Transcript How we Experience indoor and outdoor climates

2
facts govern heat flow within the body and
exchange with the environment
1.
2.

Body core temperature must remain at 37◦ C
Heat flows from hot to cold matter
Need to properly engineer environments to
maintain the core body temperature using
the physics principle that heat moves from
hot to cold matter
 Human
body has a complex control system to
maintain a stable body temperature
 Small deviations affects cellular structures,
enzymes, impairs physical work and mental
capacities
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Normal minor fluctuations
Physical activity
Metabolism (has the main impact on heat
regulation)
 Metabolically
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Skeletal muscles, organs, fat, bone, connective
tissue, nervous tissue
 Heat
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distribution
Circulatory system controls
Blood carries heat throughout the body
Blood vessel constriction and dilation controls
heat loss or conservation
 Heat
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active tissues
exchange with the environment
Lungs
skin
 Temperatures
of the core
tissues must be kept
constant
 Temperature control takes
place at the external
tissues
 Core and shell
temperature difference
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Normally 4◦C at rest
Cold environment 20 ◦C or
more
 Hot
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skin in a hot environment
Nature’s solution is to increase the skin
temperature above the environment’s
Allows for sweat evaporation
Evaporation of 1 cm³ of water requires 2440J
(580 calories)
The body must reduce its internal heat
production when it cannot disperse enough
energy
 Cold
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Reduces blood flow to the skin
Increase insulation by wearing more layers
Shivering
 Heat
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skin in a cold environment
exchanges with the environment
The body’s heat control systems must interact
with the physical components of the environment
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Convection
Conduction
Radiation
Evaporation
 Heat
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exchange by convection or conductance
Convection – transfer of heat energy to air or
water that is flowing over the skin
Conduction – transfer of heat energy to a solid
mass
As the temperature of the 2 contact surfaces
becomes equal, the energy exchange ceases
Dependant on:
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Body surface available for heat exchange
Body surface temperature
Temperature of the medium in contact with the skin
Heat conduction coefficient
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Material with heat coefficients less than skin feel
warm to touch; greater heat coefficients feel cool
Heat exchange is facilitated if the medium moves
quickly along the skin’s surface
Free convection – normal transfer of heat due to
temperature gradient
Induced convection – heat transfer due to a forced
action Figure (8.1)
 Heat
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Doesn’t depend on the temperature of air
between 2 surfaces; instead it depends on:
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exchange by radiation
The body surface
The temperature of the emitting surface
Emission coefficient of the emitting surface
Absorption coefficient of the receiving surface
Temperature of the receiving surface
Heat transfer from the window to the body
(Figures 8.2 and 8.3)
Wavelengths radiated from the human body are
in the infrared range
Emission coefficient = 1; absorption coefficient
depends on skin color
 Heat
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Occurs in only one direction
Heat loss depends on:
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Volume of sweat evaporated
Vapor pressure of the skin
The humidity of the surrounding air
Vapor pressure of the surrounding air
Can only occur if surrounding air is less humid
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exchange by evaporation
Air movement across skin helps to increase
evaporation by replacing the humidified air with dry
air
Can also occur in a cold environment when physical
work is performed and via lungs
Evaporation is more effective than convection
and radiation
4
physical factors define the thermal
environment:
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Air/water temperature
Air humidity
Air/ water movement
Temperatures of surfaces
 The
combination of these factors determines
the physical conditions of the climate, its
effects on us and how we perceive it
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Measuring temperature
 Dry bulb thermometer filled with alcohol
Measuring humidity
 Hygrometer – electrical conductivity changes with
changing humidity
 Psychrometer – consists of a DB and WB
thermometer; the WB is more cooled by the
evaporation
Absolute humidity – when further increase would lead
to water droplets
 Depends on air temperature and barometric
pressure
Relative humidity – actual vapor content in relation
to absolute
Hygrometer
 Measuring
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air flow
Anemometers have been replaced by electronic
techniques
 Measuring
radiant temperature
Measuring the temperature via
A thermometer placed in a
black globe
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 Interacting
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climate factors
Air temp affects body temp, air humidity
determines our ability to sweat, air movement
across surfaces, radiation depends on the temps
of the 2 surfaces
The combinations of the different factors can
create different effects on how we perceive
temperature
Effective temperature chart – expresses the
combined effects of environmental factors by
measuring air temp, velocity and humidity
 Combining
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instruments
Way to assess current climate by using an
instrument which weighs the different factors
and calculates a single index
Wet bulb globe temperature (WBGT) weighs the
effects of several climate parameters in warm
environments:
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WB is concerned with the humidity
GT is concerned with radiant temp
DB is concerned with dry air temp
 Microclimate
vs macroclimate
 Suitable Microclimate depends on:
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Age
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Elderly are less active, have a decreased muscle
mass/tone, sweat at higher skin temperatures
Surface to volume ratio
Fat to lean body mass ratio
 Personal
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thermal comfort
Work performed, type and intensity determine
Hard physical work is more comfortable in a cool
climate than a warm climate
 Clothing
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3 major traits
1.
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Insulation – the measure of resistance to heat
exchange by convection, conductance and radiation
Impedes energy exchange
Light colors minimizes heat gain by radiation
Dark colors absorb sun’s radiated heat
Reduces risk of injury when contacting hot or cold
objects
CLO units – the defined insulating value of clothing
Air bubbles between clothing layers increase
insulation
Permeability – measure of how clothing permits
movement of water vapor through fabric
2.
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Important in hot climates to allow evaporation from
the skin to allow cooling
Important in cold climates to prevent clammy
feeling when water is trapped under clothes
Ventilation – measure of the ability of air to
move through clothing
3.
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Good in warm climates to promote evaporation
Undesirable in cold climates
Clothing also determines SA of exposed skin
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Fingers, toes, head and neck need special
protection in cold environments
 Wind
chill – the air temp felt on exposed skin
due to wind; type of convective heat loss
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Cooling is more pronounced with higher wind
speeds
 Acclimatization
–adjustment of an
individual’s mind and body to changed
environmental conditions
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In hot conditions:
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Cold conditions
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Blood flow increases at the skin, sweating increases,
stroke volume increases
Achieved within 1-2 weeks
True physiological acclimatization is questionable
because most adaptation occurs through increased
clothing
However, blood flow is directed towards the core of
the body
Seasonal changes
 Blood
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distribution
The body produces heat which it must release to
prevent overheating
Dissipation works best when the skin temp is
higher than the surrounding temp
Circulatory system redistributes blood flow to
increase heat flow to the skin
Blood vessel dilation may bring about a 4 fold
increase of blood to the skin, greatly increasing
heat release
 Sweating
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Ability to sweat varies between individuals
Amount of sweat developed and evaporated
depends on:
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Clothing
Environment
Work requirements
Individual acclimatization
 Reducing
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Final necessary action of the body if core temp is
above an acceptable level
Body ceases muscular efforts
 Signs
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physical effort
of heat strain
Increase of circulatory activities – increase in CO
from increased HR and a decrease in BP
High sweat rate
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Normal = 50 cm³/hr
During normal work = 1L/hr
Sweat drips off the skin when generation is at 1/3 max
 Drink
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Dehydration of 1-2% body weight can affect
ability to perform bodily functions
Sweat depletes the body of water and
electrolytes
 Heat
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water!
distress
1st reaction = skin eruptions, prickly heat
associated with clogged sweat ducts
Heat cramps due to lack of Na+
 Heat
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Heat exhaustion
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Dehydration and overload of the circulatory system
Fatigue, headache, dizziness, nausea, giddiness,
excessive sweating
Heat syncope
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distress
Failure of the circulatory system
Fainting
Heat stroke
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Overload of the circulatory and sweat systems
Hot dry skin, increased core temp and mental
confusion
Medical emergency
 Working
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hard in the heat
Short, maximal muscle exertion is not
compromised
High intensity, endurance physical work is
severely reduced
Competition between the cardiovascular
functions of heat dissipation and blood supply to
the muscles
Heat dissipation wins and the person cannot
perform the physical labor
 Few
natural defenses against the cold
 2 major ways to regulate temp:
1.
2.
Blood flow redistribution
Increase in metabolic rate
 Redistribute
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Body lowers skin temp to reduce difference with
the environment
Constricts vessels near the skin’s surface
Hunting reflex – cold induced automatic
vasodilation
 Wear
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blood
gloves, caps
Blood displacement to the core may endanger
peripheral tissues
 Increase
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Involuntary: shivering
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metabolism
Begins in the neck
Muscle units firing at different frequencies of
repetition, out of phase with each other
No mechanical work is done so total activity is devoted
to heat production
Increases metabolism up to 4 x RMR
Shivering becomes violent when large muscle units
become involved
Voluntary: dynamic muscular work
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Increases metabolism to 10 x RMR
 Goose
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Retains a layer of warm stationary air close to
the skin
Insulates and reduces energy loss by the skin
 How
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cold does it feel?
Cold perception
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bumps
Surface receptors
Body core sensors
Sensation intensity increases below 35.5ºC, is
strongest at 20ºC and decreases at lower temps
Hard to separate feelings of pain from cold
 How
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cold does it feel?
Conditions of exposure influence perceived
coldness
Experiments show that subjects have a hard time
determining how cold they really are
Overshoot sensation
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When the temperature plunges, temperature receptors
respond to both the decrease in temperature and the
rate of change
Very cold water accentuates this phenomenon
 Signs
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Subjective sensation of cold is an unreliable
indicator of core and surface temp
If vasoconstriction and metabolism aren’t
enough, skin will suffer first while the body core
is protected for as long as possible
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of cold strains
Frostbite
Important temps
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Joint temps below 24ºC and nerve temps below 20ºC
reduce fine motor skills
Dexterity is reduced with finger temps below 15ºC
Tactile sensitivity is decreased below 10ºC
Touch and pressure receptors fail below 5ºC = numb
 Signs
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of cold strains
Important temps (cont.)
8ºC PN velocity = 0= nerve block; physical impairment
Apathy
hypothermia
 Core temps
 35ºC CNS coordination suffers
 32ºC = LOC
 26ºC heart failure occurs
 20ºC vital signs disappear
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 Working
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Wear appropriate clothing
 Climate
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hard in the cold
effects on mental tasks
Lack of objective testing methods
Mental performance deteriorates with temps
rising above 25ºC, 30 - 35ºC if acclimated
Motor skills are affected by temps above
30ºCWBGT; mental is not affected until
40ºCWBGT
 Comfortable
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21-27ºC in a warm climate; 18-24ºC in a cool
climate
Relative humidity between 30 – 70%
Air temp between floor and head level should
differ by less than 6ºC
Differences between body and surfaces <10ºC
Velocity of cool air < 1.5 m/s
 Comfortable
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climate conditions
climate design must consider:
Work performed
Acclimatization
Clothing
Psychological inclination
 The
body must maintain a core temp of 37ºC
 Heat energy may be gained or lost through:
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Convection
Conduction
Radiation
Evaporation (one way transfer from body to
environment)
 In
a hot environment, the body tries to keep
the skin hot; in cold the body tries to keep
the skin cold
 Wear the proper clothing!
 Thermal
environment is determined by
combinations of:

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Air humidity (evaporation)
Air temp (convection and evaporation)
Air movement (convection and evaporation)
Temperature of solids touching the body
(conduction)
Temperature of surfaces adjacent to the body
(radiation)
 Combined
effects of the physical factors are
expressed in a climate index, WBGT