Heat Stress: It’s Not Just about Drinking Water

Content Provided by National Technology and Development Program (NTDP)

Heat Stress. Heat Cramps. Heat Exhaustion.

Heatstroke…  Heat-Related Injury (HRI).

More than likely, you’ve heard these terms or possibly experienced symptoms yourself? At some point, we’ve all probably been on a hillside somewhere (listening to or) assisting with a medevac for a possible heat-related injury. If you’ve seen someone “go down” you are especially affiliated with the sense of surprise and panic that comes along with these sorts of injuries.

Each year crews discuss the signs and symptoms of HRIs and where you can find them in the IRPG, but let’s take a tactical pause to learn about what it is and what we know can increase the risk of HRIs, as well as mitigations to reduce the risk.

First off, what is a heat-related injury?

A heat-related injury (HRI) includes:

  • Heat edema (retention of water, swelling)
  • Cramps
  • Syncope (fainting episodes)
  • Heat Exhaustion
  • Heat Stress
  • Heatstroke

Symptoms occur when heat produced through physical activity or heat acquisition from the environment become higher than the body’s ability to dissipate that heat.

The following symptoms of an HRI may be difficult to recognize and could occur in no particular order (IRPG Pg 109-110): 

  • Profuse sweating with warm or cool, clammy skin leading to hot, dry skin
  • Muscle cramps and weakness
  • Dizziness, headache, and irritability
  • Rapid, weak pulse
  • Vomiting
  • Changes in mental status; as simple as not talking as much
  • Loss of consciousness


The Heat Stress Study

Beginning in 2011, the University of Montana and the National Technology and Development Program (NTDP, formerly known as MTDC) partnered to collect data from wildland firefighters (WLFF) in order to assess factors of heat stress as well as the physiological demands of the job.

In 2013,  the heat stress study began. A module of four field researchers (fellow wildland firefighters on detail to MTDC/NTDP) traveled around the country as the fire season progressed in order to collect data from firefighters (each subject volunteered to participate) during the performance of their duties.

HS Study Green Box

Prior to beginning the workday, participants swallowed a (multi-vitamin sized) capsule which contained a wireless thermometer that transmitted core body temperature readings to a heart rate (HR) monitor, which each subject wore around their chest.

Purple Pill

The “Purple Pill” (not the actual size, thankfully!) which contains a wireless thermometer to transmit core temperature.

The NTDP research module spent each day documenting the participant’s activities so that a correlation could be made between the activity performed and their core body temperature readings. In compliment, the HR monitor tracked skin temperature and respirations while a GPS device (attached to each participant’s pack) tracked distance and elevation gain, as well as monitoring ambient environmental conditions.

Knowledge Gained from the Study

Factors that Influence Internal Heat Production:

1. Fireline Packs are Heavy. This shouldn’t come as a surprise to any field going firefighter.

Pack Weight Body Weight ChartBreakdown of Pack Contents

Things to know:

The average total pack weight carried is 39 lbs., and the average percent of body weight carried is 21%. However, this is just the average pack weight, which does not account for the weight of other equipment such as fuel, tools, chainsaws, etc. Packs that exceed 25% of your body weight will add to the demand for the activity (Bastien 2005).

What does that mean? The activity becomes more challenging to accomplish due to your pack weight. It’s important to be conscientious of how much weight you and/or your people are packing.

The Equation to Remember:

Increased Pack Weight =

Increased Workload =

Increased Heat Production

2. Hiking with Weight is Associated with Increased Physiological Stress.

“Physical exertion is a primary indicator of challenges to thermoregulation in WLFFs.” (Cuddy 2015)

What does that mean?

Imagine that you and your crewmembers are geared up at the rigs and about to begin hiking into your work area. Each person essentially has their “base” core temperature to start the hike with. If the crew hiking pace is moderate and steady for the duration of the hike, then in most cases you could expect the elevation of core temperature to also be moderate and steady. However, if the crew’s hiking pace was fast and strenuous you could anticipate there to be a larger rise in core temperature.

The rise in core temperature matters because it is difficult to dissipate that generated heat when fully geared up in thick Nomex, line gear, helmets, etc. Essentially, the rise in core temperature that was experienced from the hike will become your new “base” for the day. If the shift proves to be very strenuous on a hot day with little downtime after a taxing hike to the worksite it will add a higher potential for a HRI.

Hot Hike Pictograph

Things to Know:

  • During the study, PT hikes accounted for the top 10% of the most arduous hikes that crews experienced.
  • Lesser fit individuals will be more impaired by increasing environmental temperatures (Cuddy 2013).
  • Your pre-season physical training better prepares you for adequate thermoregulation in changing environmental situations (Cuddy 2013/Sol 2018).
  • The higher the ambient temperature or relative humidity, the harder it is for the body to off-load heat.
  • High-stress situations can influence core temperature (medivacs).

 Action Items to Implement

Physical training. Pre-season preparation is crucial. A conditioned body is not only better equipped to do the work but it’s also able to better thermoregulate itself in adverse conditions. The beneficial gains come from aerobic conditioning that you do prior to the season (runs, weighted hikes, etc.) PT–it’s good for all the things.

Work/Rest.  “Work tasks contribute to increases in body temp. Both exertion levels and duration should be monitored to mitigate fatigue and heat strain” (Cuddy 2011; Sol 2018).

Nutrition 2

Strategic rest periods can do a body good. Taking a break can help dissipate heat as well as bring crewmembers back down to a resting heart rate. It may be difficult during a busy shift to feel that there is time to allow for breaks, but even during those shifts, it is possible to swap crewmembers in and out of arduous tasks to allow for some recovery.

Pack Weight. Pack weight matters. It’s important to recognize that ounces make pounds and even small, light items can add up to a lot of weight. The more you carry, the tougher your job gets. Therefore, it’s important to assess and reassess the items in your pack as the season progresses. How badly do you need to pack around that double female hose fitting “just in case”?

Monitor your Activity.  When the intensity picks up, make sure you’re paying attention to your body. When was the last time you had a snack, drank some water,  went to the bathroom?

Firefighters wearing sensors during their work shift for the Heat Stress Study.

Hydration. No water, no work, right? While focusing on hydration during a shift is important it is equally important to hydrate pre- and post-shift in order to stay “caught up.” Although hydration cannot fully eliminate the potential for a HRI, it sure does help the cause.

Nutrition. When the human body is taxed by heat it will prioritize its functions. Cooling itself and protecting vital organs will take precedence.  It’s important to focus on foods that are familiar to you, easily digestible, lower in fat, and containing carbohydrates and protein.

Trigger points.  We can identify some trigger points that make us take a step back and reassess our situation (i.e., temperature, RH, time of day, a spot on a map).  These can be utilized to pause and see where your crew is, how they are feeling physically, and to also establish reasonable expectations for the remainder of the shift.

The wildland fire community is chock-full of highly motivated individuals who almost consider it a sport to push themselves both mentally and physically.  This characteristic, prevalent among the many, is a point of pride but it can also have negative ramifications when we push ourselves too far. It’s important to remember to take care of ourselves and our people. It’s okay to not be okay, our mental and physical health should always be the top priority.

For a deeper dive, check out the studies below, or visit the National Technology and Development Page at: https://fsweb.wo.fs.fed.us/ntdp/program/fire-aviation-management/physiology-and-nutrition

Referenced Material:

  • Bastien GJ, Willems PA, Schepens B, Heglund NC. Effect of load and speed on the energetic cost of human walking. Eur J Appl Physiol. 2005;94(1-2):76-83.
  • Cuddy JS, Ruby BC. High work output combined with high ambient temperatures caused heat exhaustion in a wildland firefighter despite high fluid intake. Wilderness Environ Med 2011;22:122–5.
  • Cuddy JS, Buller M, Hailes WS, Ruby BC. Skin Temperature and Heart Rate Can Be Used to Estimate Physiological Strain During Exercise in the Heat in a Cohort of Fit and Unfit Males Mil Med. 2013 Jul;178(7):e841-7. doi: 10.7205/MILMED-D-12-00524.
  • Cuddy JS, et al. Supplemental feedings increase self-selected work output during wildfire suppression. Med Sci Sports Exerc, 2007. 39(6): p. 1004-12.
  • Cuddy JS, et al. Effects of an electrolyte additive on hydration and drinking behavior during wildfire suppression. Wilderness Environ Med, 2018. 19(3): p. 172-80.
  • Cuddy JS, et al. Work patterns dictate energy demands and thermal strain during wildland firefighting. Wilderness Environ Med, 2015. 26(2): p. 221-6.
  • Deming NJ, et al. Self-selected fluid volume and flavor strength does not alter fluid intake, body mass loss, or physiological strain during moderate-intensity exercise in the heat. J Therm Biol, 2020. 89: p. 102575.
  • Marks AN, et al. Total Energy Intake and Self-Selected Macronutrient Distribution During Wildland Fire Suppression. Wilderness Environ Med, 2020.
  • Montain SJ, et al. Efficacy of eat-on-move ration for sustaining physical activity, reaction time, and mood. Med Sci Sports Exerc, 2008. 40(11): p. 1970-6.
  • National Wildfire Coordination Group Incident Response Pocket Guide. https://www.nwcg.gov/sites/default/files/publications/pms461.pdf
  • Ruby BC, et al. Water turnover and changes in body composition during arduous wildfire suppression. Med Sci Sports Exerc, 2003. 35(10): p. 1760-5.
  • Sol JA, et al. Metabolic Demand of Hiking in Wildland Firefighting. Wilderness Environ Med, 2018. 29(3): p. 304-314.
  • West MR, et al. Risk for heat-related illness among wildland firefighters: job tasks and core body temperature change. Occup Environ Med, 2020.










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