Transcript Working in the cold

The way in which we exchange heat with
the environment is just different
Working in the heat
Working in the heat causes its problems…
If ambient air temperature (Ta) is above avg. skin
temperature (35ºC) means that all heat loss through the
avenue of convection (C) is lost.. Increase in air velocity
(v) actually WARMS you up!
Heat loss must be nearly all through evaporation (Esk)
– High relative humidity () means that driving force for
sweat evaporation is reduced
High radiant heat source causes radiative heat gain (R)
Rate of body heat storage (S) determines how long a worker
can be exposed to a hot environment
Hyperthermia
Clinical hyperthermia occurs at a core temperature of 38.5ºC
Heat stroke occurs at approximately 41ºC
– Where the body’s normal thermoregulatory functioning ceases to work
– Death soon follows at approximately 42ºC
Treatment includes rapid cooling of the areas where blood flow
is greatest near the skin surface
– Main arteries (cartoid – neck, axillary – armpits, iliac – groin)
– Water used as an artificial “sweat”
Care must be taken in order to avoid too greater thermal
gradient between cooling agent and skin surface – this could
“trick” the body into thinking it is cool when it is not
Dehydration
Sweating is the primary means of heat dissipation under
heat stress or exercise
– Maximal sweat rate of untrained individuals is ~ 1.5litres/hour
– This can improve with exercise acclimatization in hot-humid env.
When water lost through sweating is not replaced
dehydration occurs
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Most people can tolerate 3-4% decrease in body water
Fatigue and dizziness occurs after 5-8% decrease
Physical and mental deterioration at 10% decrease
Death at 15-25% decrease
The more you sweat, the more salt is lost
– Stimulates thirst
– Muscle cramps when associated with exercise
Working in the heat
Zone A
Zone B
(prescriptive zone)
Heat strain
Sweat rate
Heart rate
Core temp
Heat stress
Zone C
Protecting workers in the heat
In order to protect workers in the heat numerous heat stress
indices have been developed
– Wet Bulb Globe Temperature (WBGT) index is the most
commonly used index
– It includes all environmental parameters into a single number in
order to indicate level of heat strain
WBGT = 0.7tnwb + 0.2tg + 0.1ta
tnwb = temp of naturally ventilated
wet bulb thermometer
tg = 150mm diameter black globe
temperature
ta = air temperature
Protecting workers in the heat
WBGT index is adjusted for clothing insulation:
Clo value
0.6 – Summer work uniform
1.0 – Cotton overalls
1.4 – Winter work uniform
1.2 – Impermeable layer
WBGT correction (ºC)
0
-2
-4
-6
Critical WBGT index
(prescriptive zone) also adjusted
for metabolic heat production
Acclimatization
Acclimatization
– Adaptation produced by a change to one’s natural environment
Acclimation
– Adaptation to laboratory conditions
Heat and exercise required for optimal acclimation
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Onset of sweating occurring at a lower core temp threshold
Decrease in heart rate
Fall in NaCl conc in sweat and urine
Increase blood volume
Facilitates greater skin blood flow
SHAFTS
Sensible, Hydrated, Acclimated, Fit, Thin, Sober
Working in the cold
Working in the cold
Short-term exposures
Long-term exposures
e.g. accidental touching
of cold surfaces
e.g. military exercises
Risk of tissue damage due to
freezing and non-freezing cold
injury
Decrease in dexterity resulting
in reduced work performance INCREASED No. OF ACCIDENTS
Problems
• In order to reduce risk of freezing and non-freezing cold
injury, protective measures are taken - i.e. wearing gloves.
However, the presence of gloves can itself deteriorate
dexterity, therefore bare hands are often required
• Physiological mechanisms involved in reducing dexterity
– Physiological amputation
– Responses of receptors, nerves, muscles, joints and tendons
– Central effects
– Critical temperatures
• Prediction of cold exposure risk
What happens to our hands in the cold?
In the hand, blood flow is regulated by
the AVA’s (Arteriovenous Anatomoses)
– Body is warm: AVA’s OPEN
Blood flows in large quantities from
the arteries through the AVA’a to the
superficial veins
– Body cools: AVA’s CLOSE
Blood flow is drastically reduced due
to increased sympathetic activity
The remaining flow will return to the
body core through deep veins, which
are located close to the arteries
Physiological Amputation
• Due to reduced
blood flow, very little
heat input to the
hand remains
• Essentially the same
effect as occlusion
Dexterity tests
When the hand begins to cools, dexterity will gradually be
lost….
Below 15ºC, finger dexterity decreases sharply. The extent
to which dexterity is lost is dependent upon the task.
A range of dexterity tests are used with emphasis changing
from gross hand tasks to fine motor movements
Dexterity Test
Gross hand tasks
Fine motor movements
Filling boxes with cubes (2.5cm)
Needle and thread through cube
Fastening screws by hand
Knots in rope
Fastening screws with screwdriver
Putting rings around pins
% loss
11%
22%
26%
28%
36%
38%
Question???
Below 15ºC skin temperature finger dexterity decreases
rapidly…. Why is this?
Receptor Sensitivity
The are are two groups of receptors that affect dexterity:
There are those located in the motion apparatus (muscle
spindles, joints, ligaments)
– provide information on the position of the hand in the
environment relative to the body
There are those located at the superficial surface, in the
skin (pain, tactile, pressure and thermal)
– provides information on the structure and texture of
handled objects
Tactile sensitivity is affected by the cold
Tactile sensitivity in the cold
• Only a minor impairment of
performance is seen between 25ºC
and 8ºC skin temperature
• A nervous block occurs between
6ºC and 8ºC local skin temperature
– nerve fibres and receptors
consequently no longer initiate or
conduct nerve impulses at these
temperatures.
• This has an effect on manual
dexterity, but cannot be the full cause:
Temps for impairment lower than 15ºC
Nerve Conduction
• Nerve conduction velocity is an important factors when
considering the effects of the cold on manual performance
Normal mean conduction velocity: 60ms-1
• Between 36ºC and 23ºC a linear decrease in nerve
conduction velocity of 1.8ms-1ºC-1
• Below 23ºC there is a stronger decrease found
• A complete nervous block occurs at temperatures below
10ºC (no conduction at all below these temperatures)
– Apart from nerve endings, nerves are located in deeper structures…
therefore nerve temperature will follow skin temperature after a large
delay. Reduction in dexterity at 15ºC skin temperature may be partly
attributed to reduced conduction velocity
Muscles
The effect of muscle performance on manual dexterity can
be as a result of changes in power, contraction speed or
endurance
Optimal muscle temperature depends upon the type of task
– Endurance work with the hands shows the best performance at a
muscle temperature of 28ºC. Below this temperature performance
decreases sharply
– Maximal power and contraction speed (very short, maximal
contractions) requires an optimal muscles temperature of 38ºC
A proposed explanation for the reduction in performance for muscle
contractions in the cold is that fibres located at the periphery are eliminated
due to the effects of the environment on on superficial muscle fibres.
Therefore less fibres produce the same force level, thus producing a
quicker onset of fatigue??
Joints/Tendons
• Mobility of the fingers is mostly determined by the
movement of the joints. Cold has an critical influence upon
joints, causing the synovial fluid to become more viscous…
producing slower movements - commonly experienced as
“joint stiffness”
• There is a linear relationship between joint and skin
temperature:
– Observable decreases in manual dexterity occurs below 20ºC
skin temperature, the equivalent of 27ºC intra-articular
temperature
– A strong decrease in manual dexterity occurs below 15ºC skin
temperature, the equivalent of 24ºC intra-articular temperature
Central effects on dexterity
Psycho-physiological effects:
– thermal and pain sensation can influence manual dexterity
performance… Pain in particular has an effect on arousal and
causes a loss of attention. This can also have an effect on
reaction time
Physical effects:
– core temperature reduction itself does not affect dexterity when
isolated in experiments where the periphery has been kept
warm with a low core temperature
Core temperature does have an effect on Cold Induced
Vasodilatation (CIVD)… CIVD affects manual dexterity
Cold Induced Vasodilatation
At a certain skin temperature, AVA’s in the hand open and blood flows
through the hand increasing hand temperature. Once hand temperature
increases, AVA’s close
- cyclic behaviour
• Onset of CIVD at a skin
temperature of approx.
20ºC
• Warm core = greater
levels of CIVD
• Cold core = eliminates
effects of CIVD
5
Pain threshold
0
-5
Ts (°C)
-10
-15
-20
-25
-30
-35
-40
a) Touching time for contact temperature to reach 15 °C
-45
0
10
20
30
40
50
60
70
80
t(15) (s)
Wood
Nylon
Stone
Steel
Alum
90
100
5
Numbness threshold
0
-5
Ts (°C)
-10
-15
-20
-25
-30
-35
-40
Touching time for contact temperature to reach 7°C
-45
0
10
20
30
40
50
60
70
80
t(7) (s)
Wood
Nylon
Stone
Steel
Alu
90
100
5
Freezing threshold
0
-5
Ts (°C)
-10
-15
-20
-25
corresponding wood and nylon values above 100 s
-30
-35
-40
Touching time for contact temperature to reach 0°C
-45
0
10
20
30
40
50
60
70
t(0) (s)
Stone
Steel
Alu
80
90
100