Armored 
Medical Research Laboratory 
Fort Knox, Kenituckv 
PROJECT NO. 2 - OPERATIONS AT HIGH TEMPERATURES 
Report On 
Sub-Project No. 2-11, Influence of High Temperatures on the Efficiency 
of Personnel 
Sub-Project No. 2-12, Study of Methods of Attaining and Maintaining 
Acclimatization to High Temperatures 
Sub-Project No. 2-13, Effect of Training on the Efficiency of 
Performance at High Temperatures 
Sub-Project No. 2-17* Study of the Physiologic Effects of High 
Temperatures 
CF men in simulated desert heat 
Project Mos. 2-11, 2-12. 2-13. 2-17 
3 April 19h3 ARMORED FORCE MEDICAL RESEARCH LABORATORY 
Fort Knox, Kentucky 
Project No. 2 (2-11, 2-12, 2-13, 
2-17) 
File No. 727-2 
April 3, 19li3 
Report On 
STUDIES OF MSN IN SIMULATED DESERT HEAT 
1. PROJECT; 2; 2-11, Influence of High Temperatures on the Effic- 
iency of Personnel; 2-12, Study of Methods of Attaining and Maintaining 
Acclimatization to High Temperatures; 2-13 Effect of Training on the 
Efficiency of Performance at High Temperatures; and 2-17, Study of the 
Physiologic Effects of High Temperatures. 
a. Authority - Letter, Commanding General, Headquarters Armored 
Force, Fort Knox, Kentucky, File 1*00,112/6 GNOHD, dated September 21, 19U2. 
b„ Purpose - The purpose of these experiments was to study, under 
controlled conditions in the room, the behavior of men when 
exposed to high temperature, (as implied by the sub-project titles), and to 
enlarge upon the information obtained in previous studies in the California 
desert (Report on Desert Field Study, Project 2-3, File No* 72L.3, October 20 
19h2, and report on Water & Salt Requirement for Desert Operations, Project 
No. 2-6, File No. 333.3U, November 12, 19l2.). 
2, DISCUSSION; 
a. General. Four acclimatization experiments, utilizing a total 
of fifty-six enlisted men, were carried out in the hot room of the Armored 
Force Medical Research Laboratory. The studies extended over a period of 
four months; some men being under study for a week; a great majority 
of the men for periods of from one to two months. Forty-eight men lived 
in the hot room continuously throughout the duration of the experiments, 
being permitted to leave for only two 5-minute periods daily. 
• b. Procedure: Temperatures like those encountered during the 
hot months in the California desert were used; 120°F during the day (0300- 
1700 hours) and 90°F at night. The relative humidity ranged from 1$% to 
22% during the day. Detailed accounts of the test procedures and the re- 
sults are given in the Appendices„ 
1 3. CONCLUSIONS; 
a. Acclimatisation. 
(1) Soldiers exposed to desert heat become adapted to it by 
a process of acclimatization which enables them to carry out their duties 
more efficiently and with less risk of illness from heat than when first 
exposed. 
(2) The condition of men in respect to their ability and fit- 
ness for their tasks in the heat can be estimated by a careful observer who 
knows what to look for. Using the information in Appendix I, line officers 
who know the work capacities of their men, can determine their degree of 
acclimatization and whether or not it is safe for them to continue activity. 
(3) A man acclimatized to heat works in the heat with a lower 
body temperature, lower heart rate and a more stable blood pressure, than 
when not acclimatized. (Appendix. IIp Chart l) Nevertheless, acclimatization 
to heat cannot be measured by these criteria alone. (Appendix II, Chart 2) 
Changes in pulse and temperature accompany acclimatization but do not nece- 
ssarily correlate with the man’s behavior and ability to work. The man as a 
whole must be considered and evaluated. 
(4) Acclimatization begins with the first exposure, The pro- 
cess is rapid and a major proportion of the acclimatization is acquired by 
the third or fourth day, (Appendix II, Chart 3) 
(5) Soldiers in good physical condition acclimatize more quickly 
and are capable of a greater work output in the heat than are men in poor 
physical condition. (Appendix II, Chart 4) 
(6) Continued training in cool environments beyond that nece- 
ssary to attain good pnyslcal fitness aces not further increase the ability to 
work in tho heat nor shorten the period of acclimatization. (Appendix II, 
Charts 5, 6) 
(7) itesting for three or four clays in heat, with activity 
limited to that required for subsistence, results in definite, but only par- 
tial acclimatisation. Some work in the heat is necessary for complete accli- 
matization. (Appendix II, Charts 5, 6) 
(8) When work is begun with first exposure to the heat and pro- 
gressively increased within the limits of tolerance of the man, full accli- 
matization (the ability to perform a maximum amount of strenuous work in the 
heat) is attained most quickly0 (Appendix II, Charts 5, 6) 
(9) Strenuous v.ork on first exposure to the heat is not well 
tolerated and will often result in disability* Continuing such a degree of 
work for another day or two will incapacitate many men - the few who can con- 
tinue their labors do so ineffectively and inefficiently, (Appendix II, 
Chart 7) 
2 (10) Excessive work on first exposure, even leading tc heat 
exhaustion, does not', however, retard the rate of acclimatization or lessen 
the degree which is finally attained, provided work is discontinued upon 
the appearance of symptoms; water and salt are given; and work when later 
resumed is in keeping with the tolerance of the soldier. (Appendix II, 
Charts 7, 8) 
(ll) Three or four exposures to heat of 3 or 4 hours duration 
with two one-hour work periods during each exposure, will produce a con- 
siderable degree of acclimatization. These exposures may be separated by- 
intervals of-two days in a cool environment, (Appendix II, Chart 9) 
(12) So long as the work done is within the capacity of the 
man, the same pattern of acclimatization is produced bv short severe exer- 
tion (for ten minutes of each hour, 4 or 5 times daily) as by moderate work 
of long duration (marching 12-1/2 miles with 20 pound pack at 2-1/2 miles 
per hour). (Appendix II, Chart 10) 
(13) The well-acclimatized man deprived of adequate rest at 
night is incapable of producing his customary amount of work in the heat 
on the ensuing day or does so less efficiently. (Appendix II, Chart 11) 
(14) Once acclimatized, the soldier will retain his adap- 
tation for from one to two weeks after which it decreases at a variable 
rate. Most men lose the major portion of their acclimatization in one 
month - a few, however, are able to retain it for two months. (Appendix 
II, Chart 12) Men in good physical condition retain their acclimatization 
best, provided they remain in training after acclimatization. (Appendix II, 
Chart 12a) Repeated exposures to heat are required at intervals not exceed- 
ing one month, if a high degree cf acclimatization is to be maintained for 
long periods cf time. (Appendix II, Chart 13) 
(15) Drinking of water in amounts equal to the weight (sweat) 
lost during work increases the amount of work which can be done cn first ex- 
posure to heat. The rate and final degree of acclimatization attained, how- 
ever, are not influenced by the water intake (forced, moderately restricted, 
or taken as desired) during the first two or three days of work in the heat, 
provided that after this initial period men are permitted as much water as 
desirea. (Appendix II, Chart 14) 
(16) Suddenly restricting the water intake cf men working in 
trie heat leads to a deterioration of morale and motivation, reduces greatly 
the efficiency with which work is performed, Decreases the total work output 
causes disabling symptoms in many nen and renders others incapable of sus- 
tained purposeful action. This holds for even the well-acclimatized man. 
(Appendix II, Chert 1$) Gradual reduction of waLer intake induces changes 
similar to sudden restriction, differing only in that they are produced more 
slowly. 
3 (17) Acclimatization to hot dry (desert) environments increases 
markedly the ability of men to work efficiently and effectively in hot moist 
(jungle) environmentso (Appendix II, Chart 16) 
L. RECOMMENDATIONS: 
a. Troops brought to a hot desert should when possible be given 
at least a four day period for acclimatization, during which time they should 
be supervised carefully by medical, line and non-commissioned officers. 
b. Graded amounts of work should be done during acclimatization 
with regulated exposure to heat during the midday hours (Appendix I, Schedule 1). 
. 
c. Enough water should be drunk to satisfy thirst at all times. 
If more water is drunk during the first three days than is dictated by thirst 
alone, work Trill be accomplished more efficiently. 
d. Unnecessary exposure to sun should be avoided. It increases 
the water requirement, adds to the danger of heatstroke and may cause serious 
sunburn. 
e. All personnel should be familiarized with the signs and 
symptoms of heat exhaustion and should be instructed in methods of emer- 
gency treatment (Appendix I). 
f. All officers should be made familiar with the water and salt 
needs of their men and be acquainted with the information given in the 
Appendices. 
g. one week before and after entry into a hot desert troops 
should be given adequate rest and alcohol should be prohibited. Men who 
have had recent illness should not be exposed to heat until they have com- 
pletely recovered (and are back in good physical condition). 
Prepared'by: 
Captain William B. Bean 
Lieutenant Ludwig W. Eichna 
Major William F. Ashe 
Captain 5. M. Horvath 
Captain Horton Nelson 
APPROVED 
WILLARD MACHLE 
Lieut. Col.,.Medical Corps 
Command 1 eg 
2 IncIs. 
#1 - Appendix I 
jf2 - Appendix II, with Charts (17) 
4 APPENDIX I 
GENERAL RECOMMENDATIONS FOR SUPERVISION OF 
TROOPS IN HOT DESERTS 
1. Schedule of graded vrork during acclimatization 
Gradually increasing amounts of work should be done during 
the acclimatization period, with limited exposure to heat during the mid- 
day hours. A schedule set up according to the following plan With alter- 
nating rest and work periods is safe for practically all men; it provides 
for work during the cooler morning hours and in the hot afternoon hours. 
Local and regional variations may call for slight modifications. During 
the midday period the men should rest and keep in the shade as much as 
possible. 
Proposed Schedule of Work during Period of Acclimatization 
When Maximum Air Temp, is When Maximum Air Temp. 
90 to 105°F is 105°F and Over 
• Hours of Work 
First Day 0700 - 0900 and 1>00 - 1700 0300 - 1000 and - 1600 
Second Day 0700 - 1000 and ll;30 ■- 1630 0700 - 1000 and 1500 - 1600 
Third Day 0700 - 1000 and IpOO - 1700 0700 - 1000 and IhOO - 1600 
Fourth Day 0700 - 1100 and 1330 - 1750 0700 - 1000 and 1330 - 1630 
Fifth Day Regular Duty 0700 - 1100 and 1330 - 1630 
Sixth Day Regular Duty Regular Duty 
* 
The working period should be divided so that a man works and 
rests in alternating half-hour periods. Two .teams can be arranged to work 
in sequence. The work should equal that of marching with a 20 pound pack at 
the rate of miles per hour. Lighter work may be carried out for longer, 
and heavier work, for shorter times, 
2. Symptoms of Heat Exhaustion. 
Symptoms of heat exhaustion should serve as a warning to 
cease work immediately and lie down - if possible in a shady place. 
Plenty of water should be drunk. These warning symptoms are: flushed 
face, headache, dizziness, irritability, shortness of breath, nausea, 
occasionally vomiting and sometimes abdominal cramps or cramps in the 
muscles. A man in danger of imminent collapse can often be recognized 
by his flushed face, weakness, incoordination or stumbling gait. 
1 3o First-aid Treatment 
When a man has collapsed from the heat, first-aid treatment 
should be started at once by anyone at hand. The following should be done, 
in the order named: 
a0 Put the man in the shade. 
b. Send for a medical officer and,an ambulance. 
c. Remove the man’s clothing and sponge the body v.i,th water, 
fanning vigorously to help evaporation, 
d. ftive cool, salted water to drink, if it can be retained. 
For a day or two before prostration a soldier may appear below 
par and ha"re"a poor appetite. Men showing these early effects of heat should 
be given plenty of water and adequate salt and be relieved of work until they 
have recovered. 
h. Physical Characteristics in Relation to Heat Tolerance, 
For protracted missions or when water is restricted, men 
with the following characteristics will do best; ' 
a. Physical characteristics - men of below average to average 
stature, preferably of the lean, wiry type, having a low ratio of body mass 
to surface area. Large stocky and fat men perform at a much poorer level, 
b. Age - between 20 and 30 years, 
c. Physiologic characteristics - capable of working stren- 
uously in natural or artificial hot environments without great rise in 
body temperature or pulse rate and without complaints. 
d. Physical condition - maximum physical fitness is essential. 
2 APPENDIX II 
A. EXPERIMENTAL CONDITIONS AND I20C^U}0iS 
1. Environment - Air temperature during the day (C800 hours to 
1700 hours) during the night (1200 hours to 0630 hours) 90°F; 
relative humidity between 15$ and. 22$ during the day. One hour was 
required to change from one temperature to the other, Wall and floor 
temperatures were in equilibrium with air temperatures, Ko additional 
radiant heat was supplied and the environment, therefore, v.s not as 
severe as that of a desert with identical temperature and humidity. 
Two sources of heat-gain present in the desert were lacking in this ex- 
periment; (a) the radiant heat of the sun, (b) the heat from the desert 
terrain which reaches 140CF to 160”F, A moderate degree of air move- 
ment was obtained by-cieans of two 26-inch fans or four 10-inch fans0 
The rate of air movement was not measured. 
Since the studios were carried out during five winter months, 
the experimental environment represented an extreme change in environ- 
ment for the subjects. 
2. ixperimental sub-.lects - 56 enlisted men; 4o lived continuously 
in the hot room, 2 lived in barracks, reporting to the Laboratory for 
exposure periods. The age limits of the men were 1? and 43 years, but 
the majority were between 20 and 22 years. 
3. Clothing - Men wore what they chose; during the hot periods, 
only cotton shorts, shoes and socks; during the preliminary cool period, 
regulation fatigue clothing. 
4o Preliminary training - Before- beingQsubjectgd to the hot environ- 
ment ill men worked in cool temperatures (76 F to 76 F)for one week. Dur- 
ing this period their work was the same as that which they v.ere to perform 
later in the heat. This procedure accustomed the men to the work and ex- 
perimental procedures, and produced a mere uniform state of physical fitness 
in all men. 
5* Activity in the hot environment - For test purposes the men were 
divided into three groups according to activity: 
Group I. Hosting. 
Trie men in the resting group were permitted to rest for 3 
or four days in the heat before beginning work. 
1 Group II. Riding bicycle. 
The men in the bicycling group performed strenuous work 
for short periods; pedalling a stationary bicycle for ten minutes each 
hour, five times a day. 
Gr oup III.. W alking, 
The walking group was the largest of the three. These 
men performed work of moderate severity for long duretion, namely, walk- 
ing at ix standard array pace while carrying a 20 pound pack. A walk of 
2-1/2 miles in 47 to 50 minutes constituted a "work period". A rest of 
10 to 13 minutes was given between successive work periods. Unless 
disabled, the men walked two successive periods in the morning and three 
in the afternoon, walking a total of 12-1/2 miles a day. The data and 
conclusions of this report are based for the most part on observations 
made upon men performing this type of work. Certain exceptions are men- 
tioned in the text. 
The eight men who lived in their barracks were subjected 
to the hot environment for 4 hours in the morning, returning to their 
charters after each exposure. During each exposure to heat they walked 
for two work periods, separated by a rest period of one hour. All eight 
men had their initial exposures on the same day. They were then divided 
into pairs for re-exposure. At intervals of three days a new pair was 
re-exposed to the heat. Once a man haa received his second exposure to 
the heat he returned to the net room every third day until acclimatized. 
6, Food - Regular array fare obtained from, their regular mess, No 
record was made of the type or amount of food eaten. 
7. ?tater - Salt was added to the drinking water (final concentra- 
tion The water intake was measured and was administered according 
to one of three methods; 
. a. As much as aesired, whenever wanted. 
b, Intake regulated to equal trie total weight lost, 
c. Restricted to 4 litres, approximately one-half of the needed 
intake, and given in either of two schedules; (l) 270 ml every hour from 
6:00 A.I. to 6:00 plus 750 ml from 6:00 P.K. to 6:00 A. 11. (2) 750 
ml at 6:00 A.I . 1250 ml with noon meal, 1250 ml with evening meal and 750 
ml from 6:00 F.l , to 6:00 A.1.4• 
8. Sleep - Eight to nine hours a night. A few men had difficulty 
sleeping on the first night of an experiment; most slept well throughout. 
9. Observations made each morning on awakening - rectal temperature 
pulse and respiratory rates, weight (+ l/4 pound), arid measurement of the 
urine voided in preceding 24 hours. 
2 10, Observations made during the work periods: 
a. General appearance - noted continuously and records kept 
of vigor, flushing of the face, sweating, headache, and complaints of 
"astro-enteric or cardiovascular disturbances. 
b. Body temperature - rectal temperatures were taken at the 
beginning ana end cf each work period. 
c. Heart rate - at the beginning and end of eaca work period 
with the subject in both the supine and erect position (3 minutes in 
each) and at 15 minute intervals during the v/alking period (subject 
marking time). Ausculation over the precordium was necessary to deter- 
mine the more rapid rates. 
d. Blood pressure - at the beginning and end of each work 
period with the subject both supine and erect (3 minutes in each po- 
sition) , Change of posture was obtained by means* of a tilt table and 
also by voluntary movement by the subject. 
e, Weight - the weight within 5 grams was recorded at the 
beginning ana end of the two morning and three afternoon work periods. 
Subjects were naked and the sweat dried off. 
The water intake and urine output during each work period 
and during each 24-hour period. 
11. Special observations made at intervals throughout the studies 
included: 
a. Basal metabolic rate (Sanborn) 
b. Slectrocardiogrrm 
£. Vital Capacity 
* d, Cocle aptitude tests 
e. Attention tests 
3 p. CO?:F ARISON OF ACCLIAVTIZZD AND UNaCGLIi. ATIZED STATES 
V.ork in hot environments is at first difficult or Inpossible for 
most men. By a process of acclimatization, however, man adapts himself 
to work in the heat. He then works without subjective complaints and 
with little or no disturbance of bodily functions. Acclimatization to 
heat appears to be a complex physiologic readjustment which cannot be 
adequately defined or completely determined by a few simple physiologic 
measurements. Nevertheless, this adaptation is accompanied by certain 
physiologic changes which serve as general indices of the whole process. 
In this report some of these physiologic changes are discussed and re- 
presented by a series of charts. 
All charts are similarly plotted (see Chart l). Along the ordinates 
heart rate, rectal temperature and blood pressure are charted in turn. 
The total height of the column indicating blood pressure represents the 
systolic pressure; the white portion of this column (limited by the 
transverse line) indicates the diastolic pressure; the solid or hatched 
portion of the column gives the pulse pressure. Along the abscissae 
are indicated the day of work, the environment in which the work was 
performed and the period of work at the end of which the plotted data 
were obtained. For example, in Chart 1 the first pair of columns re- 
presents the data obtained in the erect and supine positions at the end 
of the fifth work period on the day in the cool environment; the 
next pair of columns, the data at the end of the second work period on 
the first day in the hot environment. Then, in turn, the data at the 
end of the third work period on the second day in the heat; at the end 
of the fourth work period on the third day in the heat and so on, A 
key with eacn chart interprets the hatching of the columns. The text 
associated with each chart indicates whether the data were obtained from 
single observations or averages, 
» 
I, Ihysioloric changes (Charts 1, 2}- 
The acclimatized man works in the heat with a lower pulse rate, 
a lower body temperature and a more stable blood pressure after change ir 
posture than when not acclimatized (Chart l). Compare, for example, per- 
formance on the fifth and first days in the heat0 In the unacclimatized 
state (first day in heat) change of posture causes marked alterations in 
cardiovascular dynamics, as indicated by pulse rate and blood pressure. 
In the erect posture the heart rate is .markedly accentuated, the systolic 
blood pressure falls and the pulse pressure narrows. In Chart 1 compare 
the solid black (erect) and hatched (supine) columns. As a result .of the 
lowered blood pressure the cerebral circulation at times becomes inade- 
quate, symptoms of cerebral hypoxia arise and even syncope may. ensue„ 
Lying down promptly slows the heart rata, restores the blood pressure 
and disoels the symptoms of cerebral ischemia. 
4 ERECT 
SUPINE 
SUBJECT - SCH. 
Chart - I With continued exposure to heat, acclimatization progresses 
and the heart rate, rectal temperature and blood pressure return to 
levels approximating those obtained after similar work in cool environ- 
ments. This is true with the subject erect or supine (Chart l). 
Although observations were made with the subjects both erect and 
supine, in this report acclimatization was evaluated for the subjects 
chiefly in the erect posture and all charts (unless otherwise indicated) 
refer to svch measurements. Three facts led to this decision; (a) the 
findings in the erect posture paralleled those in the supine, (b) the 
erect posture places an added strain on the physiologic functions of 
man, revealing disturbances not apparent in the supine position, (c) 
a useful man is a working man; work usually requires the upright pos- 
ture. 
Although a low heart rate, a low rectal temperature and a stable 
blood pressure generally accompany acclimatisation, one cannot define 
the process nor detect differences in the degree of acclimatization be- 
tween individuals by such simple measurements alone. This is illus- 
trated by the data for four subjects plotted in Chart 2. On the fourth 
day of exposure to heat all four men successfully completed five work 
periods at which tine subject Bel, had the most rapid heart rate and 
the highest rectal temperature. This might be taken as an indication 
of incomplete acclimatization and evidence that Bel. was not as capable 
of work in the heat as the other three men. His general appearance and 
behavior, however, indicated that he was more fit than Hanio or Gee., 
both of whom had lower heart rates and body temperatures. On the 
following (5th) day, the men worked under more severe conditions. Sub- 
ject Ham. became weak, nauseated and vomited after the second work per- 
iod and could not continue. Subject Gee. was forced to stop at the end 
of the third work period because of exhaustion. Subject Bel. with a 
heart rate and rectal temperature which were always higher than those of 
Ham. or Gee., continued energetically for another work period and on 
finishing appeared almost as fit as Lan, Thus, prediction of performance 
on the basis of heart rate and rectal temperature alone did not agree 
with the actual performance of these men. 
Undoubtedly a loan, doing a given amount of work, is less efficient 
and more prone to disability when his rectal temperature and pulse rate 
are high, than when they are low. Individual performance is influenced 
by many variables which are not evaluated by such simple measurements. 
It is necessary to consider and evaluate each man as a whole and to avoid 
focussing attention on the rectal temperature or heart rate. The man's 
subjective symptoms, his objective appearance, his behavior and his actual 
performance must receive at least equally careful consideration in any 
evaluation of his capacity to work in the heat. 
t; SUBJECT LAN 
SUBJECT GEE. 
Chart-2 
SUBJECT HAM 
SUBJECT BEL 2. Symptoms and signs - 
The acclimatized man is alert, performs his work energetically 
and without symptoms. Usually his heart rate and rectal temperature 
are low, at least not markedly elevated. On the other hand the unaccli- 
matized man working in the heat becomes dull and apathetic, performs his 
work poorly, has a rapid heart rate and a high rectal temperature and 
may manifest to varying degrees and either singly or in combinations, 
the symptoms and signs of heat exhaustion. In the present experiments 
these symptoms and signs appeared in the following order of frequency; 
Symptoms (l) fatigue (2) headache (3) dizziness, especially when erect 
(u) shortness of breath (5) loss of appetite (6; nausea (?) vomiting 
(0) abdominal cramps; Signs (l) flushing of face and neck (2) rapid pulse 
rate (140 - 200/nin) (3; lack of coordinated effort (clumsy, stumbling) 
(4) staring glazed eyes (5) mental disturbances (apathy, poor judgment, 
irritability) (6) fever over 102°F (?) collapse. 
Of interest is the marked flushing of the face, neck and upper 
chest which occurs in most men when they first work in the heat and 
which disappears as acclimatization develops. 
6 C. FACTORS IN ATTAINING AND LA I NT A HUNG 
ACCLIMATIZATION TO IL^AT 
As pointed out above, when different individuals are compared, 
the heart rate, rectal temperature and blood pressure are not in them- 
selves completely reliable determinants of acclimatization to hecit. 
Nevertheless, they may be utilized as indices of acclimatization when 
they are consistent with the ouher above-discussed evidences of accli- 
matization. It vies with these limitations in mind that the heart rate, 
rectal temperature and blood pressures were used as indices in this 
study. The factors involved in attaining and maintaining acclimatizatior 
to heat are presented in a series of charts and it is to be understood 
that the plotted changes in rectal temperature, pulse rate and blood 
pressure were consistent with the picture of the man as a whole, when 
they ’.'fere not, specific mention of the differences are wade. 
1, Course of acclimatization (Charts 1, 3) 
The process of acclimatization appears to be initiated by the 
first exposure to heat. This is indicated by Chart 1, in which are 
plotted the observations made on one man at the close of'the last work 
period of each charted day. Considerable improvement in heart rate, 
rectal temperature and blood pressure (in the erect posture) is apparent 
on the second day in the heat, that is, after one day of previous ex- 
posure. 
At the close of the last (second) work period on the first day 
in the heat this man was very tired, giddy and unable to stand erect. 
On the second day he finished three work periods feeling much better 
than on the previous day, had no difficulty in standing and maintained 
a normal blood pressure in the erect posture. 
It most men, a major portion of the acclimatization to heat is 
attained by the fourth or fifth day of work in a continuously hot environ- 
ment, This is illustrated in Chart 3 which shows the progressive changes 
recorded in two groups of men exposed to heat at different times, :Sach 
column represents the average of the data obtained on four men at the 
end of the last work period of the day During the first three 
to four days there is a progressive and rapid improvement in heart rate 
and rectal temperature, which thereafter levels off at values somewhat 
higher than those obtained under similar circumstances in the cod environ-, 
merit. 
7 ■ -GROUP-A 
0-GROUP-B 
AVERAGE OF 4 MEN (EACH GROUP) 
.-OW CHART- 3 2. Physical condition (Chart 4) 
Although exceptions are not unusual in individuals, men in 
good physical condition generally acclimatize to heat more rapidly 
than men in poor condition. Moreover, the more fit men are capable 
of a greater work-output in the heat vdth less symptoms and less dis- 
turbance of their heart rates, rectal temperatures and blood pressures 
than are the less fit men. In this study the determination of physical 
fitness was based on the work performance of the men while in a cool 
environment. In evaluating physical fitness all of the factors pre- 
viously discussed (the appearance and behavior of the man as well as th 
results of physiologic measurements) were taken into consideration. 
Those men were considered most fit who performed the prescribed work 
easily and energetically, without symptoms and with least disturbance 
of their heart rate, blood pressure and rectal temperature. 
The three men whose records are compared were from a group of 
eight men whose physical fitness was assessed before they entered the 
hot environment. Three different observers gave independent ratings. 
All three observers placed subject Sch. first. Two observers placed 
subject Kit. seventh, one observer sixth. Two observers pi .iced subject 
Lup, fifth, one observer second. Of the three men, Sch. was considered 
most fit. Kit, least fit, and Lup, intermediate between the two. 
Observations on each of these three men were obtained at the 
end of the last work period of each of six days in the hot environment 
and, for comparison, the same measurements taken on the last day in the 
cool environment are also shown in Chart 4. Tne more rapid improvement 
in the pulse rate and rectal temperature of subject Sch. is readily 
apparent. An equally rapid improvement was also apparent in the general 
appearance and behavior of this subject. By the second day he was walk- 
ing easily and with vigor. Note also the maintenance of blood pressure 
when erect. In contrast to Sch., the pulse rates, rectal temperatures 
end blood pressures of subjects Kit, and Lup. returned more slowly to- 
ward control levelso This was confirmed by the appearance of the men, 
especially Kit., who always seemed to be working with difficulty. Even 
after acclimatization had been attained by all three men the performance 
of Sch. was superior to that of the other two men (see 9th day in hot 
environment). On, the fifth hot day subject Kit. was prevented from walk- 
ing by blisters on the feet. 
e SUBJECT SCH. 
SUBJECT LUR 
Chart -4 
SUBJECT KIT. 3» Activity prior to and during acclimatization (Charts 5, 6) 
Continuing the.preliminary training period in a cool environ- 
ment beyond that required to develop a satisfactory state of physical 
fitness does not increase the subject's tolerance to heat on first 
exposureo Resting in the heat for the first three days produces a 
definite tolerance to work in heat but the acclimatization thereby 
induced is only partialo Acclimatization is developed most rapidly 
by the daily performance of work in heat from the outset, the amount 
of work being progressively increased within the tolerance of the 
individualo 
The relative effect of these three factors upon the process of 
acclimatization was determined, using twelve subjects divided into three 
equal groups. After the same preliminary training for all men, one group 
(A) continued to work in the temperate environment0 The other two groups 
were taken into the hot environment and of these, Croup B rested while the 
other (C) immediately undertook graded work which was progressively in- 
creased,* When Croup C was acclimatized (Ath day) all three groups were 
subjected to the regular work schedule in the hot environment,* 
The comparative behavior of the three groups is striking. The 
subjects in Croup C (with graded work in heat during the previous 3 days) 
had returned substantially to the normal physical state with only slight 
elevation of heart rate and temperature over the control levels„ In 
contrast, the subjects in Croup A, working in heat for the first time 
after extended preliminary training, gave no evidence of acclimatization, 
as.shown by the high heart rate and body temperature. Only two of the 
four men completed the prescribed work and the condition of these two 
was considerably below that of the other men in the other groups„ 
The performance of the four men in Group B (resting in heat 
during the previous 3 days) fell between the other two groups0 All four 
men completed the five work periods and, although the heart rate and 
body temperature were higher as compared with their control state, and 
also in comparison with Group C, the performance of these men was con- 
sistently better than that of the men in Croup ka Thus, a partial state 
of acclimatization had been induced. 
9 WORK IN COLD- GROUP A 
REST IN HEAT — GROUP B 
WORK IN HEAT- GROUP G 
AVERAGES FOR FOUR MEN, EACH GROUP 
(SUBJECTS STANDING) 
CHART -5 AVERAGES OF 4MEN, EACH GROUP 
WORK IN COLO 
REST IN HEAT 
WORK IN HEAT 
(SUBJECTS LYING) 
CHART-6 4. Strenuous work from first exposure to heat (Chart 7) - 
Strenuous work on first exposure to heat is not well tolerated 
Twelve (±2) unacclimatized men were asked to perform the full five work 
periods (12.5 miles) on their first day in the hot environment. Four 
men became exhausted (after tne third or fourth period) and were unable 
to complete the task. Tne eight who completed the work did so with 
difficulty, finishing in poor shape and with high heart rates and 
rectal teaperatures. The ability to coaplete strenuous work in the 
first exposure to heat does not necessarily indicate acclimatization 
nor the ability to continue to work in the heat. Maintaining work at 
a strenuous rate leads to progressive deterioration of performance. 
After two or three days many men become disabled and those who continue 
to work do so ineffectively and inefficiently. This is in contrast to 
the progressive improvement of men subjected to a schedule of gradually 
increasing work in heat. The performance of one of the subjects (Suo0 
Gee.) illustrates this point. 
The data were obtained at the end of each work period of the 
last day in the cool environment and of each day in the heat. During 
the first day in the heat the man completed five work periods without 
much difficulty. On the second day in the heat, however, he completed 
only three work periods. On the third day he was forced to stop in the 
middle of the first period. It is of interest to note the low blood 
pressure on this day despite the more nearly normal rectal temperature 
and heart rate as compared with the fifth period on the first day in , 
the heat at which time he was still in fair shape. After dropping out 
in the first period of the third day, this man rested the remainder of 
that day and drank plenty of water. On the next (4th) day he finished 
five work periods in good condition and with a heart rate and rectal 
temperature approximating those recorded after similar work in the cool 
environment. Despite the exhaustion resulting from too strenuous work 
during the first three days in the heat, this man had attained a large 
degree of acclimatization by the fourth day. 
10 SUBJECT GEE 
CHART-7 5. Intolerance to heat and heat exhaustion (Charts 8) 
Development of symptoms of intolerance to heat and even heat 
exhaustion during the early days of exposure to heat do not retard the 
rate nor decrease the degree of acclimatization finally attained, 
provided that when such disability occurs work is discontinued, rest 
is permitted and water and salt are given, V/hen work is resumed it 
should be within the capacity of the individual. 
Chart 7> which illustrates this, has already been discussed. 
Chart 8 represents a similar, but more severe, situation in another 
subject. The data obtained at the end of each work period of each day 
in the heat are plotted and compared with the observations on the last 
day in the cool environment. On the first day in the heat this man 
could complete but four work periods when fatigue forced him to discon- 
tinue, On the second day he finished only two work periods; on the third 
day only one. Note the high heart rates and rectal temperatures reached 
on these days and the progressively decreasing blood pressure (Chart 8). 
The appearance and behavior of the subject indicated a parallel deteri- 
oration. On the third day this subject was quite ill; exhaustion, abdominal 
cramps, nausea, vomiting and marked apathy indicating heat exhaustion. 
After a litre of physiologic salt solution was administered intravenously 
the nausea, vomiting and abdominal cramps ceased. He rested for the re~ 
mainder of that day and drank salted (0,1$) water copiously. On the next 
(4th) day and the days thereafter he completed five work periods without 
difficulty, always finishing strongly and appearing to be acclimatized. 
This improvement was accompanied by a reduction of the heart rate to 
values equalling those obtained in the cool environment but the rectal 
temperature continued to rise to high levels (102°F.). 
11 SUBJECT SMI. 
CHART- 8 6. Single exposures to heat at intervals of three days (Chart 9) 
The following conclusions are based on data derived from eight 
subjects who performed two periods of work during a four hour exposure 
to heat every third day and who .spent the intervening time in a cool 
environment: (l) A single relatively short period of work in the heat 
induces a little or no acclimatization, (2) a number of such exposures 
separated by two days in the cool environment results in acclimatization 
(3) the major portion of the acclimatization for the above work re- 
quirements is produced by three or four such exposures to heat. 
Representative observations on subject Mel,, taken at the end 
of each period during the four-hour exposures to heat are shown in 
Chart 9. During each exposure two periods of work were performed, 
separated by one hour of rest. The first exposure to heat was badly 
tolerated and caused weakness, nausea, vomiting and syncope when in 
the erect posture„ The second exposure produced similar but less severe 
symptoms and vomiting was absent. Thereafter work in the heat was 
accomplished without difficulty and with increasing ease. The associa- 
ted changes induced in the heart rate, rectal temperature and blood pressure 
and their regression paralleled the findings already described for subjects 
continuously exposed to the hot environment. (Compere Chart 9 with Charts 
1 and 3) In this subject there was a particularly striking postural 
hypotension during the first three exposures to heat with a return of 
the blood pressure to normal as acclimatization developed. 
12 SUBJECT MEL 
CHART-9 7. Short periods of severe exertion (Chart 10) 
The behavior of subjects performing severe work of short 
duration in the heat follows a pattern of change and readjustment 
similar to that for less severe work of long duration (marching). 
The severe exertion consisted of pedalling a stationary bicycle for 
ten minutes each hour. The plotted observations are for one sub- 
ject taken at the end of the first '’ride" of each day. 
The high rectal temperatures and pulse rates produced by 
this exertion become successively less marked as work in the heat 
is continued. Levelling off is attained by the fourth or fifth day. 
In one respect the readjustment differs from that observed for the 
more moderate work of marching. The resting level, rather than the 
increase cause by work, determines the final level of the pulse rate 
and body temperature. Most subjects found these short bouts of severe 
exertion less fatiguing than the prolonged but moderate work of march- 
ing. 
13 EFFECT OF TEN MINUTES OF BICYCLING IN A TRAINED SUBJECT 
CHART—10 8. Rest at nlftht (Chart 11) - 
Adequate rest at night is essential for good work perform- 
ance in the heat, even in acclimatized men. Deprived of it, men 
work inefficiently the next day. 
The data obtained on subject Mel, after the last (second) 
work period of each day are given. This subject belonged to that 
group which performed two periods of work during a four hour exposure 
to heat every third day. The night before his fifth exposure he 
failed to obtain adequate rest. His performance during the next 
day was almost as poor as that on the first exposure to heat and 
he completed the work with difficulty. Here again the true state of 
the subject is not indicated by the heart rate and rectal temperature. 
During the fifth exposure they are higher than during the first and . 
second, yet the subject* was in a better condition. There was no head- 
ache, nausea, vomiting or syncope in the erect posture, occurrences 
which had rendered him totally incapable of further effort on the first 
day in the heat. 
The poor performance during the fifth exposure to heat did not 
retard further improvement. Note the much improved performance during 
the sixth exposure. 
14 CHART-1 i 9. Duration of acclimatization (Chart 12) 
Acclimatization to desert heat after removal from the hot 
environment is well retained for at least one week and probably for 
two weeks. Thereafter, a variable but more rapid loss ensues so that 
after one month the major portion of the acclimatization is lost by 
most men. Some men, however, retained a considerable degree of accli- 
matization for two months after leaving the hot environment. 
The plotted data were obtained on seven men at the close of 
the last work period on their first re-exposure to heat (solid column) 
These are compared with the observations made at the end of the last 
work period, on the last day in the cool environment (diagonal-lined 
column), with the first day in the hot environment (cross hatched 
column), and when fully acclimatized (open-block column). The obser- 
vations for each individual .are grouped together and separated by long 
vertical lines, "Interval” indicates the time between leaving the hot 
environment and first re-exposure to it. 
* ' The sharp loss in acclimatization after three weeks was in- 
dicated not only by the high heart rates and rectal temperatures (Sko, 
Foe,, Ben,) but also by the failure to complete as many work periods 
and by the poor appearance of the subject. The vigorous and alert 
appearance of subjects Lup., Min., and Mus,, and the ease with which 
they completed their prescribed work indicated a high degree of accli- 
matization despite their higher heart rates and rectal temperatures. 
Subject Sch, worked as well in the heat after a lapse of three weeks 
as when fully acclimatized. 
15 LAST COOL 
FIRST HOT 
ACCLIMATIZED HOT 
urrr actco imtcd\/ai 
CHART—12 10c Maintenance of acclimatization (Charts 12A, 13) 
There are two requisites for the maintenance of a high 
degree of acclimatization to heat over a long period of time; a„, 
the maintenance of good physical fitness and b,,' repeated exposures 
to heat, preferably with work, at intervals of one month or less. 
Of the three subjects considered in Chart 12A, subject S 
was never in good physical condition, subject M remained moderately 
fit and subject W only fairly so. The strikingly poor performance 
of subject S when he walked in the heat after a lapse of 37 days 
indicated that he had not only lost all of his acclimatization but 
was in far worse condition than at any previous time0 This was 
attributed to loss of fitness as well as of acclimatization in the 
intervale 
The more fit subject (M) had a much better work performance 
than the less fit subject (W) when re-exposed to heat after a lapse 
of 16 days and 37 days* This was indicated not only by the lower heart 
rate and rectal temperature of subject M but also by the fact that of 
the three men re-exposed to heat after a lapse of 37 days he was the 
only one able to complete the full five work periods0 
The need for re-exposure to heat in order to maintain accli- 
matisation is indicated in Chart 13* Here are plotted the observations 
made on one subject at the close of each work period on the last (5th) 
day in the cool environment, on the first four days in the hot environ- 
ment, and on re-exposure to heat 44 and again 47 days after leaving the 
hot environment. Initial acclimatization to heat was rapid and by the 
third day, five work periods were performed without difficulty and with 
heart rate, rectal temperature and blood pressure approximating those 
for the cool environment. On the first re-exposure to heat 44 days 
after leaving the hot environment, the subject was unable to continue 
after the fourth work periodQ At that time he was almost exhausted, 
the heart rate was more rapid than at any other time and the rectal 
temperature was high. But this exposure re-induced a great deal of 
acclimatizationo During another exposure to heat three days later he 
was able to work as long and almost as efficiently as he had done when 
well acclimatized to heat 47 days previously0 
16 SUBJECT W 
SUBJECT M 
SUBJECT S 
chart—I2A SUBJECT FOE 
CHART—13 11. Increased water intake (Chart 14) 
Thirst is an inadequate guide to the fluid required for 
work in the heat. No man drank enough water voluntarily to replace 
that lost in the sweat while working and all developed water deficits. 
Increasing the water intake during work to an amount (1200 ml per 
hour) equal to the water lost by sweating increased the amount of 
work which was accomplished on the first exposure to heat. 
Twelve men were asked to work the full five periods during 
their first day in the hot room. Nine men received water in amounts 
sufficient to quench their thirst(600 ml per hour), 3 received water 
in amounts (1200 ml per hour) equal to the weight (sweat) lost. These 
three men all finished the -five work periods without great difficulty. 
In contrast, four of the other nine men became exhausted after three 
or Tour work periods and could not continue. Those who did finish 
were in poorer condition than the men whose water intake was intention- 
ally increased. 
In chart 14 one may compare the effects of slight water 
restriction (6 litres per day), and of full water replacement (9 
litres per day), on two groups of three men each, in their first and 
fourth days in the hot environment. Sach column represents the averaged 
data for the group. Observations were made at the close of each of the 
five work periods for the last day in the cool environment and the first 
day in the hot environment„ Observations are also recorded for the 
fourth day of heat exposure when both groups were permitted to drink 
as much water as they chose. Men who did not complete five full periods 
have been excluded. 
Although the group in which water was forced to full replace- 
ment showed smaller disturbances of vigor, behavior, pulse rate and 
rectal temperature during their first day in the heat, the degree of 
acclimatization attained by both groups on the fourth day was the seme. 
17 AVERAGE GROUP A (3 MEN) 
AVERAGE GROUP B (3 MEN) 
GROUP A ~ 600CC/HR. 
GROUP B — I200CC/HR. 
CHART—14 12, Water restriction (Chart 15) 
* j 
Sudden restriction of the water intake of well-acclimatized 
men at work to one-half of the optimal requirement induces changes 
similar to those which appeared in the men on first exposure to heat 
when they were unacclimatized. 
In Chart 15 are plotted observations made on each of four 
well-acclimatized men. Their performance at the close of each work 
period on the day (9th) when water was restricted to 4 litres per day 
is compared with that following the fifth work period on a day (4th) 
when they received as much water as they desired. Subject Nor. was 
incapable of continuing after the third period and the other men 
finished five periods with difficulty. Note the higher pulse rates 
and rectal temperatures and the low blood pressures. 
Important changes-which the chart does not show is the con- 
dition of the men, their low morale and lack of vigor, their glassy 
eyes, their apathetic, torpid appearance, their "don’t-give-a-damn- 
for-anything” attitude, their uncoordinated stumbling, shuffling gait. 
Some were incapable of sustained purposeful action and were not fit 
for work, let alone battle. All they wanted to do was rest and drink. 
Progressive restriction of water was tolerated better than 
sudden restriction. For sudden restriction the intake was reduced on 
1 day to 4 litres, Progressive restriction was carried out by limiting 
the intake from the optimum level of 8 L/day to 6 L for the first day, 5 
L for the second and third day and to 4 4 for the fourth day. The 
gradual restriction of water intake resulted in physiologic disturbances 
similar but less severe than those observed from sudden water restric- 
tion, Men were incapable of performing as much work as when water intake 
was adequate. 
18 SUBJECT 60N. 
SUBJECT NOR. 
SUBJECT FOE 
SUBJECT MIN. 
WATER RESTRICTED 4L/ DAY 
Chart— is 13. Cross acclimatization to .jungle heat (Chart 16) 
Acclimatization to dry (desert) heat increases markedly the 
ability of men to work efficiently and effectively in hot moist (jungle) 
environment. 
Three men were fully acclimatized to desert heat and six men 
were trained to work in a cool environment. All nine men then worked 
in a simulated jungle environment; dry bulb 90°F to 91°F, wet bulb 88°F 
to 89°F, relative humidity 90% to 96%. The averages of the data obtained 
on each group of men at the close of the last work period in the cool 
environment,and at the end of each work period of the first day ir\ the 
hot moist (jungle) environment are compared. The performance in desert 
heat, (as expressed by average data for the desert group) is plotted between 
the heavy vertical lines. The first column represents observations made 
at the close of the fifth work period of the first day in the heat, the 
second column, the data from the fifth work period of the third day, when 
acclimatized. 
On the first day in hot moist (jungle) heat five work periods 
were completed by each of the desert-acclimatized men; two men finishing 
strongly and easily and the third with some difficulty. Of the six men 
not previously exposed to a hot environment, four were able to complete 
only two periods of work, the first and third, while the other two com- 
pleted three, the first, third, and fourth. Hot only were the desert- 
acclimatized men capable of a greater work output but tie work was per- 
formed mere efficiently than was the smaller amount of work done by the 
other men. 
The performance of the desert-acclimatized men, however, was 
poorer than it had been in the cool and desert environments. 
19 DESERT ACCLIMATIZED AVERAGE OF 3 MEN 
UN ACCLIMATIZED ~ AVERAGE OF 6 MEN 
CHART—16 14. Results of special tests 
a. Basal metabolic rates did not change significantly 
during the process of acclimatization. 
b. Electrocardiographic changes associated with work and 
change in position were the same with the subjects in the heat as 
they were when the subjects were in normal temperatures. 
£. Vital capacity was not affected by acclimatization 
d. Code aptitude tests and attention tests showed a decrease 
in scores on the first day in the heat, and a gradual return to the con 
trol levels during the next three days. The initial decreases amounted 
to about 15%, All men were affected by an obvious depression in the 
heat for the first day and some instances for three days. 
20