Environmental physiology and indoor climate — Thermoregulation and thermal comfort

Energy and Buildings, 7 (1984) 29 - 34

29

Environmental Physiology and Indoor Climate Thermoregulation and Thermal Comfort YAIR SHAPIRO

Soroka Medical Center, University Center for Health Sciences, Ben Gurion University, Beersheba (Israel) YORAM EPSTEIN

Heller Institute o f Medical Research, Sheba Medical Center and The Sackler School o f Medicine, Tel Aviv University (Israel)

INTRODUCTION

Thermal comfort is achieved by keeping core temperature within a very narrow range of 36.5 - 37.5 °C and a skin temperature of 30 °C at the extremeties and 34 - 35 °C at body stem and head (Fig. 1). Any deviation from these ranges results in a sensation of discomfort which, under extreme conditions, may even be a medical threat [1]. Body temperature is the result of a fine heat balance between heat gained by the body and heat dissipated from the body (Fig. 2}. Two sources contribute to heat accumulation: internal heat and external heat. Metabolic

HEAT BALANCE I ~

, LOSS

W= MECHANICAL WORK fl R = RADIATION \ C= CONVECTION ~ E : SWEAT E V A P O R A T I ~ J

~/

i GAIN

'-) M = METABOLISM / R = RADIATION I C = CONVECTION ~ . ~

//kS=HEAT

(HEATGAIN)--(HEAT LOSS)=HEAT STORAGE

Fig. 2. Heat balance of the human body. Heat storage equals zero (0) when heat dissipation (left) equals heat accumulation (right).

Fig. 1. Regional skin temperature preference for comfort. 0378-7788/84/$3.00

processes are the main source for heat production within the body. The human body, like other organisms, is capable of transforming chemical energy, derived from carbohydrates, fats and proteins oxidation, into mechanical energy. Since the efficiencies of those processes are relatively low (10 - 20%) most of the energy produced is transformed into heat. The metabolic heat production can thus be calculated by substracting the mechanical work (W) from the total metabolic energy production (M) as ( M - W). Q Elsevier Sequoia/Printed in The Netherlands

30 The second source of heat contributing to body temperature is the "external h e a t " (environmental heat) which is absorbed from the surroundings mainly by radiation and convection (R + C).

HEAT BALANCE According to basic thermodynamics laws, heat is transferred in accordance with temperature gradients. On a warm sunny day when solar radiation is high, heat is absorbed by the body whereas on cold cloudy days when ambient temperature is below skin temperature, heat is dissipated from the body by means of radiation and convection. Heat transfer, by radiation and convection between the human body and the environment can be calculated as follows [2]: (R + C) = ( 6 . 4 7 A / c l o * ) ( T e - - Tsk)

(2)

or by using the mean radiant temperature (MRT) [3]: T~ = I/2(Ta + MRT)

(3)

where MRT = (1 + 0.22~/V)(Tg -- Ta) + T~

(4)

and V = wind velocity ms-l. It is obvious from eqn. ( 1 ) t h a t whenever Te > Tsk heat is absorbed by the body whereas with Te < Tsk heat is lost. The overall heat balance in the body is the combination between all factors mentioned (the total heat load -- metabolic and environmental heat loads -- minus heat loss) and could thus be summarized as: (M--W) +(R + C ) - - E =S

Ema × = (14.2 A )(im/clo)*(Psk -- Pa)

(6)

where (im/clo)* = permeability coefficient of water vapor through clothing, corrected for wind velocity and body movement, P~ = ambient water vapour pressure, P s k = skin water vapour pressure.

(1)

where A = surface area, Tsk = weighted skin temperature, clo* = t h e thermal insulation coefficient of clothing corrected for wind velocity and body movement (insulation reduces when air velocity increases), Te-environmental temperature. Without solar radiation, ambient temperature (T~) will equal environmental temperature. However, direct or indirect solar radiation has to be taken into consideration by adding the black globe temperature (T~). When Tg becomes higher than T~, Te is calculated as a weighted mean between T~ and T~ as follows: Te = 0.6Tg + 0.4Ta

where E = the a m o u n t of heat dissipated by evaporation of sweat and water loss through the respiratory tract, and S = t o t a l heat storage. Heat dissipation by the evaporation of sweat is limited by environmental conditions, of which vapor pressure has a major role. A negative correlation exists between ambient vapor pressure and the amount of sweat evaporated. In addition heavy, impermeable clothing reduces evaporation rate. Thus, the maximal evaporative capacity can be calculated as [2]:

(5)

THERMOREGULATORY MECHANISMS Maintaining body temperature constant requires the involvement of several thermoregulatory mechanisms. Primarily, the vasom o t o r regulatory system, by which blood flow to the skin is regulated. Peripheral vasodilatation enhances blood flow to the skin while vasoconstriction reduces it. Only when body temperature cannot be kept constant by using this mechanism solely, other physiological systems are recruited: shivering when body temperature is reduced and sweating when body temperature is increased. Thermal discomfort is associated with the deviation from the primary thermoregulatory protective system. According to eqn. (5), whenever E = 0 and S = 0, heat production equals heat dissipation and is achieved by physical means with no recruitment of sweating mechanism, then: ( M - - W ) = ( R +C)

(7)

This balance is defined as the physiological thermal comfort.

THERMAL COMFORT AND DISCOMFORT ZONES Microclimate is divided into 5 categories of thermal zones (Fig. 3). These zones are deter-

31

hot .....................

I. I N C O M P E N S A B L E H E A T Z O N E

S>>0

II. S W E A T E V A P O R A T I O N C O M P E N S A B L E Z O N E

S~-O

III. V A S O M O T O R C O M P E N S A B L E Z O N E

( M - - W) = (R + C) S=O

IV. S H I V E R I N G C O M P E N S A B L E Z O N E

S~O

warm

comfortable cool cold

V. I N C O M P E N S A B L E C O L D Z O N E

S<0

Fig. 3. Five z o n e s o f h u m a n t h e r m a l effect, its r e l a t i o n to heat storage a n d s e n s a t i o n of c o m f o r t .

mined not only by climatic conditions but also by the metabolic rate (metabolic heat production) and physical properties of clothing (insulation and water permeability). At the centre is the thermal c o m f o r t zone. Above it, when body temperature cannot be maintained, sweating is initiated (warm zone). The top zone is related to cases when heat storage is life threatening and is regarded as the incompensable heat zone. Two categories are under the comfort zone: the first, when shivering compensates for the reduction in body temperature, and the second when body temperature cannot be further maintained and heat loss from the body becomes life threatening. According to Belding [4], heat stress is determined by the term ( M - - W) > (R + C). The different factors involved in heat stress are described in Fig. 4. Maintenance of thermal c o m f o r t is achieved only when heat flow is, as described, within the borders on the upper left side of Fig. 4. Being within these borders means actually that heat stress is abolished and as defined in eqn. (7): (M--W)=(R+C).

T H E R M A L C O M F O R T IN M E T E R O L O G I C A L TERMINOLOGY

Climatological factors, as mentioned above, play a major role in thermal comfort: (a) ambient air temperature is associated with temperature gradient above skin (Te -- Tsk) (eqn. (2)): (b) black globe temperature expresses solar radiation and contributes to the T e / T s k gradient (eqn. (2)):

(c) humidity determines water vapor gradient above the skin influencing effective sweating (eqn. (6)): (d) wind velocity influences both the insulation coefficient of clothing and water v a p o r ' p e r m e a b i l i t y through clothing (eqns. (1) and (6) respectively). Thermal comfort cannot be defined by a specific meteorological factor but has to be regarded as an interaction between the metabolic state and a composition of several meteorological factors under which heat storage can be eliminated (AS = 0). Metabolic rate

The thermal comfort zone is defined as Ta = 24 -+ 3 °C, relative humidity (RH) = 50 + 15% and V = 0.2 0.7 m/sec only at rest when metabolic rate is relatively low (<100 kcal/h). Under such conditions, core temperature is 37.0 + 0.5 °C and the weighted skin temperature 33.5 + 0.5 °C. Sweat rate = 0 (except for the insensible water loss) and heart rate is 70 + 10 beats/min. Physical activity, owing to its effect on heat accumulation, is equivalent to an increase in ambient air temperature. An increased metabolic rate of 15 kcal/h equals an increase of 1 °C in Ta. Thus, the differences between a resting state and a state of moderate work load (250 kcal/h) equals 10 °C of ambient temperature, which means that the thermal comfort zone in this case will be set at 15 °C.

Wind

Increase of air movement by 0.2 m/sec equals a reduction of 1 °C in air temperature. Following the case described above, wind

32

HEAT STRESS MI[ANS HEAT LOSSES BY RAOIATIO/' AND CONVECTION ARE LESS THAN HEAT PR00UGTION BY METABOLISM

GREATER HEAT LOSS (OR LESS GAIN) tit RADIATION *,NO COHVECTION

' ~

INCREAS(o HEAT FLOW FROM BODY G O R E ~ WITH RISE IN SKIN TEMPERATURE ~

J A PROLONGED EXPOSURE / / " INCREASES BLOOD VOLUME (AGCLIMATIZATION)

SKIN TEMPERATURE J/ RISES INERVOUS RECEPTORS FOR WARMTH ARE ACTIVATED}

FIRST/ kIN( o(Ir(N'~y I v AUGMENTEO SKIN CIRCULATION (VASOOILA TION) ! J

t

INADEQUATE VENOUS RETURN TO HEART / % %

f f ale" INAOEOUATE DLOO0 FLOW TO VITAL All(AS

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OEHYDRATION ~ N20 / T H I R S T / ~ INTAKE 4~ (OFTEN AN 01MINIS HE'D IMABEOUATE • WFATtNQ |TI I"IULUg)

INABFQUA|E SKIN CIRCULATION %

I v CIRCULATORY SHOCK (RAPI0. W(AK PUL~(| LOW BLOOD PRESSURE| GORE TEMPERATURE MAY 8( NORMAL HEAT EXHAUSTION

NEAT LOSS BY I:VAPORAT ION

BASTRIG NAUS~rA

".EA. CRAMPS

RISE IN BODY T(MPEIRAtURE ~ ' . % FATIGUE ANO IMPAIRN[NT OF PERFORMANCE

INCREASED METABOLISM

I

t

P&ILURE Die CI~NTRAL CONTROL MECHANISM ISWEATING CEA.e)ES)

I

t

RAPID Rl~J[ iN ~ORE TEMPERATURE HEATSTROKE

Fig. 4. Flow chart of heat stress leading to heatstroke. Thermal comfort is achieved only when the thermoregulatory system follows the mechanisms within the frame [4 ].

speed o f 2 m / s e c w o u l d have k e p t the c o m f o r t z o n e at 25 °C even u n d e r m o d e r a t e w o r k load. Nevertheless t h e effectiveness o f w i n d s p e e d is limited, since s t r o n g winds cause a sensation of discomfort.

Cloth ing Clothing i n t e r f e r e s w i t h s w e a t e v a p o r a t i o n o n o n e h a n d a n d increases t h e r m a l insulation o n t h e o t h e r h a n d . I n s u l a t i o n values o f c l o t h i n g are d e f i n e d b y clo units. 1 clo u n i t is t h e a m o u n t o f insulation p r o v i d e d b y t h e c l o t h i n g are d e f i n e d b y clo units: 1 clo u n i t is t e m p e r a t u r e a n d refers to a business suit. A long-sleeved w o m a n ' s dress has 0.8 clo, a w i n t e r o v e r c o a t 3 clo a n d shorts + T-shirt

0.5 clo. R o u g h y , each kg o f clothing equals 0.3 clo. I n s u l a t i o n c o e f f i c i e n t of clo equals an increase o f T a b y 5 o 6 °C at a state o f rest, a n d a b o u t 11 °C w h e n p h y s i c a l l y active. Wearing light c l o t h i n g (0.3 clo) t h e r e f o r e enables o n e to m a i n t a i n Ta at a value 3 °C higher. Fanger superimposed the c o m f o r t zones on a psychrometric chart when work load a n d c l o t h i n g insulation w e r e d e t e r m i n e d [5]. Figures 5(a) and 5(b) e x h i b i t c o m f o r t z o n e s c a l c u l a t e d in t h r e e cases o f m e t a b o l i c rates a n d t w o cases o f insulation. T h e increase in p h y s i c a l activity as well as t h e increase o f insulation shift t h e r m a l c o m fort leftwards on the chart.

33 60

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Fig. 5. C o m f o r t l i n e s ( a m b i e n t t e m p e r a t u r e v e r s u s w e t b u l b t e m p e r a t u r e w i t h r e l a t i v e air v e l o c i t y as p a r a m e t e r ) a t t h r e e d i f f e r e n t a c t i v i t y l e v e l s : (a) f o r a p e r s o n n u d e (clo = O, i m / c l o = 1 ); ( b ) w i t h m e d i u m c l o t h i n g (clo = 1.0, im/clo = 1.15). (According to Fanger, 1970).

34 CONCLUSIONS

Characterization of the thermal comfort zone in meteorological terms must be based on preliminary factors; e.g. work load and clothing insulation. Climatological factors such as ambient air temperature, relative humidity and wind velocity can be changed accordingly. For microclimate planning, the challenge is to keep environmental conditions within a range where: (a) all heat dissipation will be obtained via radiation and convection; (b) sweating will be eliminated; (c) wind speed will be in the range of 0.4 - 0.7 m/sec;

(d) solar radiation will be reduced to a minimum. REFERENCES in m a n , Thermoreception and Temperature Regulation, A c a d e m i c

1 H. Hensel, T h e r m a l c o m f o r t

Press, New Y o r k , 1 9 8 1 , pp, 168 - 184. 2 B. Givoni a n d R. F. G o l d m a n , Predicting rectal t e m p e r a t u r e r e s p o n s e to w o r k e n v i r o n m e n t a n d c l o t h i n g , J. Appl. Physiol., 32 ( 1 9 7 2 ) 812 - 822. 3 N. C. M a j u m d a r , Indices of heat stress, I n t e r p r i n t , New Delhi, 1 9 7 8 . 4 H. S. Belding a n d T. F. H a t c h , I n d e x for evaluating h e a t stress in t e r m s o f resulting physiological strains, Heat Pip. Air Cond. 27 (II) ( 1 9 5 5 ) 129. 5 P. O. Fanger, Thermal Comfort, Danish Technical Press, C o p e n h a g e n , 1 9 7 0 , pp. 43 - 54.