The way in which we exchange heat The way in which we exchange heat with the environment is just differentwith the environment is just different

Working in the heatWorking 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

HyperthermiaHyperthermia

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

DehydrationDehydration

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– 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 heatWorking in the heat

Sweat rate

Heart rate

Core temp

Heat stress

He

at

str

ain

Zone A Zone B

(prescriptive zone)Zone C

Protecting workers in the heatProtecting 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 heatProtecting workers in the heat

WBGT index is adjusted for clothing insulation:Clo value WBGT correction (ºC)

0.6 – Summer work uniform 0

1.0 – Cotton overalls -2

1.4 – Winter work uniform -4

1.2 – Impermeable layer -6

Critical WBGT index (prescriptive zone) also adjusted for metabolic heat production

AcclimatizationAcclimatization

Acclimatization– Adaptation produced by a change to one’s natural environment

Acclimation– Adaptation to laboratory conditions

Heat and exercise required for optimal acclimation– 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

SHAFTSSHAFTS

Sensible, Hydrated, Acclimated, Fit, Thin, Sober

Working in the cold

Working in the coldWorking in the cold

Short-term exposures

e.g. accidental touching of cold surfaces

Long-term exposures

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

• 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

ProblemsProblems

What happens to our hands in the What happens to our hands in the cold?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 CLOSEBlood 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 AmputationPhysiological Amputation

• Due to reduced blood flow, very little heat input to the hand remains

• Essentially the same effect as occlusion

Dexterity testsDexterity 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% lossFilling boxes with cubes (2.5cm)11%Needle and thread through cube22%Fastening screws by hand26%Knots in rope28%Fastening screws with screwdriver36%Putting rings around pins38%

Gross hand tasks

Fine motor movements

Question???Question???

Below 15ºC skin temperature finger dexterity decreases rapidly…. Why is this?

Receptor SensitivityReceptor 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 coldTactile 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 ConductionNerve 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

MusclesMuscles

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/TendonsJoints/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 dexterityCentral 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 VasodilatationCold 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

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Pain threshold

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Numbness threshold

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Touching time for contact temperature to reach 0°C

Freezing threshold