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Thermal Stress: More than Core Temperature

Updated: Dec 12, 2022

SUMMARY. Thermal stress in the workplace is increasing and core body temperature sensors to measure and mitigate this risk are inadequate. Multi-dimensional approach that measures the psycho-physiological changes that occur as the body attempts to compensate for temperature changes is a more predictive and robust method to mitigate thermal stress. This paper describes the challenges with only measuring core body temperature, the science of thermal stress, impacts on the worker, and the novel solution developed and validated by GoX Labs.

Why Core Temperature is Not Enough

Razor thin margin between fit to failure. Our bodies are highly sensitive to changes in our core temperature; a 10 C change in our core temperature increases our risk for heat exhaustion. As you can imagine, the time between “fit to failure” is razor thin, having a single measure, usually a body sensor to determine a thermal stress is very challenging given sensor inaccuracy. Therefore, many of these types of solutions have very high false positives and workers ignore the alerts. As you can see, using a single body surface sensor to detect thermal stress is insufficient and generally too late!

Large individual differences. Furthermore, a single measure of core temperature is unreliable as there is a lot of variability from one worker to the next. For instance, an 8-hour march in aerobically trained, heat acclimatized, and fully hydrated soldiers increased core temperatures that varied by as much as 50 C. Using rectal temperatures, the most accurate yet impractical method, direct measures ranged from 39.9° C to 42.2° C. Even when directly measured, this single index misses a whole range of important cues that can keep your workers safe.

Core Temperature is

a Lagging Indicator. Core body temperature increases as the body’s ability to cool is overmatched by its’ heating. There are many factors that cause this mismatch to happen. Measuring these factors, which are leading indicators, are the best way to predict the future onset of thermal stress and have the time to mitigate these risks. First is sweat loss, then decrease in efficiency to do work which increases VO2, heart rate increases to maintain output given dwindling blood volume, and then finally core body temperature increases as the body is unable to cool itself effectively.

What’s the Science

Understanding Body Heat Build-up. The effectiveness of the human body’s ability to thermoregulate and defend the core temperature is influenced by the individuals’ acclimatization state, their aerobic fitness, and their hydration status. When we begin to work, our muscles contract and the energy required to complete our task can increase by as much as 5 to 15 times above our resting rate. And depending on the activity, workers can increase their energy metabolism by 70 to 100 percent; this needs to be released as heat to maintain their core temperature (Sawka, M.N. et al.). Aerobically fit workers who are heat acclimatized and fully hydrated have less body heat storage and can perform work safely for limited periods during work-heat stress.

Impacts on the Body. Our bodies are only capable of clearing 1.2 liters of water per hour which is no match to a possible 6-liter water loss in an hour. Keeping ahead of this requires frequent reminders as our thirst response only kicks in when we have already lost 2% of our body mass loss (BML). This doesn’t sound like much but already our body is working harder even though the activity remains unchanged. At 2% BML, work of the heart has increased by 9% and skin blood flow has decreased by 11%; this can lead to a dangerous increase in core temperature. Remember, our body gets rid of heat by widening our blood vessels and bringing the hot blood to the skin surface. As we become dehydrated, our body has to make a decision to either increase blood pressure to keep blood to the head or open the blood vessels and drop blood pressure to the skin to remove heat. The body will choose to protect the brain first and this is how thermal stress can quickly become dangerous. At 4% BML, which is just as common in our workforce, we experience over 16% increase in heart rate and a 33% decrease in skin blood flow (Coyle). Our ability to sweat is declining as the blood remains in the core leading to further risk of serious heat stress.

Impacts on the Brain. Heat stress does not only impact our ability to sustain our work rate but can also impact our cognitive performance. We begin to loss the capacity to make accurate decisions, our balance becomes challenged, and our awareness of our surroundings too may be inaccurate. That same 2% body mass loss increases worker fatigue, alertness and even memory. Increasing levels of dehydration only exaggerate these symptoms and can increase worker injury (Hyllier).

GoX Lab’s Solution

We use a Samsung smart watch to measure multiple physiological and environmental factors to predict the potential of thermal stress so that the interventions have the time to eliminate the risk of injury. We used our independently validated total body sweat loss model to determine the workers sweat loss during the shift. We measured VO2 and HR to assess the current work-heat stress level. Finally, we either measured the daily peak ambient temperature or the outside peak

heat index to understand future risk imposed by the environment. By combining these risk factors, we estimated the past, present, and future risk to the worker. Hydration, nutrition, rest, and cooling strategies are individualized and implemented before it is too late for your workforce.

Why you should trust GoX Lab’s Science

Independent Validation. Samsung tested 50 Korean men and women between the ages of 16 – 60 years old with ambient temperatures ranging between 50 C to 350C. Using a multifaceted approach, GoX predicted with 90% accuracy the total BML of everyone to an average of a cup of sweat, outperforming even Samsung’s own analytics! Using these leading physiological and environmental indicators, GoX Labs assures early detection of the risk for thermal stress.

Tested in the Field. In one study with 20 construction workers in the summer heat, GoX Labs was able to identify and mitigate thermal stress by 60%. Average heart rate decreased from 107 bpm to 93 bpm, metabolic cost measured with volume of oxygen consumed (VO2) decreased from 10.3 ml/kg/min to 9.8 ml/kg/min all the while the average high-low temperature increased from 80oF to 89oF. The sensitivity and accuracy to detect thermal stress across a diverse group of individuals and environments is unique and provides a tailored solution for each unique worker requirements.

Please refer to issued utility patents:



  • Patent No. 11229407, Allowed and to be Issued Jan 25, 2022, SYSTEMS AND METHODS TO DETERMINE A RISK FACTOR RELATED TO DEHYDRATION AND THERMAL STRESS OF A SUBJECT, Utility Patent, Original

Download the original document below.

Thermal Stress & Core Temperature 010622
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