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Comfort Zone
GOAL:To understand how air temperature, humidity,
mean radiant temperature (MRT), wind and
sunshine affect human comfort.
OBJECTIVE:You will be able to:
1. Describe ways that the human body reacts while
seeking its comfort zone.
2. Explain how air temperature, mean radiant
temperature, humidity, wind, and sun affect human
comfort.
3. Use the comfort zone chart correctly.
LESSON/INFORMATION:
The weather is always changing. As winter
approaches, the rabbits shed old fur and grow a new
heavier pelt, birds fly south, and bears hibernate.
When trying to maintain a body temperature close
to 98.6°F, humans have few natural controls with
which to adapt to these changes in climate.
The human body has three mechanisms to
maintain this narrow temperature range. The
first is heat generated inside the body, the
second is by gaining heat from surroundings,
and the third is by losing heat to the
surroundings. The body automatically makes
constant changes to control these three
mechanisms and regulate body temperature.
Body Heat
Heat is continuously produced by the body due to metabolism, or the
processes of food conversion and tissue building. Additional heat is
produced by muscular activity, which varies from 70 watts while
sleeping to 1100 watts for maximum heavy manual work. Of all the
heat produced, 20% is utilised, and 80% must be dissipated, in order to
maintain deep body temperature at 37°C. Any heat gained from the
environment and from solar radiation must also be dissipated. The
body can lose heat by convection, radiation and evaporation, and to a
lesser extent by conduction.
Convection is produced when heat is transferred from the body to the
air adjacent to the skin or clothing, which rises and is replaced by
cooler air. Radiant heat loss depends on the temperature of the body
surface and the temperature of opposing surfaces. Evaporative heat
loss depends on the rate of evaporation which depends on the humidity
of the air.
The following are a few of the ways the body responds in order to stay within the comfort zone:
Increased muscle activity and a higher metabolic rate increases internal heat production.
Sweating -
Blood Flow -
Reduced flow to the hands, feet and skin
surface in the winter to reduce heat loss to
surroundings and an increase in blood flow to
these areas in the summer to encourage heat
loss.
Comfort
Human beings can tolerate a fairly wide range of
climatic conditions, but comfort in the climatic sense
involves more than just avoiding the extremes of
freezing to death and dying of heat exhaustion.
Comfort depends on more than temperature; air
temperature, humidity, radiation and air movement
all produce thermal effects. Most climatic comfort
indicators are objective, i.e. they can be measured,
and acceptable ranges established quantitatively.
Factors Affecting Human Comfort Include:
Air temperature is the most significant ambient
factor which affects our internal temperature
and our level of comfort. But, it is not the only
factor involved; air speed, humidity and mean
radiant temperature must also be considered.
Each of these four factors has a direct
influence on the rate at which the body loses
or gains heat to or from the surroundings.
Air Temperature -This affects temperature difference between the body and the surroundings, consequently affecting the rate of heat loss or gain by convection.
Air Speed -This affects the rate at which the body loses heat by convection. The chill factor is one way to quantify the effects of air speed on heat loss. An air temperature of 35°F and a wind speed of 20 miles/hour combine to give a wind chill temperature of 11.2°F. This means that a body exposed to 35°F air and 20 mile/hour wind loses heat at the same rate as a body exposed to 11.2°F and no wind. Air speed is also very important during summer when the body is trying to lose heat to maintain comfort.
Mean Radiant Temperature (MRT) -
MRT is the average of the surface temperature of the
surroundings with which the body can exchange heat
by radiant transfer. Radiant heat transfer to and from
the body is quite apparent when sitting near a fireplace
(high MRT) or large cold window area (low MRT).
Humidity -
Affects the rate at which the body loses heat by
evaporation. During hot weather, high humidity
increases discomfort by making it more difficult to
evaporate perspiration into the air.
The diagram shows
the area of warmth
and humidity which
the human body
would classify as
comfortable.
The higher the
relative humidity the
lower the room
temperature must
be.
An easy way of describing the effect of air temperature, humidity, MRT,
wind and sunshine is the Bioclimatic Chart below.
The comfort zone can be pushed up by the presence of air movement, but
lowered by higher levels of radiation. The results were obtained from a
study of men in sedentary occupations, wearing clothing (suit, cotton
underwear) in a warm climate.
The air temperature is plotted on the vertical axis and
relative humidity on the horizontal axis. The shaded area
near the center of the graph shows the combination of
temperature and humidity which most humans would find
comfortable during the summer if they are sitting in the
shade. The dotted area shows the comfort zone for the
winter. It is interesting to see that the human body can
actually adjust somewhat to different seasons.
The climatic elements around the comfort zone are shown
by means of curves which indicate the nature of corrective
measures necessary to restore the feeling of comfort at any
point outside the comfort zone. For any point of known dry-
bulb temperature and relative humidity which falls within
the boundaries of the comfort zone, no corrective measures
are needed.
For example, at dry-bulb temperature, 73°F, relative
humidity of 50%, no corrective measures are needed
because this point falls within the comfort zone.
At dry-bulb temperature, 78°F, relative humidity of 70%,
it would require a wind speed of about 250 FPM to
provide comfort.
At dry-bulb temperature of 50°F, relative humidity of
55%, it would require 250 Btu/hr of sunshine to provide
comfort.
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