Physiological responses to high radiant heat exposure

ENVIRONMENTAL

RESEARCH

28,96-105

Physiological

(1982)

Responses to High Radiant Heat Exposure

ELIEZER KAMON, KAREN SOTO, AND JON BENSON No11 Laboratory

for

Human

Performance Research, The Pennsylvania University Park, Pennsylvania 16802

State

University,

Received July 27, 1981 The physiological responses to four levels of radiant heat (R) in combination with two work loads and three ambient humidity levels were studied on seven clothed young men. The globe temperature (t3 ranged from 40 to 74°C; metabolic work load (M) was either 20 or 50% of maximal aerobic capacity (vi, max); ambient vapor pressure was either 13 or 23 mm Hg; and dry-bulb temperature (t,,,,)was 38 or 49°C. The criteria for heat strain were the changes in rectal temperature (T,& mean skin temperature (TJ, heart rate (HR), and sweating (SW). Stress was defined by the calculated heat load requiring dissipation (A4 + R + C = E,,,,), the ambient evaporative capacity (E,.&), and the skin wettedness (w), defined as the ratio of E,,IE,,. The progressive increase in R resulted in a concomitant rise of T,, and HR reflecting the physiological strain. Similarly the increase in either M or in the humidity resulted in higher T,, and HR. The changes in R or E,,, were best defined by the W, thus w and the physiological responses were highly correlated. For practical application a multiple regression of the increments of HR(AHR) on t,, above neutral (25°C) and on t, above tdbwas derived as follows: ~~!XXll = 0.96(t,, - 25°C) + 0.8l(t, - tdb)- 1. It was concluded that the calculated skin wettedness is most suitable in the evaluation of heat stress.

INTRODUCTION The ambient “prescriptive zone” refers to a combination of temperatures and humidities under which an exposed worker can consistently sustain equilibrium in his physiological responses (3, 15, 19, 20). These responses are measured as core body temperature (T3 and heart rates (HR). Ambient conditions beyond the prescriptive zone indicate excessive heat loads which cannot be dissipated. The heat requiring active dissipation (E,,) is derived as the sum of metabolic (M), radiative (R), and convective (C) loads. Beyond the upper limit of the prescriptive zone, heat is stored in the body causing a rise in T, and increments in HR. These conditions can occur under two circumstances: (1) when, relative to the E,,,, the capacity of the environment for evaporation (E,,J is inadequate; and (2) when the total sweat production (SW) is less than E,,, (9). The above-mentioned circumstances were indeed used in the heat stress index (HSI), where the limits were either (1): (1) the excessive E,, relative to E,,,, or (2) the expected maximal average sweating of 1 liter hr-‘. Unfortunately, there is some difficulty in comparing HSI to the original prescriptive zone (1,5,7, 10, 18, 24), because while the first includes the total heat balance, the second refers to the 96 0013-9351/82/0300%-10$02.00/O Copyright 0 1982 by Academic Press, Inc. AU rights of reproduction in any form reserved.

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RADIANT

97

HEAT

ambient conditions (corrected effective temperature, CET). This lack of consideration for the total heat load was not resolved in the NIOSH interpretation of the threshold of heat exposure using the WBGT criteria (21, 25). Indeed, this was somewhat corrected later when a more complicated threshold limit for different M and air movements (v) was suggested (23). A thorough examination of the limit of exposure on the basis of the heat balance was reported by Belding and Kamon (2) and by Kamon and his colleagues (15). These observations indeed showed that T, and HR are a function of the inequilibrium due either to low E,,, or to E,,, which is above SW. Therefore, the use of the heat balance as an indicator of strain should be favored over other indices. This was not feasible until recently because of the need for a tedious calculation of R, C, and E,,, from the measured temperatures of dry bulb (fdb), wet bulb (tw,,), and air movement (v). However, hand-held programmable calculators can be useful in an on-site immediate evaluation of the total heat balances either for one local area or even for a quick-time weighted average for different locations (16). Although the basics of the heat balance and its relevancy to the design of work under heat stress were shown in many studies (1,2,7,8, 15, 17,20,24), there was some missing information because most of these studies failed to include extensive radiant heat as is expected in industry. This study was undertaken in order to expose subjects to work under heat stress, including radiant heat, similar to that expected in industrial situations. Indeed, the results showed that the heat balance is reflected in the rise in T, and the increments in HR, due either to inadequate E max or to insufficient sweating capacity. METHODS Subjects. Seven male college students volunteered to participate in this study and were examined by a physician who also stress-tested them for qualification. Their physical characteristics are summarized in Table 1. The extensive nature of the tests required that we divide them into two groups. One group (n = 3) participated mainly in the experiments requiring high energy expenditure (M). The setond group (n = 4) participated in the tests where R was changed frequently. Treadmill. The work was performed on a treadmill. This included the stress test where a progressive increases in the grade was used until exhaustion in order to obtain the maximal aerobic capacity (vjo2 max) for each participant (Table 1). TABLE AGE,

PHYSICAL

CHARACTERISTICS,

AND TESTS

1, 4, 6, 7, 11, 12, 13, 14, 15 1, 2, 3, 5, 7, 8, 9, 10 “ Number

refers

IN WHICH

1 CAPACITY

THEY

(P,,,max)

OF THE

SUBJECTS

AND

THE

PARTICIPATED

Height

Weight

n

(yr)

(cm)

(kg)

W)

3

20 + 2

180 + 2

77 + 17

1.95 2 0.18

3.42 k 0.34

4

23 + 3

183 2 8

77 2 9

1.98 ‘- 0.15

3.95 2 0.27

Age

Tests”

AEROBIC

to list in Table

2.

Surface

area

riotmax (liters.

min-‘)

98

KAMON,

SOTO,

AND

BENSON

The experimental workload was adjusted as follows: the speed and grade were individually adjusted to obtain 20% vi,, max for low M, and 50% F’,,* max for high M (Table 2). Environmental conditions. The combination oft db, twb, vapor pressure (PJ, and t, (globe temperature) are summarized in Table 2. Basically, two Id,, (38 and 49°C) and two vapor pressure (13 and 23 mm Hg) combinations were used. In contrast, four levels of radiant heat (R) were employed as is indicated by the t, (Table 2), R, and WBGT (Table 3). The radiant heat was obtained from infrared lamps placed in front of and behind the subject. Air movement was kept constant at 1 rn. see-‘. Clothing. The subjects wore shorts, cotton trousers, long-sleeve cotton shirts, and tennis shoes. A face shield was used for exposures to high radiant heat, Measurement. Rectal temperature (T,,) was measured with a Yellow Spring Thermistor inserted 10 cm beyond the anal sphincter. Mean skin temperature @,,J was calculated by averaging six temperature sites-at the forehead, arm, chest, back, and two thighs. Uncovered copper-constantan thermocouples were attached to the skin and connected to a digital thermocouple readout device. Heart rates were recorded on an electrocardiogram and were monitored, along with the temperature readings, every 5 min. Oxygen uptake (v,,J was determined by the open-circuit method. Expired gas was collected in a Douglas bag at 35 min into each test for 2 min. The volume and TABLE 2 DRY BULB TEMPERATURE (tdJ. NATURAL WET BULB TEMPERATURE (fnah). GLOBE TEMPERATURE (tr), AND AMBIENT VAPOR PRESSURE (P,) FOR TEST CONDITIONS GROUPED ACCORDING TO METABOLIC HEAT PRODUCTION (M)

Test No.

fdh (“a

p, (mm

M = 121 23 2 3 4

38

6

Hg) 139 W.m-” II 13

23 49

13

8 9 IO

23

tnw1, (“C)

t&! (“C)

16

23

24 25 27 28 31

41 51 60 65 58

26 28 30 30

49 62 74 48

M = 255 - 273 W,rn-’ 11

23

11

16

23

12 13

38

13 23

23 28

40 40

14 15

49

13 23

26 30

50 50

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TO

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99

HEAT

TABLE3 MEAN

VALUES

FOR WET BULB

GLOBE

TEMPERATURE

(WBGT),RADIANT

HEAT EXCHANGE

(R),

Loss(&,,A”

HEART RATE (HR): SWEATING (.%+I): REQUIRED EVAPORATIVE HEAT EVAPORATIVE CAPACITY OF THE ENVIRONMENT (E,,,); AND WETTEDNESS TEST CONDITIONS

Test

No.

Exposure time (min)

WBGT F-3

R

HR

(W.m-‘)

(bpm)

MAXIMAL (tls) FOR EACH OF THE

1,’

M = 121 - 139 W.rn-’ 1 2 3 4 5 6

90 90 90 90 90 90

18.5 28.7 32.8 37.0 39.3 38.8

-27 45 144 235 289 214

80 89 94 102 114 127

7 8 9 10

90 90 62 k 8 90

32.5 38.5 43.9 35.5

49 183 318 45

95 k 6 116k 11 129 + 14 114 + 19

11 12 13 14 15

90 90 44+ 482 362

18.5 28.4 31.8 32.9 35.9

-26 40 39 50 48

” includes

standard

” + 2 + 2 f

4 7 5 3 3 15

73k 236 2 309 i 279 + 433 2 321 ?

19 40 23 53 74 50

304 k 47 472-r-49 539 k 110 355 ” 86

44 198 295 372 440 348

2 + ? L 2 t

21 14 10 37 6 30

294 332 361 360 393 254

k 49 + 17 29 k 15 ‘-’ 9 + 21

0.16 0.60 0.82 1.03 1.12 1.37

250 385 522 237

k 2 k k

25 14 9 25

372 406 442 261

-f r t 2

19 15 9 13

0.68 0.95 1.18 0.91

173 303 314 380 389

t k k in k

14 31 23 25 24

298 336 249 385 274

t + i + k

16 13 10 4 21

0.58 0.90 1.26 0.99 1.43

M = 255 - 273 W,rn-’

15 15 9

122 145 157 154 153

?I + 2 + k

7 15 18 20

221 386 371 450 425

k + + ? 2

34 64 41 63 20

deviations

O2 concentration in the bag were determined using a dry-gas meter (Cowan-Parkinson) and an E-2 Beckman paramagnetic analyzer, respectively. Sweat loss was determined from nude body weight changes measured on a scale accurate to ?20 g, and corrected for water intake. Acclimation. Exposures to the different ambient combinations were randomized. Each subject was tested, on the average, twice a week. The experiment was carried out during the late spring and summer months. Although the subjects were not acclimated prior to the testing, the seasonal effects and the testing exposures implies that the subjects were partially acclimated, at least, later in the study. Procedures. The subjects arrived at the laboratory in the afternoon at least 2 hr after their last meal. Upon arrival, the subject’s nude weight was taken, he placed the rectal thermistor, and was rested in a neutral environment for 30 min while the chest electrodes and the skin thermocouples were placed on him. Thereafter, he got dressed, T,,, Tsk, and HR were recorded, and he entered the climatic chamber. Upon entering the chamber, clothed body weight was taken and the walk on the treadmill, set to yield the specified % voZ max, started. Clothed body weights were recorded every 30 min and at the end of the test. Termination of the test was at 90 min or when the subject reached a T,, = 39°C

100

KAMON,

SOTO,

AND

BENSON

or a heart rate of 160 beats per minute. At the end of the test, nude body weight was again taken. Since water intake was ad libitum, the intake was recorded. Calculations. Heat equivalent for sweat production or evaporation was calculated for latent heat as 0.67 W.hr+g-‘. Radiant heat exchange (Z?) was derived assuming emissivity of 1 and using the Boltzmann constant (4) R = 4.36

x

lo-“+fcZ (T”, - !?4,,J W.mP2,

where@ was corrected for clothing. T, and Tsk are absolute mean radiant heat and mean skin temperatures (“K), respectively. Thefcl was derived according to Nishi and Gagge (22) for insulative value of the clothing (I& of 0.6. The T, was derived from the t, according to Haines and Hatch (8). Convective heat exchange (C) was derived as C = 8.6 vO.~*~CI (tdb - t,,) W*m-2,

where 8.6 is the coefficient for heat transfer (12), v is air velocity in ma see-‘,fcZ is the same as for R, and &, is mean skin temperature in degrees Centigrade. The ambient evaporative capacity @,,,,A was derived as E max = 18.3 ~O.~.fpcZ (Psk - Pd W.mP2, where 18.3 is the coefftcient for evaporative heat transfer (2, 15), fpcl is the clothing permeation coefficient, which was derived from Nishi and Gagge (22) using I,,, = 0.6, and the Psk and P, are the saturated skin vapor pressure at Tsk and the ambient vapor pressure, respectively. The wettedness, w, was derived from

The wet bulb globe temperature WBGT

(WBGT) = 0.7

was calculated tnwb

+ 0.3

for indices as

t,,

where t nwbis natural wet bulb temperature. Note that the heat exchange values are adjusted for surface area, which were based on the individual values shown in Table 1. RESULTS

The ambient conditions are summarized in Table 2 for each test. The derived R, WBGT, E,,,, and Ema, are summarized in Table 3, together with some of the physiological responses for each of the ambient conditions in the order listed in Table 2. It should be noticed that R is progressively increasing for the low P, at each of the two t db. This increase in R causes progressive increases in the w (E,,,, E,,, ratio), as seen in tests 2 to 5 and 8 to 9. Tests 12 to 15 did not involve high R, but did include high M and a combination of either low or high P,. The physiological responses reflect the strain due to either high R or high w (insufficient E,,.J. This is shown in Figs. 1 to 3 for the time course of T,, and in Table 3 for the end values of HR. The rise in T,, above the pretest values for the conditions of low M and low tdb (38°C) are shown in Fig. 1. These tests were tolerated for 90 min. However, no

RESPONSES TO RADIANT

I

0

IO

101

HEAT

I

1

I

1

I

I

I

1

,

20

30

40

50

60

70

00

90

100

Time

(mid

1. Time course of the mean changes in rectal temperature (AT,& from the pretest (resting) values for the conditions of low M (130 W’m-*) and low tdb (38°C). Numbers correspond to the listed tests (Table 2) and to the skin wettedness (Table 3). FIG.

equilibrium of T,, was achieved, except for the control test and test 2 involving w of 0.6. The test number and w values are shown for each T,,. Although the conditions for tests 3 and 4 could allow for heat balance (w s f), T, did creep up mostly because of R and insufficient SW (Table 3). The sharp rise in T,, for conditions 5 and 6 reflect w > 1 with the higher T,, for the higher R (test 5) rather than for the high w (test 6). 1.4 1.2 I.0 0.8 E 0.6 I-? a 0.4 -

0.60

0.2 0.0 -0.2 i: 0

I

IO

I

I

I

I

I

I

I

I

20

30

40

50

60

70

a0

90

Time

I

100

(mid

2. Time course of the mean changes in rectal temperature (ATA from pretest (resting) values for the conditions of low M (130 W.m+) and high tab (49°C). Numbers correspond to the listed tests (Table 2) and to the skin wettedness (Table 3). FIG.

102

KAMON,SOTO,AND BENSON

G e 0.6 z a04-

I 0

IO

I 20

1 30

I 40

I 50 Time

I 60

1 TO

80

I

1

90

100

(mh)

FIG. 3. Time course of the mean changes in rectal temperature (AT,d from pretest (resting) values for the conditions of high M (264 W. mm2)and low and high t db. Numbers correspond to the listed tests (Table 2) and to the skin wettedness (Table 3).

The rise in T,, for the conditions of high fdb (49°C) and low M is shown in Fig. 2. Again, T,, was rising as R was increased (tests 8 and 9), or when P, was high (test 10). Notice the termination of test 9 at 62 min because T,, reached 39°C. This test involved large E,,, due to the contribution of R (318 We rne2). The time course of T,, for the tests involving high M and low R (Table 3, tests ll- 15) is shown in Fig. 3. Steady state was observed for the control (test 11) and the one test (test 12) involving low tdb (38°C) and low P, (13 mm Hg). However, a sharp rise in T,, and reduced tolerance time was seen for the three tests where w > 1 or w = 1 at excessive fdb and/or P,. Sweating (SW) during the first half hour was slow but peaked during the last hour of the test. As shown in Table 3, SW exceeds E,,, in almost all cases. Considerable drippage of sweat was seen in the very humid conditions and this would account for the need of large SW to attain the E,,,. Change in HR tended to follow the changes in T,,, reflecting the physiological strain of the different ambient conditions. The effects of tdb and t B(reflecting R) on the increments in HR were tested using a multiple linear regression for the pooled individual data for each test. The regression of the increment in HR above those observed under the neutral conditions for each M (AHR) against t db above neutral (tdb - 25) and t, above tdb(tg - tdb) was AHRbp, = 0.96(t,, - 25) + O.Sl(t, - tdb) -

1,

with a multiple correlation coefftcient r = 0.67. The correlation coefficient indicated that factors other than tdb and t, accounted for the HR increments. Indeed the higher P, elicited the higher-end values of HR (Table 3, tests 6, 13, 15).

RESPONSES

TO RADIANT

HEAT

103

DISCUSSION Our main purpose was to test the compatibility between the physiological responses and the computed heat balance during exposure to heat involving highradiant sources. The use of physiological responses, primarily heart rate and body temperature, to either establish the coefficients of the heat balance equations (2,6, 12, 15) or to psychrometrically define safe zones of exposure (5, 7, 11, 15, 19, 20) is not new. Indeed, in most cases the use of the heat balance equations as predictors for the prescriptive zone was proven reliable. However, all of the above-mentioned tests for safe exposure did not involve exposure to radiant heat sources similar to that experienced in many hot industries. Indeed, the observations we have made confirm the adequacy of predicting excessive physiological responses when the body is not expected to be in heat balance due to high radiant heat sources. Progressively increasing R for a given M, tdb, and P, increased the E,,, to the extent that T,, and HR did not equilibrate. These physiological responses prevailed for w 3 1. Compared to the effect of increasing R, the increase in M resulted in a steeper rise of T,, and HR (Figs. 1 and 3). This was similar to the findings reported by Kraning et al. (17), where M was twice as effective as the environmental heat (R + C) in raising HR and increasing cardiac output. The expected physiological responses were used to provide predictors for the T,, and HR based on M and the heat balance (6). In the past we suggested a work-rest schedule based on the expected change in HR due to relative M(% vo2 max) and to t d,-induced increments in HR (14). The schedule considered a previously observed regression of 1 bpm in AHR on (I,, - 25) (13), which was similar to the 0.96 derived in this study. The addition of tg to the AHR predictor was suggested here because t, is linearly related to R and is easy to measure. Thus, it is now possible to consider R in the design of work and rest for industrial hot ambient conditions. The suggested prescriptive zones (3, 15, 19, 20) require the design of new limit zones for each level of M. Similarly, the ET scales used by Gagge et al. (5) and Gonzalez et al. (7), although based on the heat balance equation, require that a new scale be constructed for different work levels. It seems, therefore, that the heat balance using the equations for all the avenues of heat exchange and an estimated M is also reliable when radiant heat sources are involved. The practitioner stayed away from such a method because of the complexity involved in the computation of E,,, and E,,,. However, a hand-held programmable calculator provides the means for on-site immediate estimate of the heat stress, the level of strain, and the limits of exposure time (16). The program defines high strain for IV > 1 and exposure limit for w > 1. The accuracy of such a new simplified system indicates that alternative indices which were suggested because of the need to simplify the process of heat stress estimates are not necessary. A recently suggested alternative index is the WBGT. Its inadequacy is clearly seen in this study. As can be seen in Table 3, similar WBGT were derived

104

KAMON,

SOTO, AND BENSON

for a wide range of ambient conditions with different physiological responses. Even with consideration to M and the OSHA-suggested threshold limits (23), it would not be possible to design for appropriate exposure and relief periods based on WBGT. On the other hand, scheduling the work will be possible using an effective heat strain index, as suggested previously, using the heat balance equations (16). REFERENCES 1. Belding, H. S., and Hatch, T. F. (1955). Index for evaluating heat stress in terms of the resulting physiological strain. HeatlPipinglAir Cond. 27, 129- 136. 2. Belding, H. S., and Kamon, E. (1973). Evaporative coefficients for prediction of safe limits in prolonged exposures to work under hot conditions. Fed. Proc. 32, 1598- 1601. 3. Dukes-Dobos, F., and Henschel, A. (1973). Development of permissible heat exposure for occupational work. ASHRAE J. 15, 57-63. 4. Fanger, P. 0. (1970). “Thermal Comfort.” Danish Technical Press, Copenhagen. 5. Gagge, A. P., Stolwijk, J. A. J., and Nishi, Y. (1971). An effective temperature scale based on a simple model of human physiological regulatory responses. ASHRAE Trans. 77, 247-262. 6. Givoni, B., and Goldman, R. F. (1972). Predicting rectal temperature response to work, environment, and clothing. J. Appl. Physiol. 32, 812-822. 7. Gonzalez, R. R., Berglund, L. G., and Gagge, A. P. (1978). Indices of thermoregulatory strain for moderate exercise in the heat. .I. Appl. Physiol: Respir. Environ. Exercise Physiol. 44,889-899. 8. Haines, G. F., and Hatch, T. (1952). Industrial heat exposure-Evaluation and control. Heat Vent. 49, 93- 104. 9. Hatch, T. F. (1963). Assessment of heat stress. In “Temperature-Its Measurement and Control in Science and Industry” (J. D. Hardy, Ed.), Vol. 3, Pt. 3. pp. 307-318. Reinhold, New York. 10. Hertig, B. A. (1973). Thermal standard and measurement techniques. In “The Industrial Environment-Its Evaluation and Control.” DHEWINIOSH, Washington, D.C. 11. Hertig, B.A., and Belding, H. S. (1963). Evaluation and control of heat hazards. In “Temperature-Its Measurement and Control in Science and Industry” (J. D. Hardy, Ed.), Vol. 3, Pt. 3. Reinhold, New York. 12. Kerslake, D. Mck. (1972). “The Stress of Hot Environments.” Cambridge Univ. Press, London/ New York. 13. Kamon, E. (1972). Relationship of physiological strain to change in heart rate during work in the heat. Amer. Ind. Hyg. Assoc. J. 33, 701-708. 14. Kamon, E. (1979). Scheduling cycles of work for hot ambient conditions. Ergonomics 22, 427-439.

15. Kamon, E., Avellini, B., and Krawjeski, J. (1978). Physiological and biophysical limits to work in the heat for clothed men and women. J. Appl. Physiol: Rrspir. Environ. Exercise Physiol. 44, 918-925. 16. Kamon, E., and Ryan, C. (1981). Effective heat strain index using pocket computer. Amer. Ind. Hyg. Assoc. J.. in press. 17. Kraning, K. K., II, Belding, H. S., and Hertig, B. A. (1966). Use of sweating rate to predict other physiological responses to heat. J. Appl. Physiol. 21, Ill- 117. 18. Lee, D. H. K. (1980). Seventy-five years of searching for a heat index. Environ. Res. 22,33 l-356. 19. Lind, A. R. (1963). A physiological criterion for setting the thermal environmental limits for everyday work. J. Appl. Physiol. 18, 51-56. 20. Lind, A. R. (1973). Prediction of safe limits for prolonged occupational exposure to heat. Fed. Proc. 32, 1602- 1606. 21. National Institute for Occupational Safety and Health (1972). A recommended standard for occupational exposure to hot environments. In “Criteria for a Recommended Standard-Hot Environments.” NIOSH, Washington, D.C.

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22. Nishi, Y., and Gagge, A. P. (1970). Moisture permeation of clothing-A factor governing thermal equilibrium and comfort. ASHRAE Trans. 76, 137- 145. 23. Occupational Safety and Health Reporter (1974). Recommendation for a standard for work in hot environments. 3, 1055. 24. Pulket, C., Henschel, A., Burg, W. R., and Saltzman, B. E. (1980). A comparison of heat stress indices in a hot humid environment. Amer. Ind. Hyg. Assoc. ./. 41, 442-449. 25. Ramathan, N. L., and Belding, H. S. (1973). Physiological evaluation of the WBGT index for occupational heat stress. Amer. Ind. Hyg. Assoc. J. 34, 375-383.