limits, and relative loss of intravascular volume. Hypernatremia
may occur if the volume loss has been primarily (water) dehydration, or
hyponatremia may be present if there has been significant loss of salt
as well. Although the core temperature is elevated, the elevation will
be somewhat lower than expected with heat stroke, usually not exceeding
105°F, and heat exhaustion is not accompanied by mental status changes.
Heat exhaustion tends to be self-limiting, the victim is incapable of
continued exertion in the heat, or the victim faints. Heat exhaustion is
treated by removal from the activity and environment, oral or
intravenous administration of salt containing fluids, and cooling. If
not treated or if the victim returns to activity without complete
recovery, heat exhaustion can easily progress to heat stroke.

**Heat cramps.** Heat cramps occur in well acclimatized individuals who
have undergone exceptionally vigorous exertion accompanied by profuse
sweating with significant sodium loss. The major symptom is extremely
painful muscle contractions which prevent continuation of exertion.
Therefore, significant elevation of core temperature is rarely part of
this syndrome. Heat cramps respond well to rest and administration of
oral salt containing fluids.

**Heat stroke.** Heat stroke is the most significant and most serious
manifestation of heat related illness. When contrasted to other forms of
heat illness, the immediate clinically distinguishing factor of heat
stroke is the profound alterations occurring in mental status. This is
caused by the extreme heat injury to brain tissue, almost always \>
106°F (41.1°C). Usually by the time physical collapse occurs the victim
has ceased sweating or sweat rates have diminished remarkably (absence
of sweating is not mandatory for the diagnosis of heat stroke). Other
significant abnormalities include extreme tachycardia, hypotension,
respiratory alkalosis, metabolic acidosis, and serum markers of tissue
damage to brain, myocardium, and liver. Rhabdomyolosis may be extreme
and associated with acute renal failure. Coagulation abnormalities are
common and disseminated intravascular coagulation is found in cases with
significant vascular intimal injury.

Heat stroke is a *medical emergency.* A successful outcome requires
swift diagnosis and therapy. Of utmost importance is to immediately
begin the process of cooling the victim because the probability of
sustaining permanent impairment is related to both the level of
hyperthermia and its duration. On the basis of theoretical
considerations and considerable clinical use, evaporative forms of
cooling are favored by most practitioners today. By misting the victim
with cool or tepid water and placing them in the air stream of strong
fans very significant quantities of heat can be removed quickly.
Formerly, immersion in iced water was common. This technique has
logistical constraints in addition to stimulating violent shivering
which counteracts dissipation of heat. Sudden ice water immersion may
also precipitate angina in victims with coronary artery disease. Almost
all heat stroke victims will have intravascular volume deficits and
require urgent administration of sodium containing intravenous fluids;
tissue perfusion may be assessed based on urine output. Adequate urine
output ([\>]{.underline} 100 ml/hr) is especially important to prevent
myoglobinuric renal failure. Other abnormalities may occur in specific
individuals and require correction, these may include hypocalcemia,
hypoglycemia, hyperkalemia, and coagulation irregularities.

**Heat Stress Predisposing Factors:**

**Risk factors in healthy persons.** Historic reviews cite many factors
thought to predispose otherwise healthy individuals to the development
of heat stress. Included are such factors as sleep deprivation, obesity,
poor physical condition (especially as manifest by low
V[o]{.smallcaps}~2~[max]{.smallcaps}), febrile illness, lack of
acclimatization, dehydration, aging (correlates with age-related decline
in V[o]{.smallcaps}~2~[max)]{.smallcaps}, relative deficiencies of zinc,
magnesium, iron, vitamin C, and skin disorders that affect sweating.
Prior episodes of heat illness may indicate an increased risk, or may
simply be a persistence of previous risk factors.

**Medications, drugs, and toxins.** Many substances are known to
predispose and individual to heat stress syndromes. Some are quite
common, such as ethanol. When abused, ethanol produces dehydration,
alters vascular reactivity, and frequently impairs judgment, all
potentially contributing to the risk for heat stress in the right
setting. Widely used over-the-counter (OTC) medications with
α-adrenergic components (phyenylephrine or phenylpropanolamine) which
limit cutaneous vasodilation and drugs such as amphetamines and cocaine
with similar actions may predispose to heat stress. Diuretics will
adversely affect intravascular volume and limit cardiac output,
especially when coupled to the volume deficits which develop with
profuse sweating. Drugs such as lithium and fluoxetine are thought to
interfere with hypothalamic thermoregulatory mechanisms.

**Medical conditions and diseases.** Many conditions can interfere with
the physiologic ability to respond to heat stress conditions and through
one or more mechanisms prevent normal heat dissipation. Many cardiac
conditions limit the maximal cardiac output. Vascular disease associated
with such common conditions as diabetes and atherosclerosis of any
etiology can interfere with vasodilation.

**Prevention of Heat Stress Illness:** Attention to several basic
factors will minimize the risk of heat induced illness.

**Physical measures.** Almost everyone has some capability for
acclimatization. This is usually accomplished through progressive
exposure to heat environments with gradually increasing levels of
exertion over a 10 -- 14 day period. Adequate hydration before, during,
and after heat exposure cannot be overstressed. This includes avoidance
of caffeinated and alcoholic beverages. Drugs known to impair heat
tolerance should be avoided if possible. Sufficient rest and balanced
diet to avoid vitamin or mineral deficiencies may be helpful. Physical
conditioning with maintenance of ideal body weight help to ensure
circulatory adequacy, remove unneeded insulating tissue, and reduce the
need for cardiac output to adipose tissue.

**Engineering controls.** These measures lie at the core of all
occupational medicine preventive strategies -- that is, physically
separate the worker or participant from the exposure whenever possible.
In occupational medicine settings, this is how primary prevention is
often put into practice. For heat stress prevention engineering controls
might include barriers between the human and the heat source,
ventilation to carry the heat away, or cooling systems to reduce the
available ambient heat load.

**Administrative controls.** This strategy requires strict control of
personnel, whereby the number and length of exposures are strictly
limited. The intensity of activity is usually regulated according to a
parameter such as the WBGT. When necessary, rest and cooling periods are
interspersed through the work cycle to allow offloading of excess heat
generated during work.

**Personal protective equipment (PPE).** In some applications the worker
can function adequately while wearing a personal cooling garment. In
many situations though, this apparatus is too bulky or restraining to be
compatible with job safety and effectiveness. In general, PPE should be
considered a solution of last resort.

**HEAT EXPOSURE, PERFORMANCE EFFECTS.**

Performance decrements are somewhat predictable for occupational and
athletic activities by having knowledge of the individuals involved
(their relative acclimatization, physical capabilities) and application
of WGBT-derived predictions. For individuals who are in a relatively
normal state of cardiopulmonary fitness, the onset of sustained
tachycardia often indicates the point at which compensatory mechanisms
to shunt heat from core to periphery through the use of increased
cardiac output is reached and a feeling of subjective fatigue is
experienced. An interesting safety consideration to keep in mind is that
heat stress insidiously degrades mental performance well ahead of
physical performance. Thus, for certain situations the onset of heat
stress will be a greater safety than productivity factor, something that
aerospace system operators need to be intimately knowledgeable about and
able to prevent or intervene prior to an incident or accident. Complex
cognitive tasks suffer from shortened simple reaction times, increased
error rates, tunneling of attention (often neglecting secondary tasks
completely), increased difficulty responding to abnormal or unusual
events.

Many investigators have noticed productivity begins to drop when the
individual perceives heat discomfort in the environment, generally well
before measurable physiologic parameters are evident. Some have noted
that "100% performance" is tolerated well up to wet bulb temperatures of
about 25°C. Irrespective of the task complexity, human performance
curtails significantly in the temperature range 27° to 30°C. Adequate
hydration for prevention of heat injury and maintenance of productivity
cannot be overemphasized. Individuals who undergo heavy physical
exertion in the heat may require 10 -- 12 quarts of water per shift.
Physiologists agree that consumption of small amounts of water at
frequent intervals, starting prior to the onset of thirst is most
effective. Often this will require a strong supervisory emphasis to
ensure or enforce a water drinking schedule which will maintain adequate
volume status. Commercial airline baggage handlers working on the ramp
and in airplane cargo spaces during summer months are at high risk for
dehydration, heat injury.

**HEAT SOURCES IN AVAITION OPERATIONS.**

Any aircraft parked in the open during hot environmental periods will
gain heat, much of it through radiant transfer to outer skins which in
turn conduct heat to inner surfaces, raising the ambient temperature of
all internal spaces. The well known term for this is, "greenhouse
effect." This creates potential heat stress for anyone entering,
particularly if the individual must remain onboard to perform
maintenance or perform extensive preflight checks. Under this situation,
adequate cooling ventilation is a must if heat stress is to be avoided.
Parking the aircraft inside a hangar or under a shade canopy can often
decrease internal temperatures by up to 40° F in desert environments.
Helpful, but to a smaller degree are external covers which block the
entry of sunlight through windows or shielding devices placed on the
inside of aircraft windows to block sunlight. Obviously, these devices
do not keep radiant energy from outer aircraft surfaces.

During flight heat is gained from several sources. Externally, radiant
energy is absorbed and has a similar effect as found on the ground.
Internally, aircrew must contend with their own metabolic heat, which
may have a considerable effect if the crew must wear constrictive
protective garments (such as crash helmets, fire retardant flightsuits,
anti-G suits, boots, survival suits, or pressure suits). While high
flying aircraft typically operate in very cold external temperatures,
the friction effect of air passing across external surfaces may raise
the temperature of those surfaces to very high levels, resulting in
conductive

heat transfer to interior spaces.

Activities related to ramp operations in hot climates must always
consider the synergistic effects brought on by the level of physical
effort, the physiologic capabilities of the individual, and the
real-time environmental factors present. Such activities include routine
maintenance, refueling, and loading tasks. Extreme heat index levels are
easily reached inside the cargo hold of an airliner parked on an outdoor
ramp in places like New Orleans or Houston on typical summer days.
Anyone working inside the cramped space of the cargo hold to physically
move luggage in an expeditious manner will generate very significant
internal heat loads. In such a semi-closed environment one'

**III. COLD RELATED INJURY AND ILLNESSES**

There are several common ways to describe and relate cold-related
illnesses and injuries. Each has its own merits. The important point for
this exercise is the potential effect on aviation safety and mission
success. From a pathophysiologic perspective, this discussion will be
based on nonfreezing and freezing conditions. These can be further
distinguished by localized and systemic categories.

**General considerations and predisposing factors related to cold
injuries and illness.** As with many physiologic challenges, individuals
at both ends of the age spectrum tend to be more susceptible, and
therefore at higher risk. Anyone with a history of previous cold injury
has already demonstrated their susceptibility and should be considered
at higher risk. Mental and physical fatigue increase the likelihood of
careless acts and the neglect of activities necessary to prevent injury.
At the same time, one of the most profound consequences of hypothermia
is a loss of normal cognitive ability (see below). Prior training and
the ability to maintain discipline has an enormous effect on the
probability of illness or injury resulting from cold exposure. The
simultaneous presence of other traumatic injuries may be synergistic
with the effects of cold exposure, injuries may physically prevent one
from taking appropriate action. Persons from warmer climates and
individuals with dark skins tend to tolerate the effects of cold
environments less well. Activity, too much or too little can effect the
risk for cold injury; increased exercise requirements due to heavy
clothing, equipment, and movement through snow can increases calorie and
fluid requirements two to three times normal. Tobacco use leads to
vasoconstriction which may predispose to the development of localized
injury or make its treatment less effective (e.g., frostbite). Alcohol
causes mental impairment (poor judgment) and peripheral cutaneous
vasodilation which increases core heat loss. Finally, one must consider
the possibility of toxic exposures which can result from faulty
equipment used to provide heat (fumes, smoke, carbon monoxide).

**Nonfreezing cold illness and injury.**

**Systemic hypothermia.** Potentially, the most serious of the cold
related illnesses is significant hypothermia. Hypothermia is normally
defined as a core temperature less than 35° C (95° F). Typically,
exposure to cold is usually present, but is not always necessary.
Prolonged exposure to water 60° F without appropriate countermeasures
will eventually result in hypothermia even with ambient air temperature
above 80° F. During exposures which may result in hypothermia, the
primary physiologic response is to shunt blood flow away from the
surface capillary beds to limit heat loss to the environment. A
prominent secondary response is to increase involuntary skeletal muscle
work in the form of shivering to cause an increase in endogenous heat
production. Obviously, any substance or medication which interferes with
one's ability to shiver would be counter productive in this situation.
Pulmonary functioning initially increases, but then falls as hypothermia
deepens. Simultaneously, cough and gag reflexes fail and the
oxyhemoglobin curve shifts to the left (decreased oxygen delivery to
tissues). Meanwhile, CNS function deteriorates as manifest by a
decreasing level of consciousness, increasing confusion and lethargy
which is often associated with a lack of appropriate response to the
situational challenge. Failure to respond appropriately causes the heat
loss to continue, the hypothermia to deepen and will eventually result
in coma and death if intervention is not forthcoming.

**Nonfreezing Cold Injuries, Chilblains and Trenchfoot (Immersion
Injury).** These conditions are most likely to occur during prolonged
exposure to cold conditions, especially when the hands and feet cannot
be kept adequately warm. The situational risk is compounded when high
humidity (or overt wetness) and winds are present. Prolonged cooling of
the distal extremities results in reduced blood flow with secondary loss
of sensation, reduced manual dexterity and agility. Unfortunately,
symptoms often go unnoticed, which can establish conditions favorable to
more severe freezing injuries with significant loss of tissue and
function.

**Chilblains** is a superficial, painful nonfreezing injury occurring
after prolonged skin exposure to humid, cold windy conditions. During
exposure affected skin exhibits a pallor and may become insensate.
**Treatment** includes removal from exposure and gradual rewarming,
during which erythema, edema, itching, and superficial blisters may
form. There is no significant tissue loss and no permanent damage.

**Trenchfoot / Immersion Injury** is a much more serious cold related
nonfreezing injury. This injury results from prolonged exposure to cold
water or cold wet footgear when dry socks are not an option. Continuous
exposure to cold wet conditions results in reduced blood flow along with
softening and maceration of the skin. Layers of skin may begin to
separate, particularly when sensation is significantly diminished and
shear stresses are applied (e.g., continued walking after boots and
socks have become saturated). Other changes include edema, waxy-white
cyanosis with bluish-purple splotches. Superficial and deep
musculoskeletal anesthesia frequently occur, making additional injury a
significant risk. This injury can occur in the absence of external
moisture when footgear precludes the evaporation of sweat. In its most
serious form, trenchfoot / immersion injury can cause significant tissue
sloughing and may require therapeutic amputation. **Treatment** consists
of removal from exposure, careful removal of wet clothing, gentle
cleansing and drying. Affected tissues should undergo gradual dry
rewarming while protecting from additional injury. Continued edema
associated with significant hyperemia usually ensues. Often, there will
be a significant need for pain control; elevation of the affected
tissues may help to alleviate discomfort. In its worst form, trenchfoot
/ immersion injury may require aggressive infection control, therapeutic
debridement and reconstructive surgery.

**Freezing cold injury: Frostnip and Frostbite.** These injuries occur
when tissue become frozen. This usually requires prolonged exposure to
very low temperatures. However, the combination of exposed skin, low
temperature, and wind can significantly shorten the exposure time
necessary to result in frozen tissue. Wind chill charts are used to
estimate such exposure times (see Wind Chill chart). Freezing limited to
the surface of the skin is frostnip. Tissue freezing which extends
deeper, below the skin and into internal flesh is called frostbite. Skin
freezes at about 28° F (-2°C). As frostbite develops, skin will become
numb and turn to a grey or waxy-white color. The area will be cold to
the touch and may feel stiff or woody. With frostbite, ice crystal
formation and absolute lack of blood flow to the frozen area will result
in necrosis.

![](media/image3.jpeg){width="3.1930555555555555in"
height="1.0055555555555555in"}

Treatment for freezing injuries begins with removal or isolation from
the cold environment. If definitive treatment will be delayed, frozen
tissue must not be allowed to refreeze during the evacuation process.
Definitive therapy begins with careful rewarming of the frozen tissue by
immersing in clean water heated to 104 - 108°F. Frequently the rewarming
process will be accompanied by severe pain so that judicious use of
analgesics and sedatives may be necessary. Once thawed, the tissues must
be kept clean and protected from addition trauma. Tetanus toxoid booster
may be needed. Prophylactic antibiotics should not be administered;
surgical consultation may be needed for debridement of dead tissues.

**Cold stress and cold injury affects on aviation operations.** The
influence that cold exerts on aviation activities is manifest in two
major categories. First, the simple mechanical bulk added by the
presence of multiple layers of extra garments has restrictive influences
over what can be done and the difficulty with which tasks are
accomplished. This is particularly true for tasks involving fine
dexterity when mittens or gloves must protect fingers. The second major
influence comes from the pathophysiologic influence imparted by
hypothermia or other cold related injuries. At best, the cold related
injuries produce a distracting factor that must be psychologically
contained by aviation personnel. More often, even subtle hypothermia
results in cognitive performance decrements that the individual does not
recognize.

![](media/image4.png){width="6.447916666666667in" height="4.53125in"}

**NOAA Wind Chill Chart.** This chart depicts the approximate time to
onset of frostbite for exposed flesh.

**Prevention of cold stress and cold related injuries.** Prevention must
start with *careful planning* for aviation activities which will occur
in cold environments. For example, one should always ascertain the worst
possible outcome for any planned activity and how that outcome can be
mitigated. Regarding ground activities, the wind chill on aircraft ramp
areas should be carefully checked and compared to expected exposure
times and available equipment, including layered garments, to protect
those exposed. Perhaps the most illustrative of the ground activities in
cold weather is deicing operations in which crews must content with
cold, wind, and moisture. Any flight during cold weather or through cold
regions should include adequate supplies and equipment for survival.
Individuals must be familiar with the effects of cold on themselves and
others. Ideally, all individuals participating in cold weather flying
activities should have knowledge and training about cold climate
survival, first aid, and pre-hospital treatment of cold injuries.
Individuals must maintain adequate hydration and limit exertion to avoid
sweating that could result in much greater heat loss. In summary,
successful aviation activities in cold environments demand careful
attention to risk assessment and adequate planning.