Hazard Category - Thermal Stress - Defense Centers for Public Health

Thermal Stress

Coordinate with the Health Hazard Assessment (HHA) Program early in the acquisition process in order to eliminate or control thermal stress exposures associated with materiel. Subject matter experts from the Industrial Hygiene Field Services Division provide input for HHAs related to thermal stress.

Cold and heat strain may result from exposure to thermal stress. The body's normal responses to cold strain include shivering and vasoconstriction in peripheral and superficial (skin) blood vessels, especially in the extremities, nose, and ears. Cold-induced injuries and illnesses include nonfreezing cold injuries (e.g., chilblains, trench foot), freezing cold injuries (e.g., frostbite), and hypothermia. Physiological responses to heat strain include hyperthermia (elevated skin and core temperatures), increased sweating rate, dehydration, increased heart rate, and compromised cardiovascular control. Heat strain may result in serious and possibly fatal heat illnesses such as heat exhaustion and heat stroke. Risk of injury is affected by multiple exposures factors such as air temperature, humidity, frequency and duration of the mission, type of clothing worn, metabolic rate, and physiological/biomedical factors. Additional health effect information is available in TB MEDs 507 and 508.^1,2

Cold Stress

Physical stress that is the product of an interaction of environmental factors (e.g., ambient air or water temperature, humidity, wind speed), mission factors (e.g., metabolic rate, clothing, equipment, work duration and intensity, terrain, exposure to moisture), and physiological/biomedical factors (e.g., medication, health, fatigue, previous cold injury).

Data Requirements

Provide the Operational Mode Summay/Mission Profile (OMS/MP) or detailed use scenario including operational environment information that is relevant to extreme thermal temperatures. Report the climate types per AR 70-38, positions occupied by Soldiers, and expected durations of exposure.³

Testing shelters requires a 24-hour analysis due to the insulation levels. If output from a commercial calculation program is provided in lieu of test data, identify the inputted data, the calculation method used, and the name of the software package to ensure that it can be adequately reviewed. Calculations must be in accordance with ANSI/ASHRAE/ACCA Standard 183-2007 (RA 2017).⁶

Health Protection Criteria

MIL-STD-1472H requires heating systems to maintain an interior T_db above 50 °F within work environments occupied during extended periods of time (including but not limited to mobile personnel enclosures), or above 68 °F when arctic clothing is worn.⁷ Temperatures around the body shall not vary by more than 9 °F. Special provisions must be made to keep hands warm when precise work is required for more than 20 minutes in an environment below 60 °F.

Heat Stress

Physical stress that is the product of an interaction of environmental factors (e.g., ambient air temperature, humidity, wind speed, radiant/solar load), mission factors (e.g., metabolic rate, clothing, equipment, work duration and intensity, terrain), and physiological/biomedical factors (e.g., medication, health, heat acclimatization, hydration, physical fitness).

Data Requirements

Provide the OMS/MP or detailed use scenario including operational environment information that is relevant to extreme thermal temperatures. Report the climate types per AR 70-38, positions occupied by Soldiers, and expected durations of exposure.³

Provide system cooling load test data according to TOP 02-2-820 for tactical vehicles or TOP 01-2-807 for truck cabs.^4,5Systems are placed inside a temperature-controlled test chamber and the air conditioning is run for at least an hour while measurements are collected. Thermocouple readings are collected at head, hand, and foot locations of the occupied positions. Wet bulb temperature and relative humidity are measured at a single point. Simulate internal heat gains from personnel and equipment if possible, or provide the number of occupants and equipment heat gain.

Health Protection Criteria

MIL-STD-1472H requires an effective WBGT within manned spaces of less than 86 °F under normal operational internal heat loads and the worst-case design climatic conditions.⁷ For prolonged work at metabolic rates above 250 watts, the American Conference of Governmental Industrial Hygienists® Threshold Limit Values® apply for the maximum effective WBGT.⁸ Temperatures between the floor and head level shall not differ by more than 10 °F. Micro-climate cooling systems are desirable when special protective clothing or personal equipment are required to be worn.

For more information and guidelines for assessing thermal stress, see Technical Guide 351C, Health Hazard Assessor's Guide.

References

⁽¹⁾^{DA/DAF. 2003. TB MED 507, Heat Stress Control and Heat Casualty Management.} External Link

⁽²⁾^{DA. 2005. TB MED 508, Prevention and Management of Cold-weather Injuries.} External Link

⁽³⁾^{DA. 2020. Regulation 70-38, Research, Development, Testing, and Evaluation of Materiel for Worldwide Use.} External Link

⁽⁴⁾^{ATEC. 2017. TOP 02-2-820, Tactical Vehicle Climate Control Testing.} External Link

⁽⁵⁾^{ATEC. 2007. TOP 01-2-807, Thermal Comfort Testing for Vehicle Operator/Passenger Workspaces (Truck Cabs).} External Link

⁽⁶⁾^{ANSI/ASHRAE/ACCA. 2017. Standard 183-2007 (RA 2017), Peak Heating and Cooling Load Calculations in Buildings Except Low-Rise Residential Buildings.} External Link
⁽⁷⁾^{DOD. 2020. MIL-STD-1472H, Department of Defense Design Criteria Standard: Human Engineering.}

⁽⁸⁾^{ACGIH®. 2020. TLVs® and BEIs® Based on the Documentation of the Threshold Limit Values for Chemical Substances and Physical Agents & Biological Exposure Indices.} External Link