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British Journal of Industrial Medicine, 1973, 30, 313-324

Assessment of man's thermal comfortin practice

P. 0. FANGERLaboratory of Heating and Air Conditioning, Technical University of Denmark, Building 402,DK 2800 Lyngby, Denmark

Fanger, P. 0. (1973). British Journal ofIndustrial Medicine, 30, 313-324. Assessment of man'sthermal comfort in practice. A review is given of existing knowledge regarding the condi-tions for thermal comfort. Both physiological and environmental comfort conditionsare discussed. Comfort criteria are shown diagrammatically, and their application is illustratedby numerous practical examples. Furthermore, the effect on the comfort conditions of age,adaptation, sex, seasonal and circadian rhythm, and unilateral heating or cooling of the bodyis discussed. The term 'climate monotony' is considered. A method is recommended forthe evaluation of the quality of thermal environments in practice.

In its Constitution, the World Health Organization(WHO) defines health as 'a state of completephysical, mental and social well-being and notmerely the absence of disease or infirmity'. The inclu-sion of man's well-being in the definition of healthreflects the increasing interest which public healthofficers, hygienists, and many physicians have shownin man's comfort.

In this connection the indoor climate warrantsspecial attention because in a modern industrialsociety man spends the greater part of his lifeindoors. A large proportion of the population spends23 out of 24 hours in an artificial climate-in dwel-lings, at the workplace, at hobby, amusement, andcultural centres, or during transport by car, train,ship or aeroplane.

This has resulted in a growing understanding andinterest in studying the influence of the indoorclimate on man, thus enabling suitable requirementsto be established, which should be aimed at inpractice.At the same time an increasing number of com-

plaints about unsatisfactory indoor climate suggestthat man has become more critical regarding theenvironment to which he is subjected. It seems that

he is most inclined to complain about the indoorclimate of his workplace (offices, industrial premises,shops, schools, etc.) where he is compelled to spendhis time in environments which he himself cancontrol only to a very limited degree.

Field studies indicate that in practice many ofthese complaints can be traced to an unsatisfactorythermal environment.

In this article the conditions for man's thermalcomfort will be discussed, as well as the thermalenvironments which should be aimed at and themethods which should be employed in practice toevaluate the quality of a given thermal environment.

Only thermal environments which cause moderatedegrees of discomfort will be dealt with. It is thesecases that bring forth by far the greatest number ofcomplaints met by physicians and engineers. Overrecent years research has led to the establishment ofrational methods for the assessment of such thermalenvironments.

Methods for determining the risk of thermaldisorders when man is exposed to extreme thermalstress (i.e., during hard physical work in mines or inhot industry) will not be discussed here. Well-established and reliable methods have long been

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available for such cases, e.g., as reviewed in themonographs of Leithead and Lind (1964) and ofKerslake (1972).

Comfort parameters

What is required in practice is that the comfortconditions are expressed in controllable factors,namely the following four main physical parameterswhich constitute the thermal environment:

1. air temperature2. mean radiant temperature3. relative air velocity4. vapour pressure in ambient air.

Besides the environmental factors, man's comfort isalso influenced by the following two factors:

5. activity level (internal heat production in thebody)

6. thermal resistance of clothing.As suggested by Gagge, Burton, and Bazett

(1941), activity is often expressed in met-units(1 met = 58 W/m2 (50 kcal/m2h) corresponding tosedentary activity) and the thermal resistance of theclothing is expressed in clo-units (1 clo = 0 155 m2°C/W (018 m2h °C/kcal)).The activity and the thermal resistance of the

clothing can be estimated with reasonable accuracyby considering the application of the room con-cerned. Values for typical activities and clothingensembles are listed in Tables 1 and 2.

TABLE 1HEAT PRODUCTION IN THE BODY DURING DIFFERENT

TYPICAL ACTIVITIES

Activity met

Sleeping .. .. .. .. .. 08Sitting .. .. .. .. .. 10Typing .. .. .. .. .. .. 12Standing .. .. .. .. .. . i 4Ordinary standing work in shop, laboratory,

kitchen1.... .. .. .. .. I6-2Slow walking (3 km/hr) .. .. 2Normal walking (5 km/hr) .. .. 2-6Fast walking (7 km/hr) .. .. .. 4Ordinary carpentering and bricklaying work .. 3Running (10 km/hr) .. .. .. 8

In practice, quantitative knowledge is needed as

to which combinations of the above-mentioned sixvariables will lead to thermal comfort for man.The existing knowledge of comfort conditions will

be discussed, including the correlation between man'sthermal sensation and his physiological reactions.But before proceeding to a discussion of comfortconditions it is important to define the concept ofcomfort.

TABLE 2THERMAL RESISTANCE OF DIFFERENT CLOTHING

ENSEMBLES

Clothing clo

Naked .. .. .. 0Bikini .. .. .. .. .. .. 0 05Shorts .. .. .. .. .. .. 01Ordinary tropical clothing: shorts, open shirt

with short sleeves, light underwear .. .. 03Light summer clothing: long light-weight trous-

ers, open shirt with short sleeves, light under-wear ... .. .. .. .. Q5

Light tropical suit .. 0'8Typical business suit .. .. ..Traditional heavy northern European suit with

waistcoat, long underwear, and singlet withlong sleeves .. .. .. .. 15

Polar clothing .. .. .. 3-4

Definition of comfort

In agreement with the American Society of Heating,Refrigerating and Air-conditioning Engineers(ASHRAE) Standard 55-66 (1966), thermal comfortfor a person is here defined as 'that condition ofmind which expresses satisfaction with the thermalenvironment'. Normally this means that the persondoes not know whether he would prefer a warmer ora cooler environment. Furthermore, it is a require-ment that the heat loss from the person is not tooasymmetrical.

People are not alike, thermally or otherwise. Ifa group of people is exposed to the same roomclimate it will therefore normally not be possible, dueto biological variance, to satisfy everyone at thesame time. One must then aim at creating optimalthermal comfort for the group, i.e., a condition inwhich the highest possible percentage of the groupare thermally comfortable.

Physiological comfort conditionsThe purpose of the human thermoregulatory systemis to maintain a reasonably constant deep bodytemperature; a requirement for this is the main-tenance of a heat balance so that the heat lost tothe environment is equal to the heat produced bythe body. Man possesses the most effective physio-logical mechanisms for maintaining a heat balance:the sensible heat loss can be altered by a variationof the cutaneous blood flow and thus of the skintemperature, latent heat loss can be increased bysweat secretion, and internal heat production canbe increased by shivering or muscle tension.These mechanisms are extremely effective and

ensure that the heat balance can be maintainedwithin wide limits of the environmental variables.



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4ssessment of man's thermal comfort in practice 315

Maintenance of heat balance is, however, far frombeing a sufficient condition for thermal comfort.Within the wide limits of the environmental variablesfor which heat balance can be maintained there isonly a narrow interval which will create thermalcomfort.

It has long been known that man's thermal sensa-tion is related to the state of his thermoregulatorysystem, the degree of discomfort being greater theheavier the load on the effector mechanisms.Experiments by Yaglou (1927) in the 1920s indicateda correlation between the skin temperature and thesensation of thermal comfort; later and morecomplete studies by Gagge, Herrington, andWinslow (1937), Gagge et al. (1941), Winslow,Herrington, and Gagge (1937), DuBois, Ebaugh,and Hardy (1952), and Nielsen (1947) showed acorrelation between thermal sensation and skintemperature, independent of whether the subjectswere nude or clothed. Therefore it was generallyaccepted for a long time that the physiologicalconditions for comfort were that a person had amean skin temperature of 33-34°C and that sweating(or shivering) did not occur. This was later confirmedin experiments by Fanger (1967) but only for seden-tary subjects. At higher activities than sedentaryoneshe showed that man prefers a lower mean skintemperature and prefers to sweat. Based on experi-mental results for 20 subjects each participating infour tests at different activity levels, and based ondata by McNall et al. (1967), the following valuesof skin temperature and sweat secretion were foundto provide comfort during steady-state conditions(Fanger, 1967):Mean skin temperature during comfort:ts= 35-7-0-0276M (OC) (1)

Sweat secretion during comfort:Esw = 0-42 (M -58) (W/m2) (2)

where M is the metabolic rate per unit body surfacearea (W/m2).

It is apparent from equation (1) that duringsedentary activity (M = 58 W/m2) man prefers askin temperature of approximately 34°C, while thepreferred skin temperature is, for example, onlyapproximately 31°C at an activity three times thatof the sedentary level.From equation (2) it can be seen that man prefers

a sweat secretion of zero during sedentary activity(M = 58 W/m2), whereas at higher activities heprefers a sweat secretion involving a latent heat lossof 42% of the increased heat production of the body.Ambient temperatures which were so low that nosweating occurred were felt to be much too cold.

It should be noted that a certain wetness of theskin in itself can produce a non-thermal discomfortdue to tactile sensations associated with dripping wetclothing, sticky skin, and skin irritation. A high wet-

ness of the skin can occur if the sweat secretion orthe vapour diffusion resistance of the clothing arehigh. Gagge (1937), who introduced the 'skin wet-ness' concept, recommends that it should be lowerthan 25% (Gagge, Stolwijk, and Nishi, 1969a).

It must be emphasized that the physiologicalcomfort conditions given in equations (1) and (2)apply only under reasonably steady-state conditions.It has not been possible as yet to establish physio-logical comfort conditions during thermal transientsbut important studies on transients have beenperformed at the Pierce Foundation Laboratory,Yale University, by Gagge, Stolwijk and Hardy(1967), Gagge, Stolwijk, and Saltin (1969b), Hardyand Stolwijk (1966), and Stolwijk and Hardy (1966).In one series of experiments nude subjects wereexposed alternately to cold and neutral and to hotand neutral environments. When proceeding fromneutral to cold or warm environments, the changingthermal sensation was found to be correlated withthe actual skin temperature and sweat rate in thesame way as under steady-state conditions. Butwhen these transients were reversed, i.e., proceedingfrom a cold or hot to a neutral environment, theyfelt almost immediately comfortable, even thoughtheir skin temperature had not yet reached the steady-state level considered comfortable. Gagge explainsthis by the rate of change of skin temperature whichmight cause a sensation that compensates for andpredominates over the sensation of discomfortcaused by the skin temperature itself. The thermalsensation 'leads' the body temperature changes inthis case and is thus 'anticipatory'.

Other transient experiments by Cabanac (1969,1971) and by Hardy (1970) with subjects immersedin a waterbath indicate that an interactionof internaland skin temperature might be important in evokingdiscomfort. The possible influence of internal tem-perature has also been suggested by Missenard(1957).The relationship between man's thermal sensation

and his physiological reactions during suddenenvironmental changes is certainly complicated, andfurther research is necessary before it will be possibleto establish quantitative physiological comfort con-ditions for thermal transients.

Environmental comfort conditions

When, in practice, artificial climates are to becreated which will provide thermal comfort for man,it is of course insufficient merely to know the physio-logical comfort conditions. What is necessary is adetailed quantitative knowledge of those combina-tions of the environmental variables which will resultin optimal thermal comfort.Two research methods which differ in principle



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316 P. 0. Fanger

have been used to establish environmental comfortconditions, but both require a large number ofsubjects in order that the optimal condition can bestated with reasonable statistical certainty.

In one of the methods the subjects are asked tovote on their thermal sensation on suitable psycho-physical scales, and at the same time measurementsare taken of the ambient thermal climate to whichthey are exposed. Subsequently the results aretreated statistically and the optimal thermal environ-ment can be determined.

This method is relatively simple and has thereforebeen used in a large number of field studies in allparts of the world. The best known study of thisnature is Bedford's (1936) classical study on factoryworkers, but recent investigations by Wyon, Lidwell,and Williams (1968), and Humphreys and Nicol(1970) have used the same method.

In many field studies activity level, and especiallyclothing, have not been specified, and in somecases it has not been possible to measure all theenvironmental variables. Even though the resultsof such field studies have often been of value inthe actual conditions under which they were mea-sured, it is difficult to make generalisations from theresults and to apply them to other conditions.The same method has been used in recent years

in extensive experimental studies under carefullycontrolled laboratory conditions at Kansas StateUniversity by Nevins, Rohles, Springer, and Feyer-herm (1966), McNall et al. (1967), McNall, Ryan, andJaax (1968), McNall and Schlegel (1968), Rohles andNevins (1971), Rohles and Johnson (1972), andRohles, Woods, and Nevins (1973). These experi-ments have involved many hundreds of subjects,who, clothed in a standard clothing ensemble, havebeen exposed to different combinations of twoenvironmental variables (e.g., air temperature andrelative humidity) while keeping constant, in so far asit is practically possible, all other factors whichinfluence thermal comfort. Large numbers ofsubjects have been asked to vote upon their thermalsensation according to special scales. From astatistical analysis combinations of the two variableswhich give optimal thermal comfort have then beendetermined.

Because of the large number of subjects and thecarefully controlled experimental conditions thismethod has given valuable results; however, theseresults are valid only for the constant values of theother four parameters prevailing when the experi-ments were performed. Since the effect of any of thesix main parameters on man's thermal comfortdepends on the level of the other parameters thisempirical method requires an enormous number ofexperiments to cover the range adequately. Themethod is thus slow, and existing data therefore do

not provide sufficient information for an overallassessment and calculation of the relative andabsolute influence of all the individual variables onman's thermal comfort.The other method for establishing environmental

comfort conditions was employed by Fanger (1967)in the derivation of his comfort equation.An underlying idea, later experimentally supported

by Olesen, Bassing, and Fanger (1972a), is that itis the combined thermal effect of all the physicalfactors which is of importance for man's thermalstate and comfort. It is therefore impossible toconsider the effect of any of the physical factorsinfluencing thermal comfort independently, as theeffect of each of them depends on the level of theother factors.The equation was derived by setting up a heat

balance equation for the human body and insertingin it the physiological comfort conditions for skintemperature and sweat secretion given in equations(1) and (2). In this way, the comfort equation estab-lishes those combinations of activity, clothing, andthe four environmental variables which will providethermal comfort. It has been known for a long timethat these six variables influence the state of comfort.But the combined quantitative influence of all theparameters on man's comfort was not known untilthe equation was introduced.

Practical application of comfort diagrams

The comfort equation is comprehensive and complexand therefore unsuitable for manual calculation, butit has been solved by the use of electronic dataprocessing and has been plotted in 28 comfortdiagrams (Fanger, 1970); these diagrams are intendedfor use in practice. In Figs. 1 to 4, examples are shownof four of the comfort diagrams. In each diagramcomfort lines have been drawn, i.e., curves throughvarious combinations of two variables which willcreate comfort providing the values of the othervariables are kept constant.For practical application of the comfort diagrams,

it is necessary to estimate the activity level and theclothing first, taking into account the use of the room(see Tables 1 and 2). From the comfort diagrams,combinations can then be found of the four environ-mental parameters which will provide thermalcomfort.

Beneath Figs. 1 to 4 nine characteristic examplesare given of the use of the comfort diagrams inpractice.

Individual differences

Everyone is not alike. How then is it possible, froman equation, to specify one particular temperaturewhich will provide comfort? The answer is that the



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Assessment of man's thermal comfort in practice 317

comfort equation does not necessarily satisfy every-one. It gives, however, combinations of the variableswhich will provide comfort for the greatest numberof people. This is exactly what should be aimed atwhen a large group of people are together in thesame room climate (optimal comfort for the group).

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FIG. 1. Comfort diagram showing the combined influenceof mean radiant temperature and air temperature. Thecomfort lines corresponding to five different velocities arecurves through different combinations of mean radianttemperature and air temperature which will provideoptimal thermal comfort. The diagram applies for seden-tary persons in medium clothing (01 clo). Relativehumidity 50 %. The slope of the lines has been reasonablywell confirmed by recent British studies (Griffiths andMcIntyre, 1972a).

Example 1 In an office, the employees are clothed at1 clo (normal business suit) and occupied with sedentarywork (1 met). Relative humidity 50%, relative airvelocity < 01 m/s. It is desired to determine the optimalair temperature, presuming that the mean radianttemperature = the air temperature. From Fig. 1,ta = tmrt = 23 0°C is found.Example 2 In a warehouse an air temperature of 140Cand a relative humidity of 50% should be maintainedowing to the nature of the goods. Air velocity is 0-2 m/s.In the warehouse, a person is occupied with sedentarywork, clothed at 1 clo. It is desired to maintain the personin comfort by means of high-intensity infrared heatersplaced above his workplace. The mean radiant tempera-ture necessary for comfort is to be calculated. FromFig. 1, tmrt = 380C is found.Example 3 Under winter conditions the mean radianttemperature in a long-distance bus is calculated to be50 lower than the air temperature. It is desired to deter-mine the air temperature necessary for comfort, thepassengers being presumed to be seated without topclothes (1 0 clo) and the velocity being 0-2 m/s (relativehumidity 50%). From Fig. 1: ta = 25 50C and tmrt =20-50C.

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15 20 25 30 35°CAir temperature=mean radiant temperature

FIG. 2. Comfort diagram showing the combined influenceof humidity and ambient temperature. The comfort linescorresponding to four different velocities are curvesthrough different combinations of ambient temperatureand humidity which will provide optimal thermalcomfort. The diagram applies for nude, sedentarypersons.

Example 4 At swimming baths with rest places it isdesired to establish the necessary air temperature whichwill maintain thermal comfort for sedentary nude persons.Relative air humidity is 80%, relative air velocity< 0-1 m/s, and air temperature = mean radiant tempera-ture. From Fig. 2, ta = 28 0C is found.

It has been found from experiments involving 1 300subjects that the best result attainable is 5% dis-satisfied (Fanger, 1970). Any deviation from thethermal conditions specified by the comfort equationwill result in an increase in the percentage of dis-satisfied.

Comfort zones or comfort pointsEarlier it was quite common to recommend so-called'comfort zones'. How can it then be that for setvalues of the parameters the comfort equationestablishes only one comfort temperature and not acomfort zone? It is true that for every person thereexists an interval of ambient temperatures withinwhich he will feel reasonably comfortable. Thus foreach individual there exists a comfort zone. But asthe comfort zone varies from person to person therewill be no common interval of temperatures for alarge group of persons which will satisfy them all.There will not even be one common temperaturewhich will provide comfort for all. But, as mentionedearlier, there will be one ambient temperature atwhich the least possible number of persons will bedissatisfied (5 %). This 'comfort point' is establishedby the comfort equation.

Variability in man's comfort conditionsfrom day to day

How reproducible are the comfort conditions for the

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50 70 800F

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5 10 20 25 300CAir temperature=mean radiant temperature

FIG. 3. Comfort diagram showing the combined influ-ence of air velocity and ambient temperature. Thecomfort lines corresponding to five different activitylevels are curves through different combinations ofrelative air velocity and ambient temperature which willprovide optimal thermal comfort. The diagram appliesfor persons wearing medium clothing (1-0 clo). Relativehumidity 50%.

Example 5 It is desired to determine the comfort tem-perature for the personnel in a shop, where the mean

activity corresponds to walking at a speed of 1 5 km/hr(activity = 1-5 met, relative velocity = 0-4 m/s); cloth-ing = 10 clo, relative humidity 50%. From Fig. 3 itwill be seen that ta = tmrt = 20 80C.Example 6 In a 'clean room' (laminar flow type) thehorizontal air velocity is 0 5 in/s and the personnel are

engaged in sedentary work clothed in a special standardsuit (10 clo). Relative humidity 50%. From Fig. 3 itwill be seen that the comfort temperature is 24 7°C.

individual? Is not the subjective thermal sensationso uncertain that large variations in comfort require-ments can be expected from day to day? This hasrecently been investigated by determining the prefer-red ambient temperature for each subject underidentical conditions on four different days (Fanger,1973). A standard deviation of only 0 60C was

found.It is concluded that the comfort conditions for

the individual can be reproduced and will vary onlyslightly from day to day.

Age

It has often been claimed that due to the fact thatmetabolism decreases slightly with age the comfortconditions based on experiments with young andhealthy subjects cannot be used as a matter of coursefor other age groups. In Table 3 the results are

summarized of recent comfort studies in Denmarkand the United States of America on different age

groups (mean age 21-84 years). Activity, clothing,and other experimental conditions have in allstudies been identical. Subjects during one experi-

ment are seen in Figure 5. It will be seen from Table3 that the thermal environments preferred by theelderly do not appear to differ from those preferredby younger people. The lower metabolism inelderly people seems to be compensated for by alower evaporative loss.The fact that young and elderly prefer the same

thermal environment does not necessarily mean thatthey are equally sensitive when exposed to cold (orheat). This has recently been discussed by Fox et al.(1973) in relation to the hypothermia problem forthe elderly.

Adaptation

It is widely believed that, by exposure to hot or coldsurroundings, people can acclimatize themselves sothat they prefer other thermal environments, and

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FIG. 4. Comfort diagram showing the combined influenceof clothing and activity. The comfort lines correspondingto six different activity levels are curves through differentcombinations of clo-value and ambient temperaturewhich will provide thermal comfort. Relative humidityis 50%, relative air velocity <0 1 m/s.

Example 7 A special uniform is to be made for personnelin the meat-packing industry who are employed in anenvironment of 12°C. The necessary clo-value of theuniform must be determined, the activity level being setat 1-6 met. From Fig. 4 it can be seen that 1 8 clo corres-ponds to the desired conditions.Example 8 In an operating theatre the surgeon is clothedat 0 9 clo and has an activity level of 1-4 met. It is desiredto determine the ambient temperature (air temperature =

mean radiant temperature) which will provide thermalcomfort. From Fig. 4, ta = 20 5°C is found.Example 9 In the same operating theatre, the othermembers of the operating team are engaged at a loweractivity level (12 met). It must be determined what clo-value their standard clothing should have in order toprovide thermal comfort. From Fig. 4, 1 15 clo is found.



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Assessment ofman's thermal comfort in practice 319

TABLE 3COMPARISON BETWEEN COMFORT CONDITIONS FOR DIFFERENT AGE GROUPS

Preferred Mean skin EvaporativeStudy ambient temp. at weight loss Number

Mean age temp. conmfort during comfort of subjects(yr) (OC) (OC) (g/m'/hr)

Nevins et al. 21 25-6 720(1966)Fanger .. 23 25-7 19-2 128(1970)Fanger .. 68 25-7 15-3 128(1970)Rohles and Johnson 74 24 5 228(1972)Technical University ofDenmark (1972)1 .. 24 25-4 33 5 19-2 32Technical University ofDenmark (1972)1 .. 84 25-4 33-2 12-4 16Comfort equation, Fanger.. 25 6(1967)

"Data not yet publishedSubjects were tested under the following standardized conditions: sedentary activity, light standardvelocity <0 1 m/s, rel. humidity 50 %, mean radiant temperature = air temperature.

clothing 0-6 clo, rel.

~~~~~~~~~~~~~~~~'jA

FIG. 5. Subjects wearing a standard uniform (0-6 do) during thermal studiesin the environmental test chamber at the Technical University of Denmark.During the studies, physical and physiological measurements are correlatedwith the subjective evaluations of the subjects.



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320 P. 0. Fanger

that the comfort conditions vary in different partsof the world, depending on the outdoor climate atthe relevant place. In Table 4 the results are shownof experiments (identical experimental conditions)involving subjects from the United States ofAmerica,Denmark, and tropical countries. The latter group

was tested in Copenhagen immediately after theirarrival by plane from the tropics where they hadlived all their lives.

Moreover, Table 4 gives experimental results fortwo groups of persons exposed daily to cold. Onegroup comprises persons who for eight hours dailyfor at least one year have been doing sedentary workin cold surroundings in the meat packing industry.The other group consists of winter swimmers whobathe daily in the sea.

It is apparent from the Table that there are onlyslight differences between the various groups asregards both the preferred ambient temperature andthe physiological parameters in the comfort con-dition. The results indicate that man cannot becomeadapted to prefer warmer or colder environments.

It is therefore likely that the same comfort con-ditions can be applied throughout the world. How-ever, in determining the preferred ambient tempera-ture from the comfort diagrams, a clo-value shouldbe used which corresponds to the local clothinghabits. A comparison of field comfort studies fromdifferent parts of the world (Nicol and Humphreys,1972) shows, as might be expected, significantdifferences in clothing habits depending, amongother things, on the outdoor climate.

Sex

In all the experiments mentioned in Tables 3 and 4,an equal number of male and female subjects partici-pated, and it is therefore possible to compare thecomfort conditions for the two sexes (Table 5). It isshown that men and women seem to prefer almostthe same thermal environments. Women's skintemperature and evaporative loss are slightly lowerthan those for men, and this balances the somewhatlower metabolism of women.

Seasonal and circadian rhythm

As it has been ascertained above that man cannotbecome adapted to prefer warmer or colder environ-ments, it follows that there is no difference betweencomfort conditions in winter and in summer. Thisis confirmed by an investigation undertaken atKansas State University where results of winter andsummer experiments showed no difference (McNallet al., 1968).On the other hand, it is reasonable to expect the

comfort conditions to alter during the day as theinternal body temperature has a daily rhythm, a

maximum occurring late in the afternoon and aminimum early in the morning.We have recently studied this by determining

experimentally the preferred ambient temperaturefor each of 16 subjects both in the morning and inthe evening. No difference was observed (Ostbergand Nicholl, 1973). We have furthermore studied

TABLE 4COMPARISON BETWEEN COMFORT CONDITIONS FOR DIFFERENT NATIONAL-GEOGRAPHIC GROUPS AND FOR

GROUPS OF PEOPLE REGULARLY EXPOSED TO EXTREME COLD OR HEAT

Preferred Mean skin Evaporative Numberambient temp. weight loss of

Group Study temp. at comfort during comfort subjects(OC) (OC) (g/m3/hr)

Americans .. .. .. Nevins et al 25-6 720(1966)

Danes .. .. .. Fanger 25-7 256(1970)

Danes .. .. .. Technical University of 25-4 33-5 19-2 32Denmark (1972)1

People from the tropics .. Technical University of 26-2 33-5 17-1 16Denmark (1972)1

Danes working in the cold Olesen and Fanger 24-7 33-6 17-1 16meat-packing industry .. (1971)

Danish winter swimmers .. Technical University of 25 0 33-3 16-6 16Denmark (1972)1

Comfort equation .. .. Fanger 25-6(1967)

'Data not yet publishedSubjects were tested under the following standardized conditions: Sedentary activity, light standard clothing 0-6 clo, rel.velocity <0-1 m/s, rel. humidity 50%, meaw radiant temperature = air temperature.



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TABLE 5COMPARISON BETWEEN COMFORT CONDITIONS FOR MALES AND FEMALES

Preferred Mean skin Evaporative Numberambient temp. weight loss of

Study Sex temp. at comfort during comfort subjects(OC) (OC) (glmllhr)

Nevins et al. (1966) and Males 25 4 488Fanger (1970) Femalcs 25 8 488(both studies combined)

Technical University of Males 25-5 33 6 21 3 16Denmark (1972)1 Females 25-3 33-4 171 16

Comfort equationFanger (1967) .. 25-6

.Data not yet publishedSubjects were tested under the following standardized conditions: Sedentary activity: light standard clothing 0 6 clo, rel.velocity <01 m/s, rel. humidity 50 %, mean radiant temperature = air temperature.

the preferred ambient temperature during a simulatedeight-hour working day (sedentary work) (Fanger,H0jbjerre, and Thomsen, 1973). It will be seen fromFig. 6 that we observed only small fluctuations inthe preferred ambient temperature during the day.There is a slight tendency to prefer somewhat warmersurroundings before lunch, but none of the fluctua-tions is significant. It is recommended, therefore,that the thermal environment be kept constantthroughout the day at a level given by the comfortequation.

Climate monotony

It has sometimes been claimed that a constantthermal environment is not ideal as it produces so-called climate monotony-increased fatigue, lowerarousal, lower performance, etc. But such claimshave not so far been supported by experimentalevidence.We have recently performed a preliminary study

of this problem by exposing subjects to temperatureswings of varying amplitudes and frequencies aroundthe comfort level (Wyon, Asgeirsdottir, Kjerulf-Jensen, and Fanger, 1973). At the same time theirthermal sensations, mental performance, and behav-iour were studied. Slight positive effects on perfor-mance were observed but only with large temperatureswings which were felt to be definitely uncomfortable.More comprehensive studies are needed. Until thenI would recommend keeping ambient temperaturesconstant at the level given by the comfort diagrams.

Unilateral heating or cooling of the body

Although a person may feel thermally neutral for thebody in general, i.e., he would preferneitherawarmernor a cooler environment, he might not be in thermal

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10 11day (hr)

0--xUsQa

12 13 14 15 16 17

FIG. 6. Mean of the preferred ambient temperature for16 subjects during a simulated normal 8-hour workingday. Sedentary activity, light standard clothing 0-6 cdo,relative air velocity <0.1 m/s, relative humidity 50%,mean radiant temperature = air temperature (Fanger,1973).

comfort if one part of the body is warm and anotheris cold. This might be caused by an asymmetricradiant field or a local convective cooling of thebody (draught) or by contact with a warm or coolfloor. Besides the comfort conditions for the body ingeneral, it is therefore essential to establish limitsfor how asymmetric the heat loss from the body canbe without evoking discomfort.

Limits for asymmetric radiation were recentlystudied experimentally by Olesen, Fanger, Jensen,and Nielsen (1972b). The following formula for



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322 P. 0. Fanger

estimating the limits of acceptable temperaturedifferences of a local radiant source for sedentarypersons in thermally neutral environments with stillair was recommended:

-2-4 - 1 81cl _ ZtwFp-w < 3 9 + 181iwhere ci = clo-value of clothing

Fpw = angle factor between person andradiant source

Itw = temperature difference ('C) betweenradiant source and mean radiant tem-perature in relation to the person.

This formula is in reasonable agreement with otherrecent data by McNall and Biddison (1970) and byGriffiths and McIntyre (1972b), while Chrenko(1953) 20 years ago established more conservativelimits for hot ceilings.

Local convective cooling is a considerable problemin practice and unfortunately it has been only super-ficially investigated so far. The mean air velocity andthe air temperature are, of course, of importance forconvective heat transfer and they should be balancedaccording to the comfort equation. But the meanvelocity is not sufficient to explain the draughtphenomenon. Man can be comfortable at quitesubstantial air velocities (i.e., 1 m/s) provided thatthe ambient temperature is adjusted to a suitablelevel; but in other cases a mean velocity of 0 3 m/smight evoke a sensation of draught. Other aero-dynamic magnitudes might be important andpreliminary experiments at the Technical Universityof Denmark indicate that the turbulence of the airis a significant factor for the sensation of draught.More studies are needed in order to establish reliabledraught limits.

Practical assessment of thermal environmentsWhen evaluating the thermal climate of a room inpractice, the thermal parameters must first bemeasured at a suitable number of points in theoccupied zone. Deviations from the optimal condi-tion given in the comfort diagrams can then bedetermined.

However, it is often of value to quantify the dcegreeof discomfort, and for this purpose an index hasbeen devised which gives the predicted mean vote(PMV) of a large group of subjects according to thefollowing psychophysical scale:

-3 cold-2 cool- 1 slightly cool0 neutral

+1 slightly warm+ 2 warm+3 hot

The PMV value is determined from tables (Fanger,1970) and it is then possible, from Fig. 7, to deter-mine the predicted percentage of dissatisfied (PPD).

0)

4)-4).-U .-

Q. 0

4)V-0.1^

4)-o4)

wa._

8060 ' o..o .... .

4030-20-p ~ 4

86 _. .

-2.0 -1.5 -1.0 -0.5 0 0.5 1-0 I-5 2.0Predicted mean vote

FIG. 7. The predicted percentage of dissatisfied (PPD)as a function of the predicted mean vote (PMV). Thecurve has a minimum value of 5% dissatisfied personsfor PMV = 0. This corresponds to optimal thermalcomfort. The curve is based on experiments comprising1 300 subjects (Fanger, 1970).

Figure 7 is based on studies comprising 1 300experimental subjects. As mentioned earlier, 5% isthe lowest percentage of dissatisfied which can beexpected. The PPD value is an appropriate andeasily understood expression for the quality of agiven thermal environment.An instrument which measures simultaneously the

PMV and PPD values has recently been developedby Korsgaard and Madsen (1971, 1973). It has asensor which integrates the comfort parameters inthe same way as the human body does (Fig. 8).Experience from practice has shown this instrumentto be a reliable and useful tool in assessing thermalenvironments in the field.

FIG. 8. Comfort-meter developed by Korsgaard andMadsen (1971, 1973). The sensor integrates the influenceof the thermal parameters in the same way as the humanbody (Comfy-test EQ-21, commercially available fromRECI, Glosemosevej, 2600 Glostrup, Denmark).

-T



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Future research

Conclusive new evidence has come to light on man'scomfort conditions as a result of extensive researchcarried out during recent years. This knowledge isquantified so that it is directly applicable in practice.However, several problems still exist which demandour efforts in this field of research in the future.

It is partly a matter of establishing comfort con-

ditions during transients (including temperature andhumidity fluctuations and sudden changes, forinstance, when a person moves from outdoors toindoors). In this connection it would be appropriateto perform more fundamental studies to clarify thecorrelation between man's thermal sensation and thefunction of his thermoregulatory system.Comfort studies on children are needed to investi-

gate whether the comfort conditions for adultsapply also to this age group.

There is also a need for further studies on theeffect on man of unilateral heating or cooling of thebody. The correlation between the sensation ofdraught and the turbulence intensity of the air shouldbe studied. Comfort limits should be established forthe floor temperature as a function of floor materialand footwear.More studies are needed on the thermal properties

of clothing ensembles and on the connection betweenclothing habits and behavioural temperature regula-tion.Another problem which has hardly been investi-

gated at all is the possible influence of non-thermalenvironmental factors on man's thermal sensation.Even though an influence from, e.g., light (colour)or sound on man's thermal sensation has not yetbeen found, the possibility of the existence of sucha pure psychological influence cannot be excluded,and an experimental investigation of the problemwould therefore seem to be relevant.

Furthermore, there is a need for studies of comfortconditions for sleeping persons. The investigation andestablishment of optimal thermal conditions forsleep is of great practical significance for the planningand operating of heating and air-conditioningsystems for hotels and hospitals.

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Received for publication June 18, 1973Accepted for publication July 4, 1973



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