Environmental Engineering Reference
In-Depth Information
firing rates at, respectively, about 40
C and 32
C) sense
heat through the skin.
The surface area of adult bodies is 1.3-2.2 m
2
, and the
highest heat loss is in the neck (
>
180 W/m
2
), the lowest
in the feet (@2 W/m
2
). Radiation is the largest heat loss
route, accounting for 60% of the total from a naked
body. According to the Stefan-Boltzmann law, the radi-
ant loss or gain (F , in W) is proportional to the product
of body surface area (A,inm
2
) and the difference of
fourth powers of ambient temperature (T
a
) and body
surface temperature (T ):
that restricted energy intake promotes longevity in
species ranging from microbes to chimpanzees and
humans (Weindruch and Sohal 1997; Fontana et al.
2004). Humans are flexible converters of food energy,
able to respond with altered metabolic efficiencies to dif-
ferent diets, environmental conditions, specific tasks, and
health states. The questions about food requirements in-
clude not simply How much? but For what? and In what
context? These questions remove the search for food
requirements from the realm of quantifiable energetic
considerations to the much larger and fundamentally
unquantifiable setting of cultural preferences and social
expectations. Human energetics is so contextual and so
value-laden precisely because it concerns humans. Borrini
and Margen (1985) summed up its challenge well: be-
fore defining specific food requirements, it is imperative
to appreciate the perceived needs and wants of the peo-
ple and their customs, the structure and dynamics of
their societies, and the ecology of their environments.
F
¼
sA
ð
dT
0
eT
4
Þ
;
where s is the radiation constant and d is the skin's ab-
sorption coefficient (0.6-0.8 for light to dark skin). Con-
duction is the least important cause of heat loss, normally
just about 3% of the total (heat loss in urine and feces is
similar). Evaporation (about 15%) and warming and wet-
ting of the air through breathing (10%) are other major
forms of heat loss. Convection is important only when
air or water move past the body.
Healthy people tolerate spikes up to 40
C-40.5
C
that follow taxing work or exercise, and fever causes
longer periods of such elevated temperatures without
any lasting damage, but temperatures above 42
C are fa-
tal. When the core temperature falls to @35
C, mental
slowness becomes extreme, and at a core temperature be-
low 33
C, life-threatening hypothermia ensues. Hypo-
thermia if brief is survivable. In contrast, the Earth's
inhabited regions have extremes of about 50
Cin
subtropical deserts and
60
C during Arctic winters. A
hairless heterotroph has only four choices to live in tem-
peratures below the zone of thermal comfort, which, for
resting unclothed people is 24
C-29
C: to insulate by
5.3 Thermoregulation
Like all other homeotherms, humans are tachymetabolic
(able to produce heat at the cellular level even in the ab-
sence of any muscular activity) and have an intricate neu-
ral regulatory system to maintain thermal homeostasis
(Hardy, Stolwijk, and Gagge 1971; Blumberg 2002). As
a result, the normal range of human temperatures is very
narrow. The lowest mean is at 35.1
C in Andean Indi-
ans; 37
C is the standard physiological cove temperature.
Infants have slightly higher normal temperatures, elderly
people slightly lower, and a daily cycle (evening peak,
morning trough) averages about 2
C. Humans have ex-
tensive sensors to guide their thermoregulation. Thermal
receptors in the brain and blood vessels sense the body
core temperature, and warm and cold fibers (maximum
