UDC 614.89:612.766.1

Candidate of Biological Sciences G. V. Bavro, I. F. Shashkov

Effect of the Weight of Personal Protective Equipment (PPE) on the Functional State of the Body During Physical Work under Different Microclimatic Conditions

The weight of equipment ensembles that protect people from adverse environmental factors can be considerable. It is determined by the weight of both self-contained life-support systems and protective clothing (coveralls or suits). The literature contains data on the effects on the body of equipment weight carried on the back and chest, as well as in the hands (S. M. Gorodinsky et al., 1967, and others). The present study examines how a weight load affects the body when distributed evenly over it, as in the load created by the weight of protective clothing.

The use of various impregnations and coatings, and changes in the structure and thickness of materials to improve their protective effectiveness, not only increase the weight of protective clothing but also affect its hygienic properties (air and vapor permeability and thermal conductivity) and, consequently, the conditions of heat exchange between the body and the environment. As a result, the load on the body caused by the increased weight of protective clothing is quite often combined with a heat load.

To distinguish the effects of clothing weight on the functional state of the body from those associated with changes in its heat exchange with the environment, we varied the weight load in our experiments not by increasing the thickness of the protective clothing, but by using calibrated weights distributed evenly over a cotton suit. The area covered by the weights amounted to only about 4% of the suit’s surface and remained constant throughout.

To study how conditions of heat exchange between the body and the environment affect tolerance of a weight load, the experiments were conducted at 23, 45, and 12°, with relative humidities of 50–60, 20–30, and 70–75%, respectively. The weight load was varied from 1 to 11 kg in increments of 2 kg. The subjects performed standardized physical work of moderate intensity. Work periods of 20 minutes alternated with 10-minute rest periods. The experiment lasted 2 hours.

To characterize the subjects’ thermal state, skin temperature was measured on the forehead, chest, hand, thigh, and lower leg throughout the experiment, and axillary temperature was measured before and at the end of the experiment. Standard formulas were used to calculate mean weighted skin temperature, mean body temperature, and changes in body heat content (\(\Delta Q\)). Heart rate, pulmonary ventilation, and energy expenditure were recorded, the latter by indirect calorimetry. Sweat loss was determined from the subject’s loss of body weight. To identify changes in physical work capacity, the amount of work the subject could perform on a hand ergograph before reaching complete exhaustion was determined before and at the end of the experiment. Work capacity was expressed as a per-

…percentages relative to the baseline, which was taken as 100. The experimental data were statistically analyzed using Student’s t-test. Confidence intervals were calculated at the 95% confidence level.

Dependence of the mean increase in indicators of physiological strain per additional kilogram of weight load on ambient temperature. Moderate-intensity work \((M \pm tm)\)

During physical work at an ambient temperature of \(23^\circ\), each additional kilogram of clothing weight was accompanied by an average increase in energy expenditure of 4.5 kcal/h. At a clothing weight of 1 kg, energy expenditure was approximately 200 kcal/h; when the weight was increased to 11 kg, it reached 250 kcal/h. As energy expenditure increased, so did thermoregulatory strain. For each additional kilogram of clothing weight, moisture loss increased by 8.5 g/h, pulmonary ventilation by 0.9 L/min, and heart rate by 1 beat per minute. Mean weighted skin temperature also rose.

A distinct, though small, increase in body heat content was observed at a clothing weight of 9 kg. The most pronounced reduction in the subjects’ physical work capacity was also observed at this weight. The decrease in work capacity when clothing weight increased from 7 to 9 kg was twice as large as when it increased from 1 to 7 kg.

At a high ambient temperature (\(45^\circ\)), that is, under conditions in which heat dissipation from the body was impeded, physiological strain increased more markedly as clothing weight rose. Thus, under these conditions, the increase in energy expenditure for each additional kilogram of clothing weight was nearly 1.5 times greater than at \(23^\circ\); the increase in body moisture loss was twice as great, and the increases in pulmonary ventilation and heart rate were three times as great (see table).

Physical work at \(45^\circ\), despite the considerable activation of the body’s thermoregulatory responses, was accompanied by a disturbance of heat balance that became more pronounced as clothing weight increased. A distinct decline in physical work capacity—20% relative to baseline—was observed at a clothing weight of only 5 kg. At 11 kg, this measure decreased by nearly 40%.

Of particular interest are the data obtained under conditions that facilitated heat dissipation from the body to the environment. In all experiments involving physical work at \(12^\circ\), the subjects’ work capacity increased relative to baseline. The magnitude of this increase grew as the weight load rose from 1 to 9 kg; only at 11 kg did work capacity begin to decline.

The increase in physical work capacity with increasing clothing weight can be explained to some extent by the increase in energy expenditure, which reduces the body heat deficit that develops during work at \(12^\circ\). As the table shows, energy expenditure per additional kilogram of clothing weight increased twice as much at \(12^\circ\) as at \(23^\circ\). Nevertheless, body heat content decreased by the end of the experiment. The disturbance of heat balance was greatest at a clothing weight of 1 kg. However, the absolute heat content value of 29.2 kcal/kg remained within the optimal range \((29.5 \pm 0.5\ \text{kcal/kg})\). At clothing weights of 9–11 kg, the substantial increase in the body’s energy expenditure kept heat content virtually at baseline; uncompensated heat losses amounted to only 8–12 kcal.

Thus, at low ambient temperatures, an increase in weight load not only has no adverse effect on the body but, on the contrary, helps normalize its thermal state and maintain a high level of work capacity. This suggests that one of the principal factors contributing to fatigue and reduced work capacity as the physical load on the body increases—in particular, the load imposed by the weight of personal protective equipment (PPE)—is disruption of temperature homeostasis. Indeed, as the experimental data presented above show, even a slight increase in body heat content leads to more rapid development of fatigue. Conversely, a small shift in heat balance toward cooling, while total body heat content remains within optimal limits, has a beneficial effect on human work capacity. These observations indicate the need for further study of the role of so-called work-induced hyperthermia (P. N. Veselkin) in maintaining high work capacity. The literature suggests that work capacity is affected not only by the amount of heat in the body but also by its distribution.

The thermal state of the body shell is also important. Thus, the tonic effects of cold receptors in the “shell,” which normalize central nervous system activity, apparently play a significant role in the mechanism by which work capacity increases at low ambient temperatures. Such effects have been demonstrated by O. P. Minut-Sorokhtina and her coworkers, by us in previous studies (G. V. Bavro and Yu. G. Pletenskii), and by other authors.

The experimental data therefore support the conclusion that the load imposed on the body by the weight of protective clothing is determined to a considerable extent by the conditions of heat exchange with the environment. Accordingly, limits on this load cannot be established without taking those conditions into account.

Taking the magnitude of the reduction in subjects’ work capacity as the criterion for determining the permissible increase in a weight load distributed uniformly over the body, it may be concluded that, during moderately strenuous physical work under conditions that maintain heat balance, PPE should weigh no more than 7 kg; under conditions leading to overheating, no more than 3 kg. When heat loss is facilitated, leading to a small heat deficit in the body, the weight load may be increased to more than 11 kg.

References

Bavro, G. V., and Pletenskii, Yu. G. Effect of inhalation of cooled air on autonomic functions and the state of the central nervous system in rabbits during hyperthermia. Byull. eksper. biol., 1971, no. 9, pp. 32–36.

Veselkin, P. N. Fever. Moscow: Medgiz, 1963.

Gorodinskii, S. M., Bavro, G. V., Pletenskii, Yu. G., et al. Investigation of the possibility of rapidly relieving human hyperthermia by local cooling. In: Medical and Technical Problems of Personal Protection. Issue 13. Moscow, 1973, p. 12.

Gorodinskii, S. M., Bavro, G. V., and Shashkov, I. F. On human work capacity in individual isolating systems. In: Proceedings of the 5th All-Union Conference on Occupational Physiology. Moscow: Nauka, 1967, pp. 82–83.

Minut-Sorokhtina, O. P. The Physiology of Thermoreception. Moscow: Meditsina, 1972.

Received March 29, 1974.

THE EFFECT OF WEIGHT OF THE INDIVIDUAL PROTECTION MEANS (IPM) ON THE STATE OF FUNCTIONING OF THE BODY IN WORK UNDER VARIOUS MICROCLIMATIC CONDITIONS

G. V. Bavro, I. F. Shashkov

The finding was that the strain produced on the body by the weight of the clothing depends to a great extent on the conditions of heat exchange with the environment. Preliminary recommendations are given on the permissible increase of the weight of clothing.