45

Prediction of Heat Disorders in Japan
Akio HOSHI1* and Yutaka INABA2
1

Research Center for Odontology, Nippon Dental University,
1-9-20 Fujimi, Chiyoda-ku, Tokyo 102-8159, Japane-mail:ahoshi@tky.ndu.ac.jp
2
Department of Epidemiology and Environmental Health, Juntendo University School of Medicine,
2-1-1, Hongo, Bunkyo-ku, Tokyo 113-8421, Japan*corresponding author

Abstract
Recently, the incidence and mortality rates from heat disorders have been increasing in Japan.
Long ago, heat disorders previously occurred most often at work, such as in mines. Although the incidence is relatively low, occurrence of heat disorders during sports activities is showing an increase. Heat
disorders can be prevented by appropriately controlling exercise, resting, and drinking water with salt,
based on a full understanding of environmental conditions. Accordingly, in Japan, criteria based on
meteorological conditions during sports activities and at work are being used to prevent heat disorders.
The Japan Sports Association proposes “exercise guidelines for preventing heat disorders,” while theJapan Society for Occupational Health recommends using “occupational exposure limits for heatstress.” Recently, as public interest in heat disorders has surged as global warming increases and urban
heat islands become more pronounced, the ability to predict heat disorders occurring in daily life activities is becoming a popular media topic. However, the guidelines and prediction models for heat disorders used now need further development. We anticipate the development of more accurate, user-friendly
guidelines and prediction models for heat disorders in daily life activities.
Key words: daily life activities, heat disorders, labor, prediction, sports

Global Environmental Research
11: 45-50 (2007)
printed in Japan

0.4

0.2

400

0

Mortality rate
(per 100,000 population)

total: deaths
male: mortality rate
female: mortality rate
total: mortality rate

600

200

2003

1999

1995

1991

1987

1983

1979

1975

1971

0
1967

The number of the deaths and mortality rate (per100,000 citizens) of heat disorders from 1959 to 2003
in Japan is shown in Fig. 1. There were 5,261 deaths
from heat disorders (mortality: 117 deaths/year, mortality rate (per 100,000 populations): 0.10 deaths/year).
Recently, however, mortalities from heat disorders are
increasing (mortality rate in 1999-2003: 0.23 deaths/

0.6

male: deaths
female: deaths

1963

2. Incidence of Heat Disorders in Japan

800

1959

In Japan, public interest in heat disorders has
surged in recent years as summer temperatures havetended to increase year by year. In order to prevent theincidence of heat disorders, it is necessary to examine
the actual conditions and important to take preventative steps against heat disorders.
Therefore, this report describes the incidence ofheat disorders in Japan, and addresses the current
status of criteria for preventing heat disorders and predicting its occurrence in three aspects of life: during
sports activities (or exercise), at work, and in daily life
activities.

year). This is attributable to higher summer temperatures as global warming progresses (Watson, 2001)
and urban heat islands intensify (Ministry of theEnvironment, Japan, 2001). The rapid increase in theelderly population and the aging of society is anotherfactor causing this trend (Hoshi & Inaba, 2002a).

No. of deaths

1. Introduction

Fig. 1 Numbers of deaths and mortality rates (per100,000 population) from heat disorders from1959 to 2003 in Japan.
(Source: Hoshi & Inaba, 2002a)©2007 AIRIES

46

A. HOSHI and Y. INABALong ago in Japan, heat disorders occurred mostoften in mines, but the incidence has declined asworking environments have improved. Recently, mostheat disorder cases at work have occurred at construction sites (Ministry of Health, Labor and Welfare,2003; Miura, 1985). Although the incidence is relatively low, heat disorders during sports activities havetended to increase (Hoshi & Inaba, 2002b). Characteristically, it is common among young people (Hoshi &Inaba, 2002a, 2004a), and occurs even in seasons
other than summer (Hoshi & Inaba, 2005). The incidence of heat disorders in daily life activities is muchhigher than the incidences at work or during sports
activities. Specifically, most elderly patients succumbto heat disorders during daily life activities (Hoshi &Inaba, 2004a, 2006).

deathsFig. 2 Numbers of deaths and mortality rates (per100,000 population) from heat disorders in eachage-range group.

The distribution of heat disorder mortalities by age
group from 1959 to 1999 in Japan (Hoshi & Inaba,
2002a) is shown in Fig. 2. The mortality rate fromheat disorder is higher among children and the elderly,
especially in those aged 65 years or older. This highmortality among the elderly is attributable to theirpoorer body temperature regulation (Iriki, 1996) andhigh morbidities from chronic diseases (Harchelroad,1993) possibly underlying heat disorders in thispopulation. The high mortality among children iscaused not only by their immature body temperature
regulation but also by care-givers’ negligence, leaving
them in automobiles in the sun (Takahashi et al.,2000). In addition, mortality from heat disorders ismuch more common in men than in women, and isconsidered to be greatly affected by gender differencesin lifestyles and occupations (Hoshi & Inaba, 2002a).
These results in Japan were similar to those ofstudies in Europe and the USA. Recently, mortalities
from heat disorders during heat waves (prolonged
periods of excessive heat and humidity) have been
increasing in Europe (Dhainaut et al., 2004;
Vandentorren et al., 2004; Conti et al., 2007) and theUSA (Naughton et al., 2002; Weisskopf et al., 2002).
Japan lacks a history of heat waves. Therefore, thenumber of hot days (when the peak temperature of the
day is over 30°C), nettaiya (when the lowest temperature of the day is over 25°C), daily maximum
temperatures, etc., are used as parameters of the heat
environment in Japan (Hoshi & Inaba, 2002a, 2004a,
2005, 2006; Nakai et al., 1999).

Tw（ºC ）

Td（ºC ）
35

～

27

～

31

～

WB GT（ºC ）(Source: Hoshi & Inaba, 2002a)

～

～

～

28

24

31

～

～

～

25

21

28

～

～

～

21

18

24

Cease Exercise

DangerCease heavy
exercise
Extreme Caution
Active restingneededCautionActive water
intake needed

～

～

～

Almost Safe
Appropriate water
intake needed

The skin temperature exceeds the ambient temperature.
In principle, exercise should be stopped.

Since the risk of heat disorders is great, heavy exercise
and long distance running should be avoided. Active
resting and water intake are needed during exercise.
Those not physically strong or unaccustomed to heatshould not exercise.
Since the risk of heat disorders increases, active
resting and water intake are needed during exercise.
Rests should be taken at about 30-min intervals during
heavy exercise.
Death from heat disorders is possible. Any signs ofheat disorders should be monitored, and water should
be taken actively during rest.

Although the risk of heat disorders is usually low,
water should be taken appropriately. Participation in
events such as citizens’ marathons requires caution.

Fig. 3 Exercise guideline for preventing heat disorders proposed by theJapan Sports Association.
WBGT: Wet Bulb Globe Temperature (Source: Kawahara, et al., 1994)

Prediction of Heat Disorders in Japan

3. Prevention of Heat Disorders during
Sports Activities (or Exercise)

4. Prevention of Heat Disorders at WorkHeat disorders can be prevented by appropriately
controlling exercise, taking rests and drinking waterwith salt, based on a full understanding of environmental conditions (Kenny, 1985; Frizzell et al., 1986;
Gisolf & Duchman, 1992; Kawahara et al., 1994).
Criteria for preventing heat disorders during sports
activities and at work have been developed based on
the relevant environmental conditions.
WBGT (Wet Bulb Globe Temperature) is known to
be useful as an index for environmental heat conditions because it involves not only temperature but also
humidity, airflow and radiant heat (Yaglou & Minard,
1957). To prevent heat disorders during sports activities, in 1994 the Japan Sports Association proposed
exercise guidelines for preventing heat disorders, using WBGT.
These guidelines proposed by the Japan Sports
Association are shown in Fig. 3. The guidelines specify five exercise levels based on WBGT (ºC): discontinuation of exercise at ≥ 31ºC, extreme caution at
28ºC-31ºC (discontinuation of heavy exercise), caution at 25ºC-28ºC (necessity for active resting), attention at 21ºC-25ºC (necessity for active water intake),almost safe at ≤ 21ºC (necessity for appropriate water
intake). Methods of performing exercise, taking restsand consuming water and salt are described for eachlevel. WBGT is not measured where sports activities
are taking place in many cases. Therefore, the dry
bulb temperature (Td) and wet bulb temperature (Tw)equivalent to WBGT are shown.
The guidelines have been compiled as a guidebook
and are used in education to prevent heat disorders
during sports activities in Japan (Kawahara et al.,1994).

The Japan Society for Occupational Health (JSOH)
has recommended adopting “occupational exposure
limit for heat stress” since 1983 (Table 1) (JSOH,
1983, 2001). The conditions applied are as follows.
Subjects: healthy young men (usual summer clothes)acclimatized to hot environments and skilled in therelevant work. Work period: continuously for 1 houror intermittently for 2 hours of work. Work conditions:water and salt taken appropriately.
The intensity of work is indicated by five RMR(Relative Metabolic Rate) levels and metabolic energy(kcal/h), and the acceptable temperature limit is specified for each level: 32.5ºC (WBGT) for very lightwork (RMR: ≤ 1, kcal/h: ≤ 130), 30.5ºC for light work
(RMR: 1-2, kcal/h: 130-190), 29.0ºC for Moderate
work (RMR: 2-3, kcal/h: 190-250), 27.5ºC for moderate work (RMR: 3-4, kcal/h: 250-310), and 26.5ºC forheavy work (RMR: 4-5, kcal/h: 310-370). RMR is anindex for evaluating work load intensity. This index
has been used in Japan for a long time. It is calculated
by METS (Metabolic Equivalents) = RMR/1.2 + 1.
The relationship between the work load intensity andacceptable level of WBGT is similar to the result of
ISO7243 (2003).

5. Prediction of Heat Disorders Occurring in
Daily Life Activities
In Japan, public interest has surged in heat disorders in recent years as summer temperatures havetended to increase year by year. Accordingly, prediction of the incidence of heat disorders is becoming
popular in the media, e.g., during weather forecasts
on TV and the radio, on web pages with information
for preventing heat disorders, such as that of theJapan Weather Association (http://www.n-tenki.jp/

Table 1 Occupational exposure limits for heat stress proposed by the Japan
Society for Occupational Health.
(Source: JSOH, 1983, 2001)

Work load intensity
RMR 1-1 (Very light)
RMR 1-2 (Light)
RMR 2-3 (Moderate)
RMR 3-4 (Moderate)
RMR 4-5 (Heavy)

47

Metabolic energy（kcal/h）-130130-190
190-250250-310310-370

Acceptable level
WBGT(°C)
32.5
30.5
29.0
27.5
26.5

WBGT: Wet Bulb Globe Temperature
Workload intensity
RMR -1; Finger work such as on a word processor or key punch
RMR 1-2; Lathe operation, automobile operation, etc.
RMR 2-3; Concrete polishing, work involving fast walking, etc.
RMR 3-4; Light to moderate carpenter work, bicycle operation, etc.
RMR 4-5; Work using the whole body, such as agricultural work, clampingof rivets, etc*RMR=(Working metabolism - Resting metabolism)/Basal metabolism

48

A. HOSHI and Y. INABA

Heat_Disorder/), and in publications, such as thehealth guidance manual for heat disorders by theEnvironment Agency (http://www.env.go.jp/health/Heat Stroke/index.html/). However, these media channels predict the incidence of heat disorders based on
the criteria of the exercise guidelines for preventing
heat disorders proposed by the Japan Sports Association (Kawahara et al., 1994). Actually, heat disorders
occur less frequently during exercise (Hoshi & Inaba,
2002b). Besides, the exercise guidelines are intended
to prevent heat disorders at the site of exercise byevaluating environmental conditions there, and are notdesigned to predict the occurrence of heat disorders.
Therefore, the guidelines should not be applied to heatdisorders occurring during activities other than exercise. Based on this recognition, Hoshi and Inaba
developed a predictive model for heat disorders occurring in all activities (Hoshi & Inaba, 2004b) (Fig. 4).
In the new prediction model, the regression line wascalculated from the correlation with daily maximum

temperature and relative humidity at daily maximum
temperature in the day of occurrence of heat disorders.
This regression line was in agreement with the WBGT
value. The +1SD of this regression line was approximated to 32ºC, and -1SD was 27ºC, -2SD was 24ºC.
This prediction model represents the risk of heatdisorders at four levels based on WBGT. Extreme danger, at ≥ 32ºC, may occur even for people staying still.
Preventive measures should be actively taken, suchas avoiding going outdoors. Danger, at 27ºC-32ºC,
occurs for the largest number of people. Adequate
preventive steps should be taken. Extreme caution, at
24ºC-27ºC, applies more often at work or in usual
activities. Sufficient caution should be exercised.
Caution, at ≤ 24ºC, may be needed during heavy exercise such as marathons. Caution should not be
neglected.
Heat disorder forecasts via certain media channels(TERUMO Health Forecast; http://kenkotenki.jp/contents) are classified into sports activities and daily life

Relative humidity at daily maximum temperature ( % )20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

100

40 30 30 31 32 33 34 35 36 37 38 39 40 41 41 42 43 44

Daily maximum temperature ( ºC )38 28 29 30 31 32 32 33 34 35 36 37 38 39 39 40 41 42
36 27 27 28 29 30 31 32 32 33 34 35 36 37 37 38 39 40
34 25 26 27 27 28 29 30 31 31 32 33 34 35 35 36 37 38
32 24 24 25 26 27 27 28 29 30 30 31 32 33 33 34 35 36
30 22 23 23 24 25 26 26 27 28 29 29 30 31 31 32 33 34
28 20 21 22 23 23 24 25 25 26 27 27 28 29 29 30 31 32
26 19 20 20 21 22 22 23 24 24 25 25 26 27 27 28 29 29
24 17 18 19 19 20 21 21 22 22 23 24 24 25 25 26 27 27
22 16 16 17 18 18 19 19 20 21 21 22 22 23 23 24 25 25
20 14 15 15 16 17 17 18 18 19 19 20 20 21 21 22 23 23
18 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21
16 11 12 12 13 13 14 14 15 15 16 16 17 17 17 18 18 19
14 10 10 11 11 12 12 12 13 13 14 14 15 15 15 16 16 17
12 8

9

9

9

10 10 11 11 11 12 12 13 13 13 14 14 15

Extreme Danger(32ºC - )May occur even among people staying still. Preventive measures
should be taken actively, such as avoiding going out.

Danger(27 - 32ºC)

Occurs in the largest number of people. Preventive measures should
be taken adequately.

Extreme Caution(24 - 27ºC)

Occurs more often at work or in daily activities. Sufficient caution
should be exercised.

Caution( - 24ºC)

May occur during heavy exercise such as marathons. Caution should
not be neglected.

In automobiles in the sun, heat disorders may occur irrespective of the season. Specifically,
temperatures in automobiles surge on sunny days. Do not leave young children in a car evenfor a few minutes.
During sports activities, heat disorders may occur even on cool days in seasons other thansummer. In particular, caution should be exercised if the WBGT increases by at least 3ºC
within one week. Take water and salt adequately.

Fig. 4 Prediction model of heat disorders using WBGT in Japan.
WBGT: Wet Bulb Globe Temperature (Source: Hoshi & Inaba, 2004b)

Prediction of Heat Disorders in Japan
Heat disorders in sports activities
Almost
Safe

Caution

ExtremeCautionDangerHeat disorders in daily activities
AlmostSafe
20

22

ExtremeCaution Danger

Caution

24

26

28

30

32 WBGT(ºC)

Fig. 5 Prediction of heat disorders according to the
TERUMO health forecast.
(Source: TERUMO Health Forecast)activities (Fig. 5). Exercise guidelines for preventing
heat disorders proposed by the Japan Sports Association are used at sports activities, and a new prediction
model of heat disorders is being used for daily life
activities. For sports activities, exercise discontinuation and danger levels were unified into one danger
level. For daily life activities, the extreme caution
level was modified to 28ºC-32ºC from 27ºC-32ºC.
The Japanese Society of Biometeorology launched
the Research Council for Heat Disorder Prevention in
2006, which began research on predicting the incidence of heat disorders and development of prediction
models. It is expected that they will develop moreaccurate, more user-friendly prediction models forheat disorders in daily life activities.

6. Conclusion
In Japan, public interest in heat disorders has
surged in recent years as summer temperatures havetended to increase year by year. Heat disorders can beprevented by appropriately controlling exercise, taking rests, and drinking water with salt based on a fullunderstanding of environmental conditions. Accordingly, in Japan, criteria based on meteorological conditions during sports activities and at work are being
used to prevent heat disorders.
For sports activities, exercise guidelines for preventing heat disorders were proposed by the JapanSports Association in 1994. For labor, occupational
exposure limits for heat stress were proposed by the
JSOH in 1983 and 2001. For daily life activities, apredictive model for heat disorders was proposed by
Hoshi and Inaba in 2004. The prediction models in use,
however, still need to be improved.
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(Received 10 November 2006, Accepted 29 January 2007)