Predicted Heat Strain index (PHS) MODEL

Transcription

1 ISO 7933 " interpretation of thermal stress using the Required Sweat Rate" Predicted Heat Strain index (PHS) MODEL J. Malchaire Unité Hygiène et Physiologie du Travail Université catholique de Louvain Brussels Belgium Main criticisms concerned: The prediction of the skin temperature The influence of the clothing on convection, radiation and evaporation The increase of core temperature linked to the activity The prediction of the sweat rate in very humid conditions The limiting criteria and in particular the "alarm" and "danger" level The maximum water loss allowed.. Predicted skin temperature Material and Methods HEAT database (3 files) containing minute by minute values of parameters of stress and strain only data from male subjects Final T sk database includes 999 data points coming from 399 conditions with 377 male subjects Predicted skin temperature Prediction model: nude subjects t sk = t a +.6 t r +.98 p a.348 v a +.66 t re clothed subjects 4 t sk =.7+. t a +.44 t r +.94 p a.53 v a +.3 M+.53 t re 4 Ranges of validity of the Tsk model Min Max t a C 5 5 P a kpa 4.5 t r -t a C 6 v a m/s 3 M W 45 I cl clo.. Observed skin temperature [ C] Predicted skin temperature [ C] Observed skin temperature [ C] Predicted skin temperature [ C] 3 4

2 . Prediction of t re from the core temperature The core temperature t co is the mean of the rectal temperature: characteristic of the muscle mass the oesophageal temperature: characteristic of the blood and influencing the hypothalamus. 3. Increase in t co associated with M Saltin (966), in a neutral condition, t cor =. M (M in watts) t co reaches t cor with a time constant of about minutes. t co = t co. k + t cor. ( - k) Edwards et al.: dtre toe = a tre + b + c dt where: k = exp(-incr/) incr = the time increment, in minutes. t co = core temperature at time i = core temperature at time (i-) tco tre = tre + t co -.96 t 9 re -.3 Heat storage associated with this increase: ds R = c sp (t co - t co ) (- α) Exponential averaging for t sk, SW 4. Maximum sweat rate: SW SW max Observed and predicted SW (using ISO 7933 and PHS) in a lab experiment with 3 sequences of work and climate. ISO 7933 assumes constant values of maximum sweat rate for acclimatised and unacclimatised subjects Araki et al. (979): SW max =.6 (M - 58) g/h for M < 3 watts : 65 g/h Sweat rate (g/h) limited to g/h for unacclimatised subjects For acclimatised subjects: sweating in a given environment greater by a factor MAXIMUM sweat rate increase by 5% (Havenith 997) Time (min) Observed PHS ISO

3 5. Maximum dehydration and water loss Szlyck (989): threshold for thirst: % loss of body weight Candas et al. (985): at 3% dehydration: increased heart rate depressed sweating sensitivity. maximum dehydration in industry (not army or sports): 3% of body mass Kampmann et al.(995): with exposure of 4 to 8 hours average rehydration rate of 6% rehydration rate greater than 4% for 95% of the subjects Maximum water loss 7.5% of the body mass for an average subject 5% of the body mass for 95% of the working population 6. Limit of internal temperature WHO document 969: Limit of 38 C commonly adopted and implicitly adopted in ISO 7933 Maximum rectal temperatures: 4 the maximum internal temperature to avoid any physiological aftereffect 39. "may rapidly lead to total disability in most men, with excessive, often disturbing, physiological changes" Wyndham et al. (965). Maximum probabilities: for 4 : < -6 : < heat stroke every 4 years among workers (5 days/year) for 39. : < -3 : < person at risk among shifts. 9 Limit of internal temperature Validation in laboratory experiments: SW Wyndham's data for non acclimatised workers 38.7 C C for p - 6 of reaching 4 C 38. C C for p - 3 of reaching 39. C for acclimatised workers 39.4 C C for p - 6 of reaching 4 C 38.3 C C for p - 3 of reaching 39. C 38.3 and 38. are closed to 38 C C (WHO document) observed SW (g/h) Observed and predicted sweat rates (95% confidence interval): 67 laboratory experiments limit at 38 C predicted SW (g/h)

4 Validation in laboratory experiments: t re Observed and predicted rectal temperature (95% confidence interval): 67 laboratory experiments Validation in field experiments: SW Observed and predicted sweat rates (95% confidence interval): 37 field experiments observed t re (C) predicted t re (C) observed SW (g/h) predicted SW (g/h) 3 4 Validation in field experiments: t re Observed and predicted rectal temperature (95% confidence interval): 37 field experiments 4. observed tre (C) Strategy for the management of the thermal working conditions predicted tre (C) 5 6

5 Climatic factors,, M, clo Predicted Heat Strain Prediction SW, Tco,, DLE 7 Symbol Term Unit Symbol in the program - code = if walking speed entered, otherwise - defspeed - code = if walking direction entered, otherwise - defdir α fraction of the body mass at the skin temperature dimensionless - α i skin-core weighting at time i dimensionless TskTcrwg α i- skin-core weighting at time (i-) dimensionless TskTcrwg ε emissivity of the bare skin dimensionless - τ time constant min - θ angle between walking direction and wind direction degrees Theta A Du Dubois body surface area square metre Adu A p fraction of the body surface covered by the clothing dimensionless Ap A r effective radiating area of the body dimensionless Ardu C heat flow by convection at the skin surface Watts per square metre Conv c e water latent heat of evaporation Joules per kilogram - C orr,cl correction for the dynamic clothing insulation for totally dimensionless CORcl clothed subjects C orr,ia correction for the dynamic boundary layer insulation dimensionless CORia C orr,tot correction for the dynamic clothing insulation as a dimensionless CORtot function of the actual clothing C orr E correction for the dynamic permeation rate dimensionless CORe c p specific heat of dry air at constant pressure Joules per kilogram of dry air - C res heat flow by respiratory convection Watts per square metre Cres c sp specific heat of the body Watts per square meter per spheat degree celsius D lim allowable exposure duration min Dlim D lim tre allowable exposure duration for heat storage min Dlimtre D limloss5 allowable exposure duration for water loss, mean min Dlimloss5 subject D limloss95 allowable exposure duration for water loss, 95% of the min Dlimloss95 working population D max maximum water loss grams Dmax D max5 maximum water loss to protect a mean subject grams Dmax5 D max95 maximum water loss to protect 95% of the working grams Dmax95 population dsi heat stored during the last time increment Watts per square metre dstorage dseq body heat storage rate for increase of core Watts per square meter dstoreq temperature associated with the metabolic rate 8 E req = M - C res - E res - C - R - ds eq E max <= E req <= E max = E req = SW req = SW max SW req = Climatic factors,, M, clo w req = E req / E max R w req >=.7 w req =.7 SW req = SW max r req = ( - w req ² / ) w req > r req = ( - w req )² / R SW req = E req / r req SW req > SW max SW req = SW max SW p =.948 SW p +.95 * SW req Black box SW p <= SW p =, E p = k = E max / SW p R3 w p = k >=.5 w p > w max w p = -k + SQR(k²+) w p = w max Prediction SW, Tco,, DLE E p = w p E max 9

6 Sophistication Cost Expertise Prevention Strategy Expertise Analyse Analysis PREVETIO When? How? Level Screening Systematically Opinions Level Observation When a "problem" is detected Qualitative observations Level 3 Analysis More complicated Cases Ordinary measurements Level 4 Expertise Very complex cases Specialised measurements Cost? Very low Low Average High Observation Dépistage Screening umber of work situations umber of risk factors Duration (order of magnitude) By whom? Knowledge - working conditions - ergonomics min hours day A few days Workers + company management Very high Low Workers + company management High Average Same + specialists Average High Same + specialists + experts Low Specialised Observation designed to: Level : OBSERVATIO Identify particular circumstances, specific tasks, unusual working conditions where a problem exists Determine what to do to reduce or eliminate these problems: straightforward solutions By or with the help of the workers themselves. Conclusion: Is the problem satisfactorily controlled or not? If not, the assistance of specialists is needed. 3 4

7 Criteria for level : OBSERVATIO Designed for the workers and their management Simple to understand by untrained people Avoid concepts or terms not readily understood Easy to use, maximum hour for a specific work situation Based on simple Observations (no measurement) Oriented towards prevention Level : OBSERVATIO. Describe the working condition known to or likely to raise a thermal problem. Evaluate the situation for each of the six parameters separately 5 6 For each of the six parameters Temperature scale. What is the situation, on a scale of discomfort?. What could be done immediately to improve the situation? 3. What is going to be the situation afterwards? AIR TEMPERATURE Generally freezing Generally between and C. Generally between and 8 C Generally between 8 and 5 C Generally between 5 and 3 C Generally between 3 and 4 C Generally greater than 4 C 7 8

8 Temperature: solutions Humidity and radiation scale AIR TEMPERATURE Locate the sources of heat or cold in the periphery Eliminate the sources of hot or cold air Insulate the hot surfaces Exhaust hot or cold air locally Ventilate without draughts Use clothes with lower or higher insulation HUMIDIT - Dry throat/eyes after -3 hours - ormal - Moist skin - Skin completely wet THERMAL RADIATIO - Cold on the face after -3 minutes - o radiation discernible - Warm on the face after -3 minutes - Unbearable on the face after > minutes - Immediate burning sensation 9 3 Humidity and radiation: solutions Air movement and work load scales HUMIDIT Eliminate the leaks of vapour and water Enclose all evaporating surface Use clothes waterproof but permeable to vapour THERMAL RADIATIO Reduce the radiating surfaces Use reflecting screens Insulate or treat the radiating surface Locate workstations away from radiating surfaces Use special protective clothes reflecting radiation AIR MOVEMETS cold strong air movements cold light air movements no air movements warm light air movements warm strong air movements WORK LOAD office work: easy low muscular constraints, occasional movements at normal speed. Moderate work with arms or legs Intense work with arms and trunk very intense work at high speed: stairs, ladders 3 3

9 Air movement and work load: solutions Clothing and opinion scales AIR MOVEMETS Reduce or eliminate air draughts Use screens to protect locally against draughts Locate workstations away from air draughts WORK LOAD Reduce the movements during work Reduce displacements Reduce the speed of movements Reduce the efforts, use mechanical assistance Improve the postures CLOTHIG - light, flexible, not interfering with the work - long, heavier, interfering slightly with the work - clumsy, heavy, special for radiation, humidity - special overalls with gloves, hoods, shoes OPIIO OF THE WORKERS - shivering, strong discomfort for the whole body - strong local discomfort overall sensation coolness - slight local cool discomfort - no discomfort - slight sweating and discomfort thirst - heavy sweating, work pace modified - excessive sweating, special clothing Clothing solutions Synthesis of the results for the present situation CLOTHIG Improve the design of the clothing Select more suitable materials Look for lighter materials Air temperature O Humidity O Radiation O Air movements O Work Load O Clothing O 35 36

10 Synthesis of the results for the future situation Level : Observation Air temperature X O Humidity X O Radiation = Air movements = Work Load X O Clothing X O 5. Determine globally how acceptable the situation is 6. Measures to be taken in the short-term: term: Hot or cold drinks Recovery periods Work organisation Clothing. 7. Decide whether a more detailed level 3, Analysis is needed to quantify and to solve the problem Level 3: Analysis Level 3: Analysis in specific conditions conducted with the assistance of an OH practitioner with adequate training to find technical solutions to define organisational solutions and short-term term protection measures Use common concepts and techniques and, if necessary, simple measurements to identify the causes of the problems and the means to solve these problems Useable in less than one day Oriented towards prevention 39 4

11 Analysis: : Procedure Analyse the sequence of activities: description of the activities mean and maximum durations period concerned by the working situation exposed workers during representative period(s) of time measurement or estimation of the mean and maximum values computation of the indices PMV-PPD, PPD, PHS t a RH t g v a M Clo PMV PPD WBGT PHS / DLE Analysis : Synthesis A ctivity... A ctivity... mean max mean max 4 4 Analysis : interpretation Risk in the present situation cold constraint PMV < - cold discomfort - < PMV < -,5 comfort -,5 < PMV <,5 warm discomfort,5 < PMV < constraint in the long term DLE < 48 min constraint in the short term DLE < min immediate constraint DLE < 3 min Analysis: : Procedure Determine the acceptability of the working condition by comparing: the mean and maximum duration of each activity with the DLEs estimated from the PHS model Define prevention - control techniques Define the optimum work organisation. Determine the residual risk after implementation of these prevention - control measures

12 Analysis : synthesis 3. Risk Class of risk If heat stress Sweating rate Water loss per day DLE 4. Acceptability 5. Prevention/control measures 6. Residual risk 7. eed for an expertise 8. Short term measures 9. Medical surveillance Activity... Activity Level 4: Expertise Level 4: Expertise Better characterise some heat or cold sources and/or some unusual circumstances Specific measurements Specific investigation techniques Characterise the overall exposure of the workers Look for sophisticated prevention/control measures Prevention Strategy Expertise Analyse Analysis Observation PREVETIO Dépistage Screening 47 48

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