Thermal EnvironmentAP Dr. Shamsul Bahari ShamsudinUniversiti Malaysia Sabah

Summaryhealth effectsheat balance equationthermal indicesa historical perspectiveinternational standards

Health effectsBody has a narrow optimal range for temperature: Core 36.1 - 37.1 oC

Control mechanismschange in blood flow sweatingshivering

Heat induced disordersBurnsPrickly heat (skin reaction/ruptured sweat ducts)Heat syncope (fainting)Heat exhaustion (core temp 37-39C)Heat stroke (core temp >39C)

Workers at risk of heat disordersFoundry workersGlass manufactureCeramic workersCoke/gas ovensFiremenMinersAsbestos removal

Cold induced disordersAccidents (slips, trips, muscular strains)Local vascular effects (chapping and chillblains)Local freezing of tissues (frostnip, frostbite)Hypothermia

Workers at risk of cold disordersOffshore industry (oil/gas exploration)ConstructionFood/warehousing industryFishingAgriculture

Heat balance equation...

M - W = K + C + R + E +S

M = metabolic heatW = workK = conductionC = convectionE = evaporationS = storageR = radiation

Thermal indicesThermal Comfort IndicesPredicted Mean Vote (PMV) Predicted Percentage Dissatisfied (PPD)Heat Stress IndicesWet bulb globe temperature (WBGT) index (ACGIH TLV)Predicted Four Hour Sweat Rate (P4SR)Heat Stress Index (HIS)Effective and Corrected Effective Temperature (ET and CET)

WBGTWet bulb globe temperaturedeveloped in the 1950s by Yaglou and Minard for the US Navysimple and easy to calculate...WBGT = 0.7 Twb + 0.3 Tg indoors

WBGT = 0.7 Twb + 0.2 Tg + 0.1 Ta outdoors

WBGT continued...incorporated into ACGIH TLV and ISO 7243note it is the natural wet bulb which is required in this indexlimited range of application (i.e. not high air velocity with high radiant heat or high air temperature)but it is conservative

Clothing and WBGT

Clothing

Clo value

WBGT correction

Summer clothes

0.6

0

cotton overalls

1

-2

jacket+trousers

1.4

-4

permeable vapour barrier suit

2

-6

chemical protective suit

>2

-10

Clothing Clothing insulation is measured in Clo units (Icl) 1 clo = 0.155 m2 C/W clo value ranges from 0 (naked body) to 4 Summer clothing ~ 0.6 clo Winter clothing ~ 1 clo Icl ~ 0.15 x weight of clothes in lbs. 1 clo maintains sedentary person indefinitely comfortable at 21C, 50% RH, 0.01 m/sec.

Effective Temperature

Assessing thermal environmentWhat do we measure?

Air temperatureRadiant temperatureRelative humidityAir velocity

Air temperatureDry bulb thermometerssimple liquid thermometerresistance thermometerthermistorsthermocoupledifferential expansion effect

Mean radiant temperatureMeasured indirectly using nomogramGenerated fromglobe temperatureair temperatureair velocityGlobe temperature: dependent on radiant heat and convection

Air velocity High speeds use anemometersresistance anemometerthermistor anemometerLow speeds use Kata thermometerhow long does heated fluid take to coolair movement calculated from air temperature, kata factor and cooling time of fluidTracer methods also used to identify draughts and flow patterns

Heat stressOur temperatures are influenced by the interaction of:EnvironmentWorkrateClothing

Control of thermal environmentEnvironmentmodify air movementmodify air temperatureshielding from hot surfacesWork criteriaphysical workloadfrequency of work breaksacclimitisationeducationsurveillance during exposure

Control of thermal environmentIndividual fitness for workscreening for health problemsbody sizefluid balanceSuitable PPEpermeabilityreflectivityinsulationabsorbtion

Controlwork planning to avoid or minimise stressengineering controls at sourceengineering control using ventilation or air conditioningengineering control using radiation barriersmanagerial controlsuse of personal protective clothing

Engineering controls at sourcereduce the temperature of the surfaceprovide insulation for the surfacereduce the radiant heat emission

Ventilationgeneral ventilation

local ventilationincreases heat loss from convection and evaporation

Radiation barrierschoice of materialsshould we use steel, aluminium or something else?a good barrier has low emissivity and good insulationbarriers can also incorporate water cooling

Managerial controlsimproved education and trainingbetter supervisionrestriction on the amount of time spent in hot environmentsself-pacing of workdrinking rulesmedical surveillance

Personal ProtectionThe criteria for good protective clothing are:sufficient permeability to water vapour and / or sufficient ventilationacceptability to those wearing itcomfortdexterity/mobilitysufficient insulation to keep the body thermally comfortable

Active coolingdry iceair refrigeratedwater cooled

Cold environmentsclothing, windbreak and waterproof environmental modifications shelterswork-rest regimeeducation, supervision etc..

Signs of Hypothermia

Core temperature

Signs

37.6 oC

normal core temperature

36 oC

shivering

32 oC

conscious clouded, low blood pressure

30 oC

loss of consciousness, muscular rigidity, decreased respiration

28 oC

risk of heart problems

Special Recommendations for hands... fine work with bare hands below 16oC for more than 10 -20 minutes should have warm air jets or radiant heaters

Gloves needed...

Special provisions below -12oCconstant supervisionworkrate controlled to avoid sweatingacclimatisation period for new workersconsider the bulk of clothing when working out workrateminimise sitting and standinginstruction and training

SummaryMeasurement of the thermal environment involves 4 key parametersAir temperature, Radiant temperature, Relative humidity and Air velocityThe general approach to control follows a hierarchysource, transmission and ppeClothing is not an important control for moderate heat stress situationsClothing is most important for cold stress