Environmental Engineering Reference
In-Depth Information
BMR, as vasoconstriction lowered skin temperature (feet
minima down to 17
C) and reduced thermal conduc-
tance. Conserving heat the Aboriginal way is much
more effective than shivering (thermogenesis), which
only accelerates convectional heat loss (Taylor 2006).
The vasoconstriction response would be grossly inade-
quate to handle seasonal thermal deprivations, even in
temperate latitudes where clothing and heating must be
used to protect against the cold. Interestingly, the Inuit,
whose harsh environment forced them to develop ex-
traordinary multilayered clothing protection replicating
the furry adaptations of Arctic animals, appear to have
the poorest naked-body insulation (the highest thermal
conductance) of any population studied. The microcli-
mate of the Inuit's body was constantly almost tropical,
and hence there was no need of metabolic adaptations.
Other supposedly adaptive Inuit characteristics also have
plausible energetic explanations (Rennie et al. 1962; So
1980). Their relatively high BMR is explained as being
due to the specific dynamic action of diets rich in fat and
protein.
Still, boreal populations have evolved a variety of mi-
nor but important adaptations: sweat glands more abun-
dant on the face and greatly reduced on the body trunk
and legs, and higher finger temperatures, advantageous
when warm mitts have to be slipped off for outdoor
activities. But intelligence (an enterprising and impres-
sively effective mimicry of hunted mammals) was always
the key to Arctic adaptation. Animal precursors can also
be seen in the common heat adaptation of wearing dark-
colored clothes in desert environments. As in the case of
dark bird plumage, dark robes in the desert absorb a very
large part of incoming radiation, which is subsequently
lost by convective heat transfer without impinging on
the body.
Adaptation to hot climates is not only fast but also very
effective for all healthy people, and it is retained even by
populations that have lived for generations in temperate
or cold climates. Hanna and Brown (1983) include the
ability to tolerate heat as a distinctly human characteristic
alongside bipedalism, hairlessness, a large brain, and a
symbolic linguistic ability. Both the rapidity and the ef-
fectiveness of the process can be perfectly illustrated by
Wyndham's (1969) revealing experiments with South Af-
rican white males with no recent exposure to heat during
work. They had to perform moderate work in a hot
(33
C) and humid environment, and the acclimatization
period consisted merely of ten consecutive days of such
a regime. Their response was remarkable: rapidly rising
core temperatures reaching a danger zone, heartbeats
approaching tolerable maxima, and low sweating rates
were swiftly transformed to levels nearly identical to
those of highly acclimatized native Bantu.
Other studies have demonstrated that such adaptive
training requires just an hour or so per day regardless of
the thermal environment for the rest of the time. A Bra-
zilian study found increased sweating during a short-
duration exercise in a temperate environment after only
nine days of 1-h exercises on a cycle ergometer at 50%
VO
2max
(Machado-Moreira et al. 2005). The initial re-
sponse to heat is the dilation of peripheral skin blood ves-
sels, compensated by vasoconstriction elsewhere, and the
shifting of large volumes of blood into the hands and
feet. The magnitude of skin vasodilation is striking: rest-
ing skin flow (in thermoneutral surroundings) is about
250 mL/min (dissipating up to 100 W), but vasodilation
can increase skin blood flow to 6-8 L/min (or 60% of
cardiac output) during severe hyperthermia (Charkou-
dian 2003). Perspiration begins, first on the trunk, then
on the extremities, at 28
C-32
C of skin temperature.
