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DHA Public Health Experts Track Climate Change, Heat Trends Impacting Service Members, Training

Image of DHA Public Health Experts Track Climate Change, Heat Trends Impacting Service Members, Training. Heat risk data trends for U.S. Army bases worldwide show that the five-year average of annual heat risk days exceeded the 15-year average at 84%of the 44 bases tracked by Defense Health Agency Public Health, indicating that heat risk is steadily increasing. (DHA Public Health graphic illustration by Joyce Kopatch)

The World Meteorological Organization, which monitors climate, weather, and water resources for the United Nations, has declared that 2023 was the hottest year in recorded history. This comes as no surprise to Lisa Polyak, an environmental engineer at Defense Health Agency Public Health, who has been tracking heat data and reporting the results annually for the U.S. Army’s Health of the Force report. The report is published by Defense Centers for Public Health - Aberdeen, formerly the Army Public Health Center, part of DHA Public Health.

“It’s not just that 2023 was the hottest year in human history, it’s that the recent trends indicate that 2023 is not an outlier year,” said Polyak. “Heat risk trends for Army bases worldwide show that the 5-year average of annual heat risk days exceeded the 15-year average at 84%of the 44 bases we track, indicating that heat risk is steadily increasing as we move into the future.”

Heat Risk Trends

As defined in Health of the Force, a heat risk hour is counted when the National Weather Service heat index is greater than 90 degrees Fahrenheit. A heat risk day is a day with one or more heat risk hours.Many locations in the U.S, especially in the South and Southeast, routinely experience 25 or more heat risk days during the summer months of July and August, said Polyak. However, July and August 2023 differed from prior years in the number of heat risk hours experienced during the month. The rising number of heat risk hours contributes to heat illness risk when there is an insufficient interval for lower temperatures to facilitate the body’s recovery from heat stress.

According to the latest DOD Heat Illness Report, 720 service members were diagnosed with heat illness (635 heat exhaustion cases, 85 heat stroke cases) in July 2023. The data for this report are obtained from the Defense Health Agency’s Weather-related Injury Repository, which captures a selection of ICD-10-CM codes in inpatient and outpatient medical encounter records from the Military Health System Data Repository and medical event reports of heat exhaustion and heat stroke submitted through the Disease Reporting System internet, or DRSi.

Here’s the July heat illness breakdown by military service:

  • U.S. Army: 72%
  • U.S. Marine Corps: 16%
  • U.S. Air Force/Space Force: 7%
  • U.S. Navy: 5%

“The Army and Marines consistently have the highest burden and rate of heat illness among the services,” said Polyak.

In military training settings, the Wet Bulb Globe Temperature, or WBGT, index is used to measure risk of adverse health effects associated with exposure to ambient heat, said Polyak. The WBGT index measures heat stress in direct sunlight and is a function of temperature, humidity, wind speed, sun angle, and cloud cover. WBGT values are categorized and assigned a color (flag) that represents varying levels of heat risk. Flag colors include white, green, yellow, red, and black, where white represents less risk (lower WBGT values) and black represents the greatest risk (higher WBGT values).

Polyak says ambient heat as measured by WBGT readings is not the only thing leaders should monitor when assessing heat risk.

“Ambient heat is not the only factor, and may not always be the most important factor, in managing heat illness in training settings,” said Polyak. “Recent studies show that far more military heat illness occurs during red and yellow flag conditions, rather than black flag conditions.”

This also tracks with data from the Army Heat Center.

“We track the environmental conditions (temperature, humidity, WBGT) for every heat casualty seen at Martin Army Community Hospital,” said U.S. Army Lt. Col. David DeGroot, director of the Army Heat Center, located at Martin Army Community Hospital at Fort Moore, Georgia, and co-director of the Warrior Heat Illness Collaborative. “Leaders do a great job planning high-heat illness risk events for the coolest time of day. However, as heat illness occurs not just because of environmental conditions but also due to mission-related and individual risk factors, most heat casualties occur at heat category 1 (white flag) or cooler conditions."

In their 2023 Heat Illness Report to Congress, the Undersecretary of Defense Office for Personnel and Readiness reported heat illness events experienced by active duty service members serving at bases in the continental United States between 1996 and 2019. The analysis found that 84% of heat illness cases occurred on days that did not experience black flag conditions. Further, 20%of heat illness cases occurred when the daily maximum WBGT was below 78 degrees Fahrenheit, corresponding to no flag condition.

“This suggests underestimation of the effects of heat stress in moderate conditions, or insufficient application of the work-rest cycle and heat illness prevention measures,” said Polyak.

In July 2023, seven U.S. Army training installations experienced more than one-third of all hours in the month as heat risk hours:

  • JB San Antonio - 56 % of hours
  • Fort Bliss - 43 % of hours
  • Fort Jackson - 41 % of hours
  • Fort Sill - 36 % of hours
  • Fort Novosel - 35 % of hours
  • Fort Moore - 34 % of hours
  • Fort Eisenhower - 34 % of hours

There were similar outcomes in August 2023.

“Our installation-level tracking can be used to flag locations experiencing the worst conditions and prioritize them for intervention,” said Polyak.

“The data should be used to spur enhanced health surveillance, as well as adaption and resilience measures in anticipation of worsening conditions—particularly in training settings but also in everyday working, living, and housing conditions for service members and their families.”

What Can Leaders Do?

DeGroot explains that over the last five years, 85% of all exertional heat stroke, or EHS, casualties at the Maneuver Center of Excellence occur during either foot march or run events. Targeting prevention strategies during those events will have the maximum impact on EHS incidence.

“As heat illness incidence has many factors and there is no one-size-fits-all solution, it is our objective to provide unit leaders with a ‘toolkit’ of resources that they can select from which best meets their needs,” said DeGroot. “For example, taking extra rest breaks and/or carrying a lighter ruck sack are good options in the initial entry training environment, but those options are off the table in a setting such as Ranger School. We provide resources and training on hydration strategies, proper use of arm immersion cooling to ‘dump heat,’ risks associated with concurrent illness and medication use, how to acclimatize to the heat, and other known risk factors and risk mitigation strategies.”

The goal of this continued monitoring is to help military leaders and planners manage their operational training when heat risk exceeds identified thresholds.

“Implications for training and readiness include the need for increased vigilance and intervention to modify training practices when heat risk exceeds doctrinal thresholds,” said Polyak. “In future, adaptive measures like ‘reverse-cycle’ training, where the training mission is conducted during cooler night hours or shifting basic combat training away from the summer months, may be necessary. Further, certain locations could become unsuitable to continue training missions due to the persistence and worsening of outdoor heat risk conditions exacerbated by climate change.”

DeGroot says he also has a “newfound appreciation” for the role of motivation in heat illness.

“The reality is that human performance will always be reduced when it’s hot outside, compared to more temperate or mild conditions,” said DeGroot. “Motivation to excel is good, but excess motivation, in a quest to attain the unattainable, can set up an individual for heat illness. The universal precaution to mitigate heat stress is to move slower! Of course, that is not always an option, so unit leaders need to consider other mitigation strategies.”


Resources are available to help military leaders and service members reduce exertional heat illness:

The Defense Health Agency supports our nation by improving health and building readiness—making extraordinary experiences ordinary and exceptional outcomes routine.

NOTE: The mention of any non-federal entity and/or its products is for informational purposes only, and is not to be construed or interpreted, in any manner, as federal endorsement of that non-federal entity or its products.

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Last Updated: March 25, 2024
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