Biophysical Assessment and Predicted Thermophysiologic Effects of Body Armor

Heat Stress Management in the Military: Wet-Bulb Globe Temperature Offsets for Modern Body Armor Systems

OBJECTIVE

The aim of this study was to model the effect of body armor coverage on body core temperature elevation and wet-bulb globe temperature (WBGT) offset.

BACKGROUND

Heat stress is a critical factor influencing the health and safety of military populations. Work duration limits can be imposed to mitigate the risk of exertional heat illness and are derived based on the environmental conditions (WBGT). Traditionally a 3°C offset to WBGT is recommended when wearing body armor; however, modern body armor systems provide a range of coverage options, which may influence thermal strain imposed on the wearer.

METHOD

The biophysical properties of four military clothing ensembles of increasing ballistic protection coverage were measured on a heated sweating manikin in accordance with standard international criteria. Body core temperature elevation during light, moderate, and heavy work was modeled in environmental conditions from 16°C to 34°C WBGT using the heat strain decision aid.

RESULTS

Increasing ballistic protection resulted in shorter work durations to reach a critical core temperature limit of 38.5°C. Environmental conditions, armor coverage, and work intensity had a significant influence on WBGT offset.

CONCLUSION

Contrary to the traditional recommendation, the required WBGT offset was >3°C in temperate conditions (<27°C WBGT), particularly for moderate and heavy work. In contrast, a lower WBGT offset could be applied during light work and moderate work in low levels of coverage.

APPLICATION

Correct WBGT offsets are important for enabling adequate risk management strategies for mitigating risks of exertional heat illness.

Can smaller body armour improve thoracolumbar range of motion and reduce interference when female soldiers perform dynamic tasks?

Most female soldiers report that in-service body armour systems are too large. We investigated whether a smaller prototype body armour system could improve thoracolumbar range of motion (ROM) and reduce interference when female soldiers performed dynamic postures. 97 female soldiers completed three ROM tasks and seven dynamic postures wearing no armour, an in-service body armour system, and a smaller prototype system. Feedback on comfort of the prototype system was also obtained. Thoracolumbar ROM and dynamic posture completion were both hindered by using body armour, although the participants' performances were significantly less impeded when they wore the smaller prototype system compared to the in-service system. A smaller body armour system that is better matched to the anthropometric dimensions of female soldiers appears to improve overall fit and function. An increased range of body armour sizes and female-specific designs should be systematically explored to further enhance fit and function of body armour.

Bra-body armour integration, breast discomfort and breast injury associated with wearing body armour

This study investigated whether female soldiers experience bra integration or breast discomfort/injuries related to body armour use and whether these issues were associated with breast size. Ninety-seven Australian Defence Force female soldiers completed a questionnaire and had their breast volume assessed (range: 91-919 ml/breast) using three-dimensional scanning. Twenty-two percent (n = 21) of participants reported integration issues between their bra and body armour, 63% (n = 61) reported breast discomfort while wearing body armour and 27% (n = 26) reported experiencing a breast injury related to wearing body armour. Although bra-body armour integration was not dependent upon breast size, female soldiers with medium-large breasts reported significantly more breast discomfort and injuries when using body armour compared to participants with small breasts. These findings highlight the importance of developing body armour systems that cater to the range of breast sizes of female soldiers in order to improve bra-body armour integration and reduce breast discomfort and injury. Practitioner summary: This exploratory research provides evidence of bra integration issues, breast discomfort and breast injury experienced by female soldiers when wearing body armour. Given the growing representation of women in military organisations, strategies to alleviate these issues for female users of body armour, particularly those with larger breast sizes, are required.

Identifying problems that female soldiers experience with current-issue body armour

Despite female soldiers representing a growing user population, military body armour systems are currently better suited to the anthropometric dimensions of male soldiers. The aim of this study was to explore issues that female soldiers experience with current Australian Defence Force (ADF)-issue body armour. Following a sequential exploratory design, an initial questionnaire was completed by 97 Australian female soldiers. Subsequently, 33 Australian female soldiers participated in one of three focus groups. Descriptive statistics of questionnaire data considered alongside thematic analysis of focus group transcripts revealed problems with the design (fit, form and function) of current ADF-issue body armour, as well as problems with the issuance and education surrounding use of the system. It is recommended that anthropometric data of female soldiers be better incorporated into future body armour designs, that these data inform processes surrounding both acquisition and issuance of body armour and that training protocols for body armour use be reviewed.

Field validation of The Heat Strain Decision Aid during military load carriage

OBJECTIVES

We aimed to determine the agreement between actual and predicted core body temperature, using the Heat Strain Decision Aid (HSDA), in non-Ground Close Combat (GCC) personnel wearing multi terrain pattern clothing during two stages of load carriage in temperate conditions.

DESIGN

Cross-sectional.

METHODS

Sixty participants (men = 49, women = 11, age 31 ± 8 years; height 171.1 ± 9.0 cm; body mass 78.1 ± 11.5 kg) completed two stages of load carriage, of increasing metabolic rate, as part of the development of new British Army physical employment standards (PES). An ingestible gastrointestinal sensor was used to measure core temperature. Testing was completed in wet bulb globe temperature conditions; 1.2-12.6 °C. Predictive accuracy and precision were analysed using individual and group mean inputs. Assessments were evaluated by bias, limits of agreement (LoA), mean absolute error (MAE), and root mean square error (RMSE). Accuracy was evaluated using a prediction bias of ±0.27 °C and by comparing predictions to the standard deviation of the actual core temperature.

RESULTS

Modelling individual predictions provided an acceptable level of accuracy based on bias criterion; where the total of all trials bias ± LoA was 0.08 ± 0.82 °C. Predicted values were in close agreement with the actual data: MAE 0.37 °C and RMSE 0.46 °C for the collective data. Modelling using group mean inputs were less accurate than using individual inputs, but within the mean observed.

CONCLUSION

The HSDA acceptably predicts core temperature during load carriage to the new British Army non-GCC PES, in temperate conditions.

Thermoregulatory Responses with Size-matched Simulated Torso or Limb Skin Grafts

Skin grafting following a burn injury attenuates/abolishes sweat production within grafted areas. It is presently unknown whether the thermoregulatory consequences of skin grafting depend on anatomical location.

PURPOSE

To test the hypothesis that a simulated burn injury on the torso will be no more or less detrimental to core temperature control than on the limbs during uncompensable exercise-heat stress.

METHODS

Nine non-burned individuals (7 males, 2 females) completed the protocol. On separate occasions, burn injuries of identical surface area (0.45 ± 0.08 m2 or 24.4% ± 4.4% of total body surface area) were simulated on the torso or the arms/legs using an absorbent, vapor-impermeable material that impedes sweat evaporation in those regions. Participants performed 60 min of treadmill walking at 5.3 km·h-1 and a 4.1% ± 0.8% grade, targeting 6 W·kg-1 of metabolic heat production in 40.1°C ± 0.2°C and 19.6% ± 0.6% relative humidity conditions. Rectal temperature, heart rate, and perceptual responses were measured.

RESULTS

Rectal temperature increased to a similar extent with simulated injuries on the torso and limbs (condition-by-time interaction: P = 0.86), with a final rectal temperature 0.9 ± 0.3°C above baseline in both conditions. No differences in heart rate, perceived exertion, or thermal sensation were observed between conditions (condition-by-time interactions: P ≥ 0.50).

CONCLUSION

During uncompensable exercise-heat stress, sized-matched simulated burn injuries on the torso or limbs evoke comparable core temperature, heart rate, and perceptual responses, suggesting that the risk of exertional heat illness in such environmental conditions is independent of injury location.

Burn Injury Does Not Exacerbate Heat Strain during Exercise while Wearing Body Armor

INTRODUCTION

Although evaporative heat loss capacity is reduced in burn-injured individuals with extensive skin grafts, the thermoregulatory strain due to a prior burn injury during exercise-heat stress may be negligible if the burn is located underneath protective clothing with low vapor permeability.

PURPOSE

This study aimed to test the hypothesis that heat strain during exercise in a hot-dry environment while wearing protective clothing would be similar with and without a simulated torso burn injury.

METHODS

Ten healthy individuals (8 men/2 women) underwent three trials wearing: uniform (combat uniform, tactical vest, and replica torso armor plates), uniform with a 20% total body surface area simulated torso burn (uniform + burn), or shorts (and sports bra) only (control). Exercise consisted of treadmill walking (5.3 km·h; 3.7% ± 0.9% grade) for 60 min at a target heat production of 6.0 W·kg in 40.0°C ± 0.1°C and 20.0% ± 0.6% relative humidity conditions. Measurements included rectal temperature, heart rate, ratings of perceived exertion (RPE), and thermal sensation.

RESULTS

No differences in rectal temperature (P ≥ 0.85), heart rate (P ≥ 0.99), thermal sensation (P ≥ 0.73), or RPE (P ≥ 0.13) occurred between uniform + burn and uniform trials. In the control trial, however, core temperature, heart rate, thermal sensation, and RPE were lower compared with the uniform and uniform + burn trials (P ≤ 0.04 for all).

CONCLUSIONS

A 20% total body surface area simulated torso burn injury does not further exacerbate heat strain when wearing a combat uniform. These findings suggest that the physiological strain associated with torso burn injuries is not different from noninjured individuals when wearing protective clothing during an acute exercise-heat stress.

Heat strain in chemical protective ensembles: Effects of fabric thermal properties

An ongoing challenge in material science has been to reduce heat strain experienced by individuals wearing chemical protective ensembles. The objective of this study is to analyze the relationship between the thermal properties of eight chemical protective fabrics and heat strain in ten chemical protective ensembles constructed with those fabrics. The fabric samples were tested on a sweating guarded hot plate to measure fabric thermal and evaporative resistance. The ensembles were then tested on thermal manikins to measure ensemble thermal and evaporative resistance. An empirical thermoregulatory model, the Heat Strain Decision Aid (HSDA), was used to predict thermal responses of core temperature and endurance times. Model inputs included ensemble thermal and evaporative resistances, four environmental conditions and a metabolic rate of 400 W. The fabric intrinsic thermal and evaporative resistances ranged from 0.01 to 0.05 m² °C·W^-1 and from 5.9 to 12.82 m² Pa·W^-1, respectively. Ensemble intrinsic thermal and evaporative resistances ranged from 0.23 to 0.31 m² °C·W^-1 and 51.7-67.8 m² Pa·W^-1, respectively. Predicted endurance times varied from 170 to 300 min at 20 °C/50% RH/2 m s^-1 and 26 °C/55% RH/9 m s^-1 conditions, and varied from 91 to 98 min at 30 °C/75% RH/2 m s^-1 and 40 °C/20% RH/2 m s^-1 conditions. Improved fabric thermal properties reduced heat strain and extended endurance times, but the magnitude of the extended times is dependent on the environmental conditions. Consequently, the benefits of improved fabric thermal properties may only be observed under certain environmental conditions.

Heat strain imposed by personal protective ensembles: quantitative analysis using a thermoregulation model

The objective of this paper is to study the effects of personal protective equipment (PPE) and specific PPE layers, defined as thermal/evaporative resistances and the mass, on heat strain during physical activity. A stepwise thermal manikin testing and modeling approach was used to analyze a PPE ensemble with four layers: uniform, ballistic protection, chemical protective clothing, and mask and gloves. The PPE was tested on a thermal manikin, starting with the uniform, then adding an additional layer in each step. Wearing PPE increases the metabolic rates [Formula: see text], thus [Formula: see text] were adjusted according to the mass of each of four configurations. A human thermoregulatory model was used to predict endurance time for each configuration at fixed [Formula: see text] and at its mass adjusted [Formula: see text]. Reductions in endurance time due to resistances, and due to mass, were separately determined using predicted results. Fractional contributions of PPE's thermal/evaporative resistances by layer show that the ballistic protection and the chemical protective clothing layers contribute about 20 %, respectively. Wearing the ballistic protection over the uniform reduced endurance time from 146 to 75 min, with 31 min of the decrement due to the additional resistances of the ballistic protection, and 40 min due to increased [Formula: see text] associated with the additional mass. Effects of mass on heat strain are of a similar magnitude relative to effects of increased resistances. Reducing resistances and mass can both significantly alleviate heat strain.

Ebola Response: Modeling the Risk of Heat Stress from Personal Protective Clothing

Introduction

A significant number of healthcare workers have responded to aid in the relief and containment of the 2013 Ebola virus disease (EVD) outbreak in West Africa. Healthcare workers are required to wear personal protective clothing (PPC) to impede the transmission of the virus; however, the impermeable design and the hot humid environment lead to risk of heat stress.

Objective

Provide healthcare workers quantitative modeling and analysis to aid in the prevention of heat stress while wearing PPC in West Africa.

Methods

A sweating thermal manikin was used to measure the thermal (R_ct) and evaporative resistance (R_et) of the five currently used levels of PPC for healthcare workers in the West Africa EVD response. Mathematical methods of predicting the rise in core body temperature (T_c) in response to clothing, activity, and environment was used to simulate different responses to PPC levels, individual body sizes, and two hot humid conditions: morning/evening (air temperature: 25°C, relative humidity: 40%, mean radiant temperature: 35°C, wind velocity: 1 m/s) and mid-day (30°C, 60%, 70°C, 1 m/s).

Results

Nearly still air (0.4 m/s) measures of R_ct ranged from 0.18 to 0.26 m² K/W and R_et ranged from 25.53 to 340.26 m² Pa/W.

Conclusion

Biophysical assessments and modeling in this study provide quantitative guidance for prevention of heat stress of healthcare workers wearing PPC responding to the EVD outbreak in West Africa.