Journal of Heat Island Institute International Vol. 9-2 (2014)

Academic Article

Experimental Analysis of Human Thermal Condition
During Outdoor Exercise under Summer Conditions
Yasuhiro SHIMAZAKI*1

Atsumasa YOSHIDA*2

Shohei TAKETANI*1*1Department of System Engineering for Sports, Okayama Prefectural University*2Department of Mechanical Engineering, Osaka Prefecture University
Corresponding author: Yasuhiro SHIMAZAKI, shimazaki@ss.oka-pu.ac.jp
ABSTRACT
Outdoor walking is a common activity in daily life. In order to estimate the human thermal environment, weinvestigated the relationship between physically measured human thermal states (based on human thermal load) andsubjective thermal perceptions. The experiments were performed for 50 min, including 10 min of rest before andafter 30 min of exercise, at three distinct velocities: 0 m/min standing, 55 walking, and 167 running, under summer
conditions. The results provide valuable information for discussing urban street comfort in relation to humanactivity. For outdoor walking or running, although radiation is significant, metabolic heat generation has a greater
impact on human thermal states. Human thermal load is a useful tool because human thermal states, and changes inthem, are herein expressed quantitatively by dividing the energy flow around humans into components. However,there are some limitations to estimating thermal sensation for high-intensity-activity applications.
Key Words: Thermal comfort, Human energy balance, Perception, Pedestrian, Transient

outdoors. Therefore, these indoor standards have limitations

1. Introduction

when applied to outdoors. Moreover, outdoor human thermal
Factors in climate change, such as global warming and theperception is considered to be different from indoor humanurban heat-island phenomenon, have become widely known.

thermal perception.(5) In addition, there is a lack of available

Increased temperature has also occurred in Japan and because

data for establishing or verifying any new index for theof this, an increasing number of uncomfortable situations,outdoor thermal environment.

including heat-related disorders and even mortality, have been

Radiation has always been the focus in assessments of the(1)

Therefore, muchoutdoor thermal environment, because of the large amount ofresearch has addressed adaptation measures, particularly

heat received. Metabolism increases with increasing exercise

regarding human thermal perception. In particular, humanintensity, and sometimes high-speed walking generates larger

comfort and thermal sensation have attracted attention as

amounts of heat than is received in radiation. Thus, in thisevaluation criteria for assessing the thermal environment.

study, our goal was to obtain information about human

Human thermal perception is generally considered to be

thermal states, as well as physiological and mental responses

predictable from calculations of the human energy balance.

during outdoor walking in summer, related to pedestrianThis balance is considered to be closely associated with

comfort and safety.

reported recently outdoors in summer.

human thermal conditions, of which the six predominantcomponents are known to be temperature, humidity, wind

2. Procedures

condition, radiation, clothing insulation, and metabolism.(2) It
is advisable to include as many factors as possible in studies

Theof thermal conditions, particularly human factors.

physiological reactions and mental actions of human subjects,

International thermal comfort standards such as SET*(3) andas the surrounding weather factors and the physiological

PMV(4) were developed based on relatively neutral indoor

responses of the human body were measured. The global solar

thermal environments and are commonly used. In reality,

radiation (EKO MR-60), reflected solar radiation from thehumans experience extremely hot situations when active

ground (EKO MR-60), infrared radiation from the atmosphere- 33 -

experimentwasperformedbyobservingtheand ground (MR-60), air temperature (thermistor), 3-D windon the skin; sublingual for thermistor) were installed 15 minvelocity(capacitive

before the measurements began, at which point the subjects

hygrometer), and ground temperature (thermo couple T) were

(Young

CYG-81000),

humidity

had become accustomed to the devices and returned to theirmeasured in 1-min intervals. Field measurements wereoriginal states. A heart-rate meter (Polar RS800CX) andperformed in three distinct outdoor spaces, and within an

exhaled gas analyzer (SMe VO2000) for evaluating theindoor space at three distinct walking velocities, in themetabolism were also installed at this time. All this equipment

summer of 2013. The thermal-environment assessments were

was synchronized. The body mass was measured before anddone in two different ways: (a) “Human thermal load,” wasafter the experiment to observe any weight change frombased on estimating the human energy balance in an outdoor

sweating.

environment(6) and (b) “Thermal perceptions,” was based on

A total of 30 healthy males participated in the study (Heightthe sense of satisfaction or dissatisfaction under the prevailing1.72 ± 0.06 m, Weight 63.3 ± 7.0 kg, Age 21.0 ± 1.3 yr). The

climatic conditions in the outdoor spaces. Measurements wereresearch was conducted during a hot season in Okayama, withperformed to include the effect of surfaces such as black

approval from the Research Ethics Committee of Okayamaasphalt concrete, green concrete, and natural grass. The

Prefectural University.

surfaces considered were typical of outdoor exercise scenarios.
As a reference, the experiment was also performed indoors ina similar manner.
The human thermal load is in itself the heat flux under a given
condition and is calculated from Equation (1) as the amountremaining after summing each item of an energy balance. It is

Cold

Hot

Uncomfortable

Comfortable

Thermal sensation
Thermal comfort
Not at all

Extremely

Not at all

Extremely

Perceptions of perspiration

an objective value based on an energy-balance formula for theThirst sensation

Fig. 1 Topics covered in subjective questionnare

physiological factors:

600

Q = M – W + Rnet – E – C

Human thermal load[W/m2]

human body, which incorporates varying degrees of(1)

where M quantifies the metabolic variable proposed by Weir
[W/m2] (7), W is the workload [W/m2], Rnet is the net radiation[W/m2], E is the latent heat loss [W/m2], and C is the sensible
heat loss [W/m2]. In a steady state, and even in an unsteadystate due to variations in weather and human factors, the

Rest

500

Excercise

RecoveryStanding (Outdoor)
Walking (Outdoor)
Running (Outdoor)

400
300
200
100
0
0

10

20
30
40
Elapsed time[min]50

Fig. 2 Time-dependent changes in human thermal load

thermal condition can generally be obtained using the overall

Table 1 Time-averaged human thermal load

human thermal load.
Subjects were asked whether they experienced any thermal
sensation, and if so, whether they experienced it as comfort orStanding

discomfort on a 5-point scale from “discomfort (-2)” toOutdoorRest

Exercise

Recovery

120

115

113

Walking

152

195

133

“comfort (+2)”. The subjects were also instructed to note their

Running

144

406

244

mental reactions every 2 min on a fixed report that employedStanding

31

25

23

a 7-point scale from “cold (-3)” to “hot (+3)”, as proposed by

Indoor

ASHRAE.(8) At the same time, subjects were asked about their

Walking

39

73

32

Running

34

220

81

perceptions of perspiration, from “not at all (0)” to “extremely(+3)”, and thirst, from “not at all (0)” to “extremely (+3)”.
The experiments were performed for 50 min, including 10
min of rest before and after 30 min of exercise, at velocities of
0 m/min standing, 55 walking, and 167 running, undersummer conditions. The clothing insulation was 0.41 clo
throughout the experiment.
Sensors (thermistors and perspiration meter; at seven points

3. Results
3.1 Human thermal states
For reference, the average air temperature, humidity, windspeed，and solar radiation were approximately 31.0 °C, 53.9%
RH, 0.9 m/s outdoors (calm indoors), 704 W/m2 outdoors, and
0 W/m2 indoors, respectively.

- 34 -

500

Standing (outdoor)
Walking (outdoor)
Running (outdoor)

Standing (indoor)
Walking (indoor)
Running (indoor)

600

Heat flux[W/m ]400

2

Metabolism[W/m ]2

Net radiation
Metabolism
Workload

300
200
100

Latent heat loss
Sensible heat loss
Human thermal load

400
200
0

-200 Outdoor Outdoor Outdoor Indoor00IndoorIndoor(standing) (walking) (running) (standing) (walking) (running)

10

20
30
40
50
Elapsed time[min]Fig. 3 Time-dependent changes of metabolism

Fig. 4 Human-thermal-load components for exercise

The time-dependent changes in total human thermal load
Mean skin temperature[℃] Mean skin temperature[℃]

outdoors are shown in Fig. 2. During their rest periods, thesubjects behaved similarly, just standing, and their thermalloads were almost the same (approximately 140 W/m2). The
total human thermal load, with subjects standing, remained
almost constant and a thermal steady state was reached.
Human thermal loads for walking and running rapidly
increased after 10 min from the time exercise started. After ashort while, human thermal loads settled and thermal steadystates were reached. During walking, the human thermal load
wasapproximately

200

W/m2,

and

duringrunning,2

approximately 410 W/m . After 40 min from the time walking
exercise terminated, the human thermal load rapidly decreased
and returned to initial levels. The human thermal load forrunning also rapidly decreased, however it settled into anotherlevel at approximately 220 W/m2. Human thermal loads
indoors showed the same tendencies. The time-averaged

37
36
35
34
Standing (Outdoor)
Walking (Outdoor)
Running (Outdoor)

33
32
37

Standing (Indoor)
Walking (Indoor)
Running (Indoor)

36
35
34
33
32
0

10

20
30
40
Elapsed time[min]50

Fig. 5 Time-dependent flux of mean skin temperature

human thermal load in each period, and under each exercise
condition, is shown in Table 1. Even during rest periods whensubjects only stood, there was approximately 100 W/m2

state was reached for walking. The time-averaged metabolism

difference between outdoor and indoor human thermal loads.

during walking was 125 W/m2 outdoors and 115 W/m2

During exercise periods, the outdoor/indoor difference inindoors. Metabolism for running also settled at a certain level;2

human thermal load for walking was approximately 120 W/m ,however, the value continued to increase gradually. The

and for running, approximately 190 W/m2. The differencetime-averaged metabolism during running was 419 W/m2

between outdoor and indoor human thermal loads increased

outdoors and 337 W/m2 indoors. After finishing the walking

with increasing intensity of exercise. Even when the intensity

exercise, metabolism quickly returned to previous levels. Inof exercise was the same, the human thermal load outdoors

contrast, during recovery after running, metabolism graduallytended to be larger than that indoors.

decreased but did not reach initial levels in the rest period.

The time-dependent metabolic changes are shown in Fig. 3.

Metabolism tends to increase with increasing intensity ofMetabolism was almost constant against time for standing,

exercise. At the same time, metabolism tends to become

and was almost constant for other intensity-levels of exercise

higher outdoors because humans experience environmental

as well. When subjects stood during rest periods, theheat stress when outdoors during hot periods.

2

metabolic value was approximately 55 W/m . There were no

Breakdowns of total human thermal load are shown in Fig. 4.

significant outdoor/indoor differences in metabolism. The

Since there were notable differences in human thermal loads

metabolism rapidly increased after walking or running for 10

during the exercise period, time-averaged values during themin. Then, metabolic conditions settled and thermal steady

exercise period are shown in the figure. During this period,- 35 -

values for each human-thermal-load component fell within askin temperature when standing was 35.9 °C outdoors andcertain range. Outdoors, for example, metabolic values were34.0 °C indoors. Outdoors, the mean skin temperatureapproximately 52, 125, and 419 W/m2 for standing, walking,increased gradually during the rest period. This is because, inand running, respectively. Metabolism tended to increase withhumans, the skin temperature is adjusted according to theincreasing intensity of exercise. On the other hand, theoutdoor environment. When walking, the mean skinamount of net radiation (as a heat source) was almost constant,temperatures decreased slightly after 10 min. The reduction of2

with values of approximately 120 W/m outdoors and 10

the maximum mean skin temperature was approximately

W/m2 indoors. This is the reason that outdoor/indoor human

0.3 °C outdoors and 0.5 °C indoors. A short while later, thethermal loads were different throughout the experiment. In themean skin temperature during walking reached an almost

experiment, or even under real living conditions, metabolism

constant level. The mean skin temperature increased after theand net radiation are normally the only heat sources received

walking exercise ended. The mean skin temperature foroutdoors. When you look at the ratio of metabolism and net

running subjects made similar moves; however, there wasradiation, it is easy to realize which is the more influential

larger variation over time. After the running exercise started,

factor. As the intensity of activity increases, the effect ofmean skin temperatures dropped because of the decline ofmetabolism becomes stronger. For outdoor walking or

blood flow and because of sweating. In addition, heat transfer

running,

metabolic

was promoted by the increased relative wind speed due togeneration of heat has a greater impact on human thermal

body motion. The maximum decrease in the mean skinstates. Indoors, net radiation has little effect.

temperature was approximately 0.5 °C outdoors and 0.9 °C

Changes in time-dependent mean skin temperature are shownindoors. After running finished, the mean skin temperaturein Fig. 5. The mean skin temperature is calculated accordingrose temporarily. The mean skin temperature was higher

althoughradiationis(9)significant,

You need to understand theoutdoors than indoors. The decline in the mean skin

skin temperature differences in advance because of personaltemperature at the start of exercise was larger for running thanand environmental ranges. Mean skin temperature is almost

for walking. Since latent heat loss or convective heat transfer

constant against time when standing. The time-averaged meanis influenced by the difference between the human-skin and

to Hardy and DuBois’s formula.

3Standing
Walking
Running8

Sweat rate[g/min]Sweat rate [g/min]

10

6
4
2
0
0

10

20
30
40
Elapsed time [min]
(a)

Head
Abdomen
Arm
Hand
Thigh
Leg
Foot

2

1

0
0

50

10

20
30
40
Elapsed time[min]
(b)

50

400

1

2

500

2

500

Human thermal load [W/m2]
Psychological sensation

Psychological sensation

2

400

1

300
0

300
0

200

-1
-2
0

100
10

20
30
40
Elapsed time [min]050

Human thermal load [W/m ]Fig. 6 Time-dependent changes of sweat rate: (a) Total outdoor sweat rate, (b) Regional sweat rate running outdoors

200

-1
-2
0

100
10

20
30
40
Elapsed time [min]

(a)

0
50

Thermal sensation
Thermal comfort
Perspiration sensation
Thirst sensation
Human thermal load(b)Fig. 7 Time-dependent changes of thermal perceptions and human thermal load: (a) Running outdoors, (b) Standing outdoors- 36 -

ambient temperatures, and because the ambient temperature3

was almost constant during the experiment, the time variation
Thermal sensationof latent heat loss can be determined from changes in themean skin temperature.
The time-dependent sweat rate changes outdoors are shown in
Fig. 6 (a). Evaporative heat release is important, especially forhigh thermal loads such as occur outdoors. During the rest

increasedbecauseof0Standing
Walking
Running-2-100 0 100 200 300 400 500
2Human thermal load [W/m ]exposure. After 10 min, the sweat rates for walking andsharply1-1

period, the sweat rate gradually increased due to heatrunning2

increasing(a)metabolic-heat generation. Sweat rates for standing and2walking settled at a certain level. The sweat rate for running
Thermal comfort

showed another increase during the recovery period. Thisoccasionally happened when the intensity of the exercise washigh. The sweat rate increased with increasing intensity of
exercise. The maximum sweat rate was 5.3 g/min standing,
5.9 walking, and 8.8 running. The time-dependent regional

Standing (Outdoor)
Walking (Outdoor)
Running (Outdoor)
Standing (Indoor)
Walking (Indoor)
Running (Indoor)

1
0

-1

sweat rates for outdoor running are shown in Fig. 6 (b). Eachregional sweat rate made similar moves; however, the feet-2
-3

were not sweating at all. The abdomen, arms, thighs, and legs
had up to 2 g/min higher sweat rates. The head and hands

-2 -1
0
1
2
3
Thermal sensation(b)

have relatively lower sweat rates, because they are exposed

Fig. 8 Thermal perceptions: (a) Relationship between thermal

directly to wind, and because their sensible heat loss provides

sensation and human thermal load, (b) Relationship between

a significant advantage for cooling.

thermal comfort and thermal sensation

The workload in general varies according to intensity of
exercise. In our experiment, the workload values, both2

human thermal load and thermal sensation for standing andoutdoors and indoors, were approximately 0 W/m standing,walking. However, another trend was found in running during

25 walking, and 90 running.

exercise and afterward, during the recovery period. The
thermal sensation for running shows relatively low values

3.2 Thermal perceptions

during the recovery period, even when humans experience

Examples of time-dependent changes in thermal perceptions

large thermal loads due to substantial generation of metabolic

outdoors are shown in Fig. 7. As a reference for humanheat. The relationship between thermal comfort and thermal

thermal states, the human thermal load is also drawn in thesensation is also shown in Fig. 8 (b). There is a good linear

figures. Since the thermal stress, or human thermal load, isrelationship between thermal comfort and thermal sensation inalmost constant while standing, all sensations remain almost

general, therefore one possible reason for not estimating

constant.

temperature,

thermal sensation based on thermal load, is because of theperspiration, and thirst increased during the exercise period.

outmost transient response of thermal perception. Another

Then at the time the exercise finished, sensation of heatreason may be due to the ability to calculate human thermal

dropped rapidly. However, the perceptions of perspiration andloads directly.

thirst continued to increase. Thermal comfort decreased until

The main cause of large human thermal loads is increased

the exercise finished; then rose rapidly.

metabolism during exercise and the recovery period.

Forrunning,

perceptionsofConvective heat release from humans may be induced,
4. Discussion

because relative wind speed exists and air-flow becomesturbulent around moving humans. To calculate the latent heat

In order to analyze the interconnection between objectively

loss, Yang’s convective heat-transfer coefficient(10) was

measured human thermal states and subjective thermal

adopted and expressed as
h = 3.6 × 6.97 v 0.89

perceptions, the relationship between them is shown in Fig.

(2)
28(a). In a previous study, a linear relationship between human

where h is the convective heat transfer coefficient [W/m /K],

thermal load and thermal sensation was confirmed. In thisand v is wind speed [m/s]. A 167 m/s body movement isstudy, our results revealed a linear relationship between

simply equivalent to approximately 2.8 m/s wind. Considering- 37 -

the increase in the loss of latent heat by movement, its effect(5) P. Hoppe, Different Aspects of Assessing Indoor andon overall human thermal load is limited. Therefore, there are

Outdoor Thermal Comfort, Energy and Buildings, 34

some limitations for estimating thermal sensation using
human thermal load during transient periods of high-intensity(2002), pp. 661-665.
(6) Y. Shimazaki, A. Yoshida, R. Suzuki, T. Kawabata, D.

exercise, at this stage.

Imai, S. Kinoshita, Application of Human Thermal Load

The delayed responses of thermal comfort and thermal

into Unsteady Condition for Improvement of Outdoor

sensation were observed just after exercise started, as shown

Thermal Comfort, Building and Environment, 46-8 (2011),

in Fig. 7 (b). Metabolism sharply increases; however, thermal

pp. 1716-1724.

comfort and thermal sensation change gradually. Changes in(7) J.B.V. Weir, New Methods for Calculating Metabolic Ratethe perception of perspiration and thirst also are not consistent

with Special Reference to Protein Metabolism. Journal ofwith human thermal load. Perspiration constantly increased

Physiology, 109 (1949), pp. 1-9.

throughout the experiment (Fig. 6), in this sense, perspiration(8) ASHRAE, Standard 55-2013 - Thermal Environmental

offers an indication of sweat or latent heat loss. A good

Conditions for Human Occupancy (2013), ASHRAE,

relationship was observed between the perceptions ofAtlanta.

perspiration and thirst. Therefore, there are limitations in(9) J.D. Hardy, E.F. DuBois, The Technique of Measuring

estimating thermal comfort and thermal sensation for

Radiation and Convection. Journal of Nutrition, 15-5

high-intensity activity applications.

(1938), pp. 461-475.
(10) J.H. Yang, S. Kato, S, Murakami, T. Takahashi, T.
Hayashi, Measurement of Convective Heat Transfer

5. Conclusions

Coefficient with Using a Experimental and Computational
In order to measure and understand human thermal conditions

Thermal Manikin (Part 2): Convective Heat Transfer

during outdoor exercise, the participants in the study

Coefficient of Each Part of a Human Body in Outdoor andperformed for 50 min, including 10 min of rest before andIndoor Environment, Proceedings of The Society ofafter 30 min of exercise, at three distinct velocities. We

Heating, Air-Conditioning Sanitary Engineers of Japan,

measured the physical and mental responses, and then(2001), pp. 777-780. (in Japanese)

estimated human thermal states by using human thermal load.
First, we obtained valuable human information with which to(Received Nov. 12, 2014, Accepted Dec. 27, 2014)

discuss urban street comfort in relation to human activity.
Our results indicate that when walking or running outdoors,
although radiation is significant, metabolic heat generation
has a greater impact on human thermal states. Human thermalload is a useful tool, because human thermal states, andimprovements in them, may be expressed quantitatively bydividing the energy flow around humans into components.
However, there are some limitations to estimating thermal
sensation for high-intensity-activity applications.
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Heatstroke This Summer in Japan after Fukushima
Nuclear Disaster, Environmental Health and Preventive
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(3) A.P. Gagge, A.P. Fobelets, L.G. Berglund, A Standard
Predictive Index of Human Response to the Thermal
Environment, ASHRAE Transactions, 92-2B (1986), pp.
709-731.
(4) P.O. Fanger, Thermal Comfort (1973), McGraw-Hill, NewYork.
- 38 -