Environmental Engineering Reference
In-Depth Information
Sweat (99% H
2
O, NaCl, KCl, traces of urea, lactic and
fatty acids, and proteins) is produced by about 3 million
ecrine glands (from
<
100/cm
2
on the thigh and leg to
>
300/cm
2
on the forehead), and studies have found no
significant differences in their density among populations
of different continents and climates (Taylor 2006).
Without active sweating, the average person loses
about 12 W/m
2
of body surface, equally split between
respiration and skin diffusion. Above 28
C, evaporative
losses climb exponentially, so that even at rest in still air
they remove about 130 W/m
2
at 48
C, or a total of up
to 230 W. With work, the perspiration rates go up to sur-
pass greatly those of sweating mammals: a horse can lose
every hour 100 g/m
2
and a camel up to 250 g/m
2
, but
a person can perspire more than 500 g/m
2
(Folk 1976).
Perspiration of 500 g/m
2
translates to heat loss of
550-625 W for most adults, sufficient to regulate tem-
peratures even in extremely hard-working individuals.
Designers of heating and ventilating plants use 586 W
(@325 W/m
2
) as the maximum heat output of a manual
laborer, and under normal free-conversion conditions,
with air moving at just 0.45 m/sec, virtually all of this
load can be lost through evaporation of sweat. Similarly,
pedaling (with adequate cooling) can sustain output of
nearly 600 W without any noticeable increase in body
temperature no matter how long the activity continues
(D. G. Wilson 2004).
Acclimatized individuals can produce sweat up to 1.1
L/m
2
/h, enough to remove 5 MJ of heat. At a rate of
1390 W, this exceeds all but the most strenuous athletic
exertions. The highest reported short-term peak sweating
rates are 4 L/h. At such levels rehydration is an acute
necessity because humans can neither tolerate substan-
tial dehydration nor store large volumes of water. How-
ever, limited voluntary dehydration (drinking less than
is perspired) is common during heavy exertions, with
the deficit gradually replaced within a day. Most no-
tably, elite-level marathon runners ingest only about
200 mL/h during the race, much less than the recom-
mended volume of 1.2-2 L/h, and the latest guidelines
for fluid replacement during marathon running urge
slower runners to drink no more than 400-800 mL/h in
order to avoid risks of hyponatraemia (IMMDA 2001).
5.4 Limits of Human Performance
There are three fundamentally different, time-dependent
ways to energize physical performance. The first one is
the anaerobic, alactic mode. ATP, the direct source of
energy for muscular contractions, is stored in muscles at
minuscule levels, averaging a mere 5 mmol/kg of wet
tissue. With 20 kg of active muscles and 42 kJ/mmol of
ATP, this is equivalent to 4.2 kJ, the total sufficient to
energize contractions for 0.5-0.75 s of maximum effort.
The most rapid recharge is through the breakdown crea-
tine phosphate (CP). But CP's muscle stores are also lim-
ited (20-30 mmol/kg of wet tissue), and the recharge
will last only 5-8 s. Maximum metabolic power achiev-
able by this route is large, 3.5-8.5 kW for a 65-70-kg
average man, and as much as 12.5 kW for a trained
man, but the overall capacity averages just 20-40 kJ and
reaches no more than 55 kJ for the best-adapted bodies.
Somewhat longer exertions are energized anaerobically
by muscular glycogen, whose glycosyls break down into
pyruvate and hydrogen. The pyruvate is converted to lac-
tate (its accumulation in active muscles leads to the well-
known weakness and pains), and a convenient indicator
of anaerobic threshold is the breaking point when lactate
blood levels start increasing exponentially. In sedentary
people this is invariably when the workload is 50%-70%
of the maximum aerobic power; in endurance athletes
