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

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Presentation on theme: "The way in which we exchange heat with the environment is just different."— Presentation transcript:

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

2 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 (E sk ) –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

3 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

4 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 –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

5 Working in the heat Sweat rate Heart rate Core temp Heat stress Heat strain Zone A Zone B (prescriptive zone) Zone C

6 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 = 0.7t nwb + 0.2t g + 0.1t a t nwb = temp of naturally ventilated wet bulb thermometer t g = 150mm diameter black globe temperature t a = air temperature

7 Protecting workers in the heat WBGT index is adjusted for clothing insulation: Clo valueWBGT correction (ºC) 0.6 – Summer work uniform0 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

8 Acclimatization 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

9 SHAFTS Sensible, Hydrated, Acclimated, Fit, Thin, Sober

10 Working 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

11 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 Problems

12 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

13 Physiological Amputation Due to reduced blood flow, very little heat input to the hand remains Essentially the same effect as occlusion

14 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 Gross hand tasks Fine motor movements

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

16 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

17 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

18 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 -1 ºC -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

19 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??

20 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

21 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

22 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




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