Thermal comfortThermal comfort

Thermal comfort is a complex Thermal comfort is a complex entityentity.. Much work was done to Much work was done to determine what constitutes “thermal determine what constitutes “thermal comfort”. Several comfort”. Several indices indices have been put have been put forward from time to time forward from time to time to express to express thermal comfort and heat stress.thermal comfort and heat stress. These These are as follows: are as follows:

AIR TEMPERATUREAIR TEMPERATURE – it was – it was used for a long time as an index of used for a long time as an index of thermal comfort, but it was realized that thermal comfort, but it was realized that air temperature alone was not an air temperature alone was not an adequate index.adequate index.

AIR TEMPERATURE AND AIR TEMPERATURE AND HUMIDITYHUMIDITY - later, air temperature and - later, air temperature and humidity were considered together to humidity were considered together to express thermal comfort, even this was express thermal comfort, even this was found to be satisfactory.found to be satisfactory.

COOLING POWERCOOLING POWER – still – still later, air temperature, humidity and air later, air temperature, humidity and air movement were considered together and movement were considered together and expressed as “cooling power” of the air. expressed as “cooling power” of the air. These indices plus mean radiant heat are These indices plus mean radiant heat are used by the Bulgarian standards to evaluate used by the Bulgarian standards to evaluate

the thermal comfort.the thermal comfort.

EFFECTIVE TEMPERATUTEEFFECTIVE TEMPERATUTE – an – an arbitrary index, which combines into a single value arbitrary index, which combines into a single value the effect of temperature, humidity and air movement the effect of temperature, humidity and air movement on the sensation of warmth or cold felt by the human on the sensation of warmth or cold felt by the human body. The numerical value of effective temperature body. The numerical value of effective temperature is that of the temperature of still, saturated air which is that of the temperature of still, saturated air which will induce the same sensation of warmth or cold as will induce the same sensation of warmth or cold as that experienced in the given conditions. For that experienced in the given conditions. For example, if the environment has an ET value of 30 example, if the environment has an ET value of 30 deg.C, it implies that the subjective sensation will be deg.C, it implies that the subjective sensation will be the same as in a saturated atmosphere of 30 deg.C the same as in a saturated atmosphere of 30 deg.C with no air movement. A criticism of the ET is that it with no air movement. A criticism of the ET is that it ignores the effect of a radiation from the surrounding ignores the effect of a radiation from the surrounding

structuresstructures..

CORRECTED EFFECTIVE CORRECTED EFFECTIVE TEMPERATUTETEMPERATUTE – it deals with all the four – it deals with all the four factors namely, air temperature, humidity, velocity factors namely, air temperature, humidity, velocity and mean radiant heat.and mean radiant heat.

Thermal comfort is a function of many Thermal comfort is a function of many variables, including the variables, including the season of the yearseason of the year, , cultural practices and habitscultural practices and habits, differing from , differing from country to country. Nevertheless, describing country to country. Nevertheless, describing comfort zones is necessary for the proper design of comfort zones is necessary for the proper design of heating and air conditioning systems. heating and air conditioning systems.

In choosing optimal conditions for In choosing optimal conditions for comfort, knowledge of the comfort, knowledge of the energy energy expended during the course of expended during the course of routine physical activitiesroutine physical activities is is necessary, since body heat production necessary, since body heat production increases in proportion to exercise increases in proportion to exercise intensity.intensity.

Evaluation of the information relating Evaluation of the information relating the the physiology of a personphysiology of a person to the physical to the physical aspects of the environmentaspects of the environment is not a simple is not a simple task. Considerably more is involved than task. Considerably more is involved than simply taking a number of air temperature simply taking a number of air temperature measurements and making decisions on the measurements and making decisions on the basis of this information. basis of this information.

Whenever temperature differences Whenever temperature differences exist between two bodies, heat can be exist between two bodies, heat can be transferred. Net heat transfer is always transferred. Net heat transfer is always from the body (or object) with higher from the body (or object) with higher temperature to the body with lower temperature to the body with lower temperature and occurs by one or more of temperature and occurs by one or more of the following mechanisms.the following mechanisms.

ConductionConduction – the transfer of heat from one point to – the transfer of heat from one point to another within a body, or from one body to another when another within a body, or from one body to another when both bodies are in physical contact. Normally, this term is both bodies are in physical contact. Normally, this term is insignificant and can be disregarded except in special insignificant and can be disregarded except in special cases, such as swimming.cases, such as swimming.

ConvectionConvection - the transfer of heat from one place to - the transfer of heat from one place to another by moving gas or liquid. Natural convection another by moving gas or liquid. Natural convection results from differences in density caused by temperature results from differences in density caused by temperature differences.differences.

Radiation Radiation – the process by which energy, – the process by which energy, electromagnetic (visible of infrared) is transmitted through electromagnetic (visible of infrared) is transmitted through space without the presence or movement of matter in or space without the presence or movement of matter in or through this spacethrough this space

There are two sources of heat that are There are two sources of heat that are important to anyone working in a hot environment: important to anyone working in a hot environment: 1) 1) internallyinternally generated metabolic heat and 2) generated metabolic heat and 2) externallyexternally imposed environmental heat. imposed environmental heat.

The net heat exchange between a person and The net heat exchange between a person and the ambient environment can be expressed by:the ambient environment can be expressed by:

H = M ± R ± C – EH = M ± R ± C – EWhere: H = body heat storage loadWhere: H = body heat storage load

M = metabolic heat gain M = metabolic heat gain R = radiant or infrared heat load R = radiant or infrared heat load C = convection heat load C = convection heat load E = evaporative heat loss E = evaporative heat loss

Metabolic heat gainMetabolic heat gain is composed of is composed of the basal or resting metabolism that the basal or resting metabolism that provides the energy necessary to keep the provides the energy necessary to keep the body functioning, as well as the working body functioning, as well as the working metabolism that provides the energy metabolism that provides the energy necessary for the body to accomplish necessary for the body to accomplish specific tasks. specific tasks.

Metabolism can only add energy to the Metabolism can only add energy to the body; therefore, body; therefore, M is always positiveM is always positive..

Radiant heat loadRadiant heat load is energy in the is energy in the form of wavelengths that are form of wavelengths that are transformed into heat when they strike transformed into heat when they strike an object. Whether the human body an object. Whether the human body emits or receives radiant energy emits or receives radiant energy depends on temperature of the body, depends on temperature of the body, and the surrounding objects. and the surrounding objects. Thus, R Thus, R can be either positive or negativecan be either positive or negative..

Convective heat loadConvective heat load is the amount is the amount of heat energy transferred between the skin of heat energy transferred between the skin and air. Air temperature excess of skin and air. Air temperature excess of skin temperature will warm the body; air temperature will warm the body; air temperature less than skin temperature will temperature less than skin temperature will cause the body to be cooled.cause the body to be cooled.

The evaporative heat lossThe evaporative heat loss from the from the body (perspiration) reduces body heat, body (perspiration) reduces body heat, therefore therefore its value is always negativeits value is always negative. The . The use of fans to increase E is a common use of fans to increase E is a common method of cooling workers.method of cooling workers.

The body tries to maintain a balance The body tries to maintain a balance between the heat gained by work, radiant between the heat gained by work, radiant and converted heat imposed on the body, and converted heat imposed on the body, and the heat loss by sweating (evaporation). and the heat loss by sweating (evaporation). Ideally, the change in body heat content Ideally, the change in body heat content should be zeroshould be zero. If this balance cannot be . If this balance cannot be maintained by evaporation, then heat can maintained by evaporation, then heat can build up in the body, causing a rise internal build up in the body, causing a rise internal temperature.temperature.

MeasurementMeasurementThermometers are the instruments used for Thermometers are the instruments used for

measuring measuring 1. temperature1. temperature. Mercury thermometers . Mercury thermometers are widely used, as mercury boils at a high are widely used, as mercury boils at a high temperature, has a regular expansion and its level temperature, has a regular expansion and its level can be easily seen. The essential conditions for the can be easily seen. The essential conditions for the use of thermometers are that 1. the air should have use of thermometers are that 1. the air should have access to the bulbs of the thermometers and 2. the access to the bulbs of the thermometers and 2. the thermometer should be protected against radiant thermometer should be protected against radiant heat.heat.

Dry bulb thermometerDry bulb thermometer - this is an ordinary - this is an ordinary thermometer shielded from direct radiant energy thermometer shielded from direct radiant energy

sources, which measures the air temperature.sources, which measures the air temperature.

Wet bulb thermometerWet bulb thermometer - precisely the - precisely the same as the dry bulb thermometer excepting same as the dry bulb thermometer excepting that the bulb is kept wet by a muslin cloth. that the bulb is kept wet by a muslin cloth. The evaporation of water from the muslin The evaporation of water from the muslin cloth lowers the temperature of the mercury. cloth lowers the temperature of the mercury. The wet bulb thermometer therefore shows a The wet bulb thermometer therefore shows a lower temperature reading than the dry bulb lower temperature reading than the dry bulb thermometer.thermometer.

The drier the air, the lower the wet The drier the air, the lower the wet bulb reading. It the wet and dry bulb bulb reading. It the wet and dry bulb thermometers record the same temperature, thermometers record the same temperature, it means that the air is completely saturated it means that the air is completely saturated with moisture, which is rare. with moisture, which is rare.

In conclusion, the wet bulb In conclusion, the wet bulb thermometer measures the effect of thermometer measures the effect of humidity on evaporation and effect of air humidity on evaporation and effect of air movement on ambient temperature.movement on ambient temperature.

2) Humidity2) Humidity,, the amount of water the amount of water vapour in a given space is commonly vapour in a given space is commonly measured as measured as relative humidityrelative humidity (RH). That is (RH). That is the percentage of moisture present in the the percentage of moisture present in the airair, complete, complete saturat saturation being taken as 100ion being taken as 100. . It could be determined using aIt could be determined using an n AssmannAssmann psychrometerpsychrometer giving accurate giving accurate measurements measurements ofof the the wet and dry bulb wet and dry bulb temtemperature of the air. perature of the air.

In this instrument,In this instrument, air air is drawn at a is drawn at a speed higher than 5 m/s by a click-work speed higher than 5 m/s by a click-work fanfan. The bulbs of the thermometers are . The bulbs of the thermometers are protected from the effects of solar protected from the effects of solar radiation. By use of suitable psychrometric radiation. By use of suitable psychrometric charts or tablescharts or tables the RH of the air may be the RH of the air may be obtained from the readings of the obtained from the readings of the psychrometerpsychrometer..

Kata thermometerKata thermometer. The word “kata” is . The word “kata” is a Greek word meaning “down”. The kata a Greek word meaning “down”. The kata thermometer is an alcohol thermometer with a thermometer is an alcohol thermometer with a glass bulb. The readings on the stem are glass bulb. The readings on the stem are marked from 35 to 38. Before taking the marked from 35 to 38. Before taking the readings, the bulbs are immersed in hot water readings, the bulbs are immersed in hot water to warm them when the alcohol rises into a to warm them when the alcohol rises into a small reservoir at the top of the instrument. small reservoir at the top of the instrument. Then the instrument is suspended in air at the Then the instrument is suspended in air at the point of observation.point of observation.

The timeThe time in seconds required for the spirit in seconds required for the spirit to fall from 38 to 35 is noted with a stop watch. to fall from 38 to 35 is noted with a stop watch. The length of time depends upon the “cooling The length of time depends upon the “cooling power” of the airpower” of the air. Each Kata has a “factor” . Each Kata has a “factor” called called Kata FactorKata Factor marked on the stem. This marked on the stem. This factor is determined for each instrument by the factor is determined for each instrument by the manufactures. Kata Factor divided by the manufactures. Kata Factor divided by the cooling time gives the rate of cooling. The cooling time gives the rate of cooling. The instrument instrument is used is used for recordingfor recording low low 3) air 3) air velocityvelocity. . High air velocityHigh air velocity is measured by an is measured by an instrument, called instrument, called anemometeranemometer andand isis expressed expressed in metersin meters per second per second (m/s(m/s).).

The The globe thermometerglobe thermometer is usedis used for direct for direct

measurement of the mean radiant temperature of measurement of the mean radiant temperature of

the surroundings. The instrument consists of a the surroundings. The instrument consists of a

hollow copper bulb 15 cm in diameter and is hollow copper bulb 15 cm in diameter and is

coated outside with mattblack paint which absorbs coated outside with mattblack paint which absorbs

the radiant heat from the surrounding objects. A the radiant heat from the surrounding objects. A

specially calibrated mercury thermometer is specially calibrated mercury thermometer is

inserted, with its bulb at the center of the globe. inserted, with its bulb at the center of the globe.

This thermometer registers a higher This thermometer registers a higher

temperature than the ordinary air one temperature than the ordinary air one

because it is affected both by the air because it is affected both by the air

temperature and radiant heattemperature and radiant heat. The globe . The globe

thermometer is also influenced by the thermometer is also influenced by the

velocity of air movement.velocity of air movement.

Location of thermal sensorsLocation of thermal sensors

Thermal-sensing instruments should be Thermal-sensing instruments should be

located at the workstation so that the actual located at the workstation so that the actual

conditions of heat exposure are measured. In those conditions of heat exposure are measured. In those

zones where the worker spends substantial amount zones where the worker spends substantial amount

of time, measurements should be taken of time, measurements should be taken

periodically, three or four times for a work shift periodically, three or four times for a work shift

could be adequate. could be adequate.

Where the employee moves through a Where the employee moves through a

large area, several zones may be involved. large area, several zones may be involved.

In such cases the thermal sensors could be In such cases the thermal sensors could be

located in several points to collect data located in several points to collect data

from these different zones.from these different zones.

Heat stress indicesHeat stress indices

Heat stress isHeat stress is the load of heat that must be the load of heat that must be

dissipated if the body is to remain in thermal dissipated if the body is to remain in thermal

comfort. comfort.

The guidelines currently used for worker The guidelines currently used for worker

exposure to heat are based on indices developed exposure to heat are based on indices developed

through subjective and objective testing of through subjective and objective testing of

workers or from combinations of external heat workers or from combinations of external heat

measurements. measurements.

Three of the more commonly used indices Three of the more commonly used indices

are are

- - EFFECTIVE TEMPERATUTEEFFECTIVE TEMPERATUTE, the , the

- - WET BULB GLOBE WET BULB GLOBE

THERMOMETER INDEX (WBGT)THERMOMETER INDEX (WBGT) and the and the

- - HEAT STRESS INDEXHEAT STRESS INDEX..

For For indoor exposure, or outdoor indoor exposure, or outdoor exposure without a solar loadexposure without a solar load, the formula is:, the formula is:

WBGT = 0.7twb + 0.3tgWBGT = 0.7twb + 0.3tg The necessary measurements require The necessary measurements require

relatively simple instrumentation. Heat stress relatively simple instrumentation. Heat stress monitors that measure all three temperatures monitors that measure all three temperatures and calculate WBGT index are also available. and calculate WBGT index are also available. Sensors should be at least the mean height of Sensors should be at least the mean height of the worker, or at the levels of the ankles, the the worker, or at the levels of the ankles, the abdomen and the headabdomen and the head..

A practical applicationA practical application of the WBGT of the WBGT index is to recommend the index is to recommend the percentage of percentage of time that the individual is permitted to time that the individual is permitted to perform the taskperform the task according to the according to the severity of the environment and severity of the environment and metabolic demands of the task metabolic demands of the task

For example, a task requiring light to For example, a task requiring light to moderate work of 200 kcal/hr could be moderate work of 200 kcal/hr could be performed continuously in environment up performed continuously in environment up to a WBGT of 30 C, but only 25 % of the to a WBGT of 30 C, but only 25 % of the time at a WBGT of 32.2 C.time at a WBGT of 32.2 C.

The The WBGT index – (ISO 7243) WBGT index – (ISO 7243) is is commonly used because is easy to determine. commonly used because is easy to determine. It is It is recommended by NIOSH in the United States.recommended by NIOSH in the United States. The WBGT index is calculated from the The WBGT index is calculated from the measurements of the wet bulb (WB), the black measurements of the wet bulb (WB), the black globe (G) and the dry bulb air (A) temperatures. globe (G) and the dry bulb air (A) temperatures.

For For outdoor exposure with a solar heatoutdoor exposure with a solar heat source, the WBGT formula is:source, the WBGT formula is:

WBGT = 0.7twb + 0.2tg + 0.1taWBGT = 0.7twb + 0.2tg + 0.1tawhere: twb = wet bulb temperaturewhere: twb = wet bulb temperature

tg = globe temperature tg = globe temperature tgtg = dry bulb air temperature = dry bulb air temperature

Heat stress index (HSI)Heat stress index (HSI)The determination of HSI results in The determination of HSI results in

more knowledge about the environment more knowledge about the environment and possibility to perform efficient control and possibility to perform efficient control measures than the use of the WBGT. To measures than the use of the WBGT. To calculate the HSI, measurements of the wet calculate the HSI, measurements of the wet bulb (WB), the black globe (G), the dry bulb (WB), the black globe (G), the dry bulb air temperatures and the air velocity bulb air temperatures and the air velocity are necessary for each jobsite, and are necessary for each jobsite, and estimation of the metabolic rate (M) of the estimation of the metabolic rate (M) of the workers.workers.

Once measurements have been made, rates of Once measurements have been made, rates of heat exchange between the worker and the heat exchange between the worker and the environment by convection (C) and radiation (R) environment by convection (C) and radiation (R) are calculated, and with M, are used to estimate the are calculated, and with M, are used to estimate the amount of sweating required to stay at equilibrium amount of sweating required to stay at equilibrium (E req). Calculations of C, R, E req, and E max, (E req). Calculations of C, R, E req, and E max, and estimation of M lead to a and estimation of M lead to a knowledge of the knowledge of the relative contribution of each, and hence, may well relative contribution of each, and hence, may well suggest possible means of solving the problemsuggest possible means of solving the problem. . This can not be done with WBGT. This can not be done with WBGT.

An interpretation of the HIS An interpretation of the HIS values is given below:values is given below:

0 No thermal stress0 No thermal stress10-30 Moderate to mild heat 10-30 Moderate to mild heat

strainstrain40-60 Severe heat strain40-60 Severe heat strainVery severe heat strainVery severe heat strainUpper limit of heat toleranceUpper limit of heat tolerance

Subjective and physiological method Subjective and physiological method for thermal comfort evaluationfor thermal comfort evaluation

Inquiry method of Begford –Inquiry method of Begford – method method to collect votes from a group of people, to collect votes from a group of people, exposed to certain thermal environment exposed to certain thermal environment about their thermal sensation, estimated about their thermal sensation, estimated according to the 7-point thermal sensation according to the 7-point thermal sensation scale.scale.

Index of Fanger (PMV and PPD indices) Index of Fanger (PMV and PPD indices) (ISO 7730).(ISO 7730). It could be calculated using It could be calculated using mathematical equations, as well as using an mathematical equations, as well as using an integrating sensor - Bruel & Kjäer mod. 1212. integrating sensor - Bruel & Kjäer mod. 1212.

The PMV-index can be determined when the The PMV-index can be determined when the activityactivity (metabolic rate-1 metabolic unit = (metabolic rate-1 metabolic unit = 1 met1 met = = 58 W/m2) and the 58 W/m2) and the clothingclothing (thermal resistance – 1 (thermal resistance – 1 unit of thermal resistance of clothing = unit of thermal resistance of clothing = 1 clo1 clo =0,155 m2 . ºC/W) are estimated – from tables, the =0,155 m2 . ºC/W) are estimated – from tables, the following environmental parameters are measured: following environmental parameters are measured: air temperature, mean radiant temperature, relative air temperature, mean radiant temperature, relative air velocity and partial water vapor pressure.air velocity and partial water vapor pressure.

In moderate environment man’s In moderate environment man’s thermoregulatory system will automatically try thermoregulatory system will automatically try to modify the skin temperature and the sweat to modify the skin temperature and the sweat secretion to maintain heat balance. In the PMV-secretion to maintain heat balance. In the PMV-index the physiological response of the index the physiological response of the thermoregulatory system has been related thermoregulatory system has been related statistically to thermal sensation votes collected statistically to thermal sensation votes collected from more than 1 300 subjects. from more than 1 300 subjects.

It is recommended to use the PMV-index It is recommended to use the PMV-index only only for values of PMV between –2 and + 2 and for values of PMV between –2 and + 2 and the main parameters of the equation are inside the main parameters of the equation are inside

fixed intervalsfixed intervals..

The PMV-index predicts the mean The PMV-index predicts the mean

value of the thermal votesvalue of the thermal votes of a large of a large

group of people exposed to the same group of people exposed to the same

environment. But individual votes are environment. But individual votes are

scattered around this mean value and it is scattered around this mean value and it is

useful to useful to predict the number of people predict the number of people

likely to feel uncomfortably warm or cool.likely to feel uncomfortably warm or cool.

The PPD-indexThe PPD-index establishes a establishes a

quantitative prediction of the number of quantitative prediction of the number of

thermally dissatisfiedthermally dissatisfied persons among a persons among a

large group of people. It is recommended large group of people. It is recommended

that the that the PPD be lower than 10 %PPD be lower than 10 %.. This This

corresponds to the following criteria for the corresponds to the following criteria for the

PMV: PMV:

- 0,5 < PMV < +0,5- 0,5 < PMV < +0,5

Preventive measures and control of heat Preventive measures and control of heat stressstress

Modifying one or more of the following Modifying one or more of the following factors can reduce heat stress: metabolic heat factors can reduce heat stress: metabolic heat production, heat exchange by convection, heat production, heat exchange by convection, heat exchange by radiation, or heat exchange by exchange by radiation, or heat exchange by evaporation. evaporation.

Environmental heat load (C, R and E) can Environmental heat load (C, R and E) can be modified by be modified by engineering controlsengineering controls (e. g. (e. g. ventilation, air conditioning, screening, and ventilation, air conditioning, screening, and modification of process or operation) and modification of process or operation) and

protective clothing and equipment.protective clothing and equipment.

Engineering approaches to enhancing Engineering approaches to enhancing covective heat exchange covective heat exchange are limited to are limited to modifying air temperature and air modifying air temperature and air movementmovement. When the air temperature is less . When the air temperature is less than the mean skin temperature, increasing than the mean skin temperature, increasing air movement across the skin by increasing air movement across the skin by increasing either general or local ventilation will either general or local ventilation will

increase the body heat loss.increase the body heat loss.

Metabolic heat productionMetabolic heat production can be can be modified by modified by work practiceswork practices and and application of labor - reducing devices application of labor - reducing devices – – mechanizationmechanization of the physical of the physical components of the job, components of the job, reduction of reduction of work timework time (reduce work day, increase (reduce work day, increase rest time) to reduce the duration of rest time) to reduce the duration of exposure to a hot environment, exposure to a hot environment, increased work forceincreased work force..

Work and hygienic practices and Work and hygienic practices and administrative controls.administrative controls.

Situations in industries exist where the Situations in industries exist where the complete control of heat stress by application complete control of heat stress by application of engineering controls may be of engineering controls may be technologically impossible or impractical, technologically impossible or impractical, where the level of heat stress can be where the level of heat stress can be unpredictable. In such cases other solutions unpredictable. In such cases other solutions could be applied. Preventive practices could be applied. Preventive practices

include:include:

1. 1. Limiting of modifying the Limiting of modifying the duration of exposure timeduration of exposure time - when - when possible, schedule hot jobs for the possible, schedule hot jobs for the cooler part of the day (early morning, cooler part of the day (early morning, late afternoon), provide cool areas for late afternoon), provide cool areas for rest and recovery etc. rest and recovery etc.

2. 2. Enhancing the heat toleranceEnhancing the heat tolerance by heat acclimatization.by heat acclimatization.

3. 3. Adequate water supplyAdequate water supply to maintain the to maintain the electrolyte balance. It is widespread that extra electrolyte balance. It is widespread that extra salt intake is needed to prevent the ill-effects of salt intake is needed to prevent the ill-effects of heat. The normal intake of salt in some national heat. The normal intake of salt in some national diets is far more than is actually needed. diets is far more than is actually needed. Therefore there is no need to add salt to water; Therefore there is no need to add salt to water; only unacclimatized persons need extra salt only unacclimatized persons need extra salt during the first some days of their exposure to during the first some days of their exposure to heat.heat.

4. 4. Protective clothing and equipmentProtective clothing and equipment – – they should be light, loose and of light colors.they should be light, loose and of light colors.

5. 5. Protective devicesProtective devices – goggles, – goggles, shields, helmets.shields, helmets.

6. 6. Medical screeningMedical screening of workers to of workers to discern individuals with low heat tolerance discern individuals with low heat tolerance and/or physical fitness. The screening and/or physical fitness. The screening should include a history of any previous should include a history of any previous incident of heat illness or other pathological incident of heat illness or other pathological conditions that could influence the health of conditions that could influence the health of workers in unfavorable thermal workers in unfavorable thermal environment.environment.