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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