KEEPING WARM 
Clothing for Cold Climes 
Will synthetics duplicate the insulating properties of natural fibers? 
by John V. E. Hansen 
Considering the many functions we 
expect it to perform, the design of the 
body can hardly be faulted. In one area 
alone — the regulation of temperature — 
the body provides engineers and physi- 
ologists with a unique system for study- 
ing a remarkably successful combina- 
tion of materials and mechanisms that 
maintains temperature stability within 
the limits necessary for human survival. 
This is all the more impressive given the 
variation in the amount of energy that 
must be dissipated by the body. A rest- 
ing human has a metabolic level, or 
work rate, on the order of 350 BTUs per 
hour, which must be dissipated to avoid 
heat buildup. (BTU stands for British 
thermal unit; one BTU is equal to the 
amount of heat required to raise the 
temperature of one pound of water by 
one degree Fahrenheit.) When an indi- 
vidual walks, the work rate goes up to 
approximately 900 BTUs per hour, and 
a very rapid walk up a slight incline 
would raise the work rate to approxi- 
mately 1,200. The work rate of an ath- 
lete competing in a long race would be 
in excess of 5,000 BTUs per hour. 
Because the body does not dissipate 
heat in a uniform fashion, it can accom- 
modate a wide range of heat loads. A 
closer look at the body reveals an ingen- 
ious array of flow controls and heat- 
transfer mechanisms that accommodate 
this tremendous range in energy dissipa- 
tion requirements. These mechanisms 
are worth examining because they hold 
the clues to the manner in which we can 
keep ourselves warm. In other words, if 
we know how the body controls its heat 
system, we can probably choose clothing 
materials and designs that will comple- 
ment these controls and protect the 
body beyond its own natural limits. 
The extremities play a large role in 
the several mechanisms by which the 
body dissipates heat. Human limbs and 
fingers represent an interesting compro- 
mise between carrying out necessary 
mechanical body functions and at the 
same time presenting a fairly large area 
through which heat can be released. 
Heat is removed from the skin surface 
by means of four processes: evaporation. 
convection, conduction, and radiation. 
Of these, evaporation through the pores 
of the skin plays a major role when there 
is a large amount of heat to be removed. 
The normal evaporation of the body’s 
perspiration can be accelerated by re- 
placing moist air near the skin with drier 
air. The extent of the resultant evapora- 
tive cooling is easily demonstrated. A 
swimmer leaving the water on a day 
when there is even a faint breeze will 
quickly feel chilled. 
The evaporation effect, of course, 
works best when the surrounding air is 
relatively dry and there is a good deal of 
body moisture to evaporate. The latter 
occurs when blood flow increases near 
the surface and the blood vessels in the 
body’s skin expand, permitting the pores 
to open. The result of this process, called 
vasodilation, is to present a warm skin 
area that offers a maximum amount of 
perspiration. In order to keep the body 
warm, the process must be reversed, 
that is vasoconstriction must take place: 
the blood vessels in the skin and the 
pores constrict, and evaporation and 
blood flow are reduced. 
A rich blood flow near the surface 
and the dilation of blood vessels in the 
skin not only help the evaporation proc- 
ess but also help to carry heat away 
from the body by convection — a process 
whereby heat from the warm body is 
transmitted to colder air moving around 
the body. Here again, in order to keep 
the body warm, the process is reversed 
by reducing the temperature of the skin, 
which in turn reduces the heat loss by 
convection. 
The body can also lose heat by con- 
duction — a process in which heat is con- 
veyed from a warm object to a static 
colder object. (The major difference be- 
tween convection and conduction is that 
convection involves motion whereas con- 
duction involves static elements.) The 
higher the skin temperature, the greater 
the rate at which the body’s heat will be 
reduced by conduction through cooler 
objects in touch with the skin. Con- 
versely, if the skin’s temperature is re- 
duced, the rate of heat loss by conduc- 
tion will also be reduced. 
The last process is radiation, or the 
emission of energy, which is also influ- 
enced by skin temperature because 
transfer by radiation between two ob- 
jects is dependent on the temperature 
difference between the objects. When 
skin temperature is reduced on a cold 
day, the skin will radiate less heat, and 
here again, the constriction of blood 
vessels helps conserve body heat. The 
amount of blood flowing from the heart 
to the hands, for example, is not only 
reduced as the blood flow to the extrem- 
ities is restricted but the blood is also 
cooled through the exchange of heat 
with the blood returning to the heart 
from the cold extremities. This is some- 
thing everyone has experienced on a 
cold day. As the temperature of and the 
flow of blood to the hand decrease, the 
skin of the hand becomes colder and the 
heat loss to the environment is reduced. 
This mechanism applies to the feet as 
well as the hands. The feet constitute 
approximately 10 percent of the total 
body surface, but under hot conditions, 
approximately 1 3 percent of the body’s 
heat can be lost through the feet. Under 
cold conditions, when the heat loss 
might be expected to increase, it may 
actually be diminished and amount to 
only 7 percent of the total. Here again, 
reduced blood flow and reduced blood 
temperature are the contributing fac- 
tors. In some cases the temperature of 
the arterial blood flowing to the hands or 
feet may be reduced by as much as 1 5 to 
25 degrees before the blood reaches the 
extremities. The hands and feet may 
even attain the same temperature as the 
surrounding environment. 
The manner in which the body regu- 
lates temperature by controlling its 
Copper man ( with electric power 
leads attached to eye sockets) 
is used to measure the insulating 
capability of different systems 
of cold weather clothing. 
Photos by U S Army Natick Research and Development Laboratories 
90 
