https://en.m.wikipedia.org/wiki/Insensible_perspiration
Insensible perspiration is the loss of water through the skin which does not occur as perceivable sweat. Insensible perspiration takes place at an almost constant rate and reflects evaporative loss from the epithelial cells of
the skin.
Unlike in sweating, the fluid lost is pure water, i.e. no solutes are
lost. For this
reason, it can also be referred to as "insensible water loss".
The amount of water lost in this way is deemed to be approximately 400ml
per day. Some sources broaden the definition of insensible perspiration to include not only the water lost through the skin, but also the water lost through the epithelium of the respiratory tract, which is also approximately 400ml per day.
Insensible perspiration is the main source of heat loss from the body, with
the figure being placed around 480 kCal per day, which is approximately 25%
of basal heat production.Insensible perspiration is not under regulatory control.
https://www.energy.gov/energysaver/evaporative-coolers
"In low-humidity areas, evaporating water into the air provides a natural
and energy-efficient means of cooling. Evaporative coolers, also called
swamp coolers, rely on this principle, cooling outdoor air by passing it
over water-saturated pads, causing the water to evaporate into it. The
15°- to 40°F-cooler air is then directed into the home, and pushes warmer
air out through windows."
https://www.ccohs.ca/oshanswers/phys_agents/humidex.html
Canadian Centre for Occupational Health and Safety
"What is the importance of humidity?
The body attempts to maintain a constant internal temperature of 37°C at
all times. In hot weather, the body produces sweat, which cools the body
as it evaporates. As the humidity or the moisture content in the air
increases, sweat does not evaporate as readily. Sweat evaporation stops entirely when the relative humidity reaches about 90 percent. Under these circumstances, the body temperature rises and may cause illness."
https://engineering.mit.edu/engage/ask-an-engineer/why-do-we-sweat-more-in-high-humidity/
"As the water evaporates, it transfers the body’s heat to the air. Because water has a high latent heat, which is the heat required to change liquid
water to vapor, this process usually carries away enough heat to do a good
job of cooling the body.”It’s a fabulous system,” says Christie.
"But the rate at which water — or in this case, sweat — evaporates depends on how much water is already in the air. On dry days, sweat evaporates
quickly, which means it also carries away heat faster. On humid days, when
the air is already saturated with water, sweat evaporates more slowly.
"This explains why it feels so much hotter in high humidity. When relative humidity reaches a high enough level, the body’s natural cooling system simply can’t work. Sweat evaporates very slowly, if at all, and the body heats up. In extreme cases, people begin to suffer from heat cramps or
heat stroke, which is basically organ failure as the body begins to cook itself.
"A metric called the heat index provides warnings for weather conditions
that will make heat stroke more likely. For instance, the body experiences 88-degree (Fahrenheit) weather with 85 percent humidity as if it were 110 degrees. Working outside, even in the shade, is dangerous in these
conditions. At 40 percent humidity or lower, however, 88 degrees feels
like 88 degrees, and gardening is once again a perfectly safe activity.
"To help athletes and laborers stay cool in extremely hot and humid
conditions, engineers have developed special clothing that wicks moisture
away from the skin. Wearing these fabrics “is like standing in a wind tunnel,” says Christie. The clothing pulls sweat off the skin through tiny channels in the fabric and deposits it on the outside of the fabric where
it evaporates. Fabrics that do not wick moisture away from the skin, such
as cotton, simply soak up the moisture and retain it — leaving you feeling soggy and hot."
https://phys.org/news/2021-04-chillest-ape-humans-evolved-super-high.html
"Scientists broadly assume that humans' high density of sweat glands,
also called eccrine glands, reflects an ancient evolutionary
adaptation. This adaptation, coupled with the loss of fur in early
hominins, which promoted cooling through sweat evaporation, is thought
to have made it easier for them to run, hunt, and otherwise survive on
the hot and relatively treeless African savannah, a markedly different
habitat than the jungles occupied by other ape species."
Repeated mutation of a developmental enhancer contributed to human thermoregulatory evolution.
https://www.pnas.org/content/118/16/e2021722118
Significance
One of the most distinctive physiological traits differentiating
humans from other primates is a reliance on sweating to cool off. The effectiveness of human thermoregulatory sweating is underlain by the
evolution of a dramatically increased density of water-secreting
eccrine sweat glands in human skin relative to that of other primates.
Here, we show that the accumulation of human-specific mutations in a developmental enhancer collectively promoted the production of eccrine
glands in humans by up-regulating the expression of the Engrailed 1 transcription factor in the skin. This study reveals a mechanism that contributed to the evolution of humans’ signature thermoregulatory capabilities and underscores the importance of regulatory evolution in generating the modern human form.
Abstract
Humans sweat to cool their bodies and have by far the highest eccrine
sweat gland density among primates. Humans’ high eccrine gland density
has long been recognized as a hallmark human evolutionary adaptation,
but its genetic basis has been unknown. In humans, expression of the
Engrailed 1 (EN1) transcription factor correlates with the onset of
eccrine gland formation. In mice, regulation of ectodermal En1
expression is a major determinant of natural variation in eccrine
gland density between strains, and increased En1 expression promotes
the specification of more eccrine glands. Here, we show that
regulation of EN1 has evolved specifically on the human lineage to
promote eccrine gland formation. Using comparative genomics and
validation of ectodermal enhancer activity in mice, we identified a
human EN1 skin enhancer, hECE18. We showed that multiple epistatically interacting derived substitutions in the human ECE18 enhancer
increased its activity compared with nonhuman ape orthologs in
cultured keratinocytes. Repression of hECE18 in human cultured
keratinocytes specifically attenuated EN1 expression, indicating this
element positively regulates EN1 in this context. In a humanized
enhancer knock-in mouse, hECE18 increased developmental En1 expression
in the skin to induce the formation of more eccrine glands. Our study
uncovers a genetic basis contributing to the evolution of one of the
most singular human adaptations and implicates multiple interacting
mutations in a single enhancer as a mechanism for human evolutionary
change.
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