Cooling Vests and Dehydration
In research literature, cooling vests like the Qore Performance ICEPLATE® and products like ICEFLASK are classified as “phase change material” (PCM) microclimate systems. In this context, PCM simply means cooling power is provided when a substance (e.g., ice) changes from a solid to a liquid, and “microclimate” is a pseudonym for “personal device”. Research has consistently found microclimate cooling “an effective measure to alleviate heat stress and improve human performance in the military, firefighting, and sports fields” and PCM systems specifically are “applicable for the majority of occupational workers” (quotes from Chan et al).
Phase Change Physics
A phase change is the process that occurs when a substance goes from one state of matter to another, such as solid ice melting to liquid water or liquid water boiling to steam (a gas). Qore Performance products use ice as a phase change material, but other materials, like paraffin wax, can be used. In wax-based cooling products, the phase change occurs when the wax melts; like ice, the wax changes from a solid to a liquid as it draws in heat. Wax-based materials have the advantage of freezing at higher temperatures than water, making it (theoretically) easier to refreeze after use. In practice, this consideration is largely irrelevant- access to any freezer (nominal temperature, 0oF) is plenty sufficient to refreeze the water inside an ICEPLATE® Curve or ICEFLASK, and replacement frozen ICEPLATE® Curves can be stored in coolers and changed out during longer operations.
Wax and other non-ice phase change materials have significant disadvantages compared to ice. Understanding why requires a brief explanation of phase change physics. “Heat of fusion” refers to the quantity of heat needed to melt a solid. Water has a high heat of fusion- 79.7 calories per gram (334 joules per gram). Water’s heat of fusion is especially high compared to wax-based materials, which commonly have a 30% lower heat of fusion (around 55 calories, or 230 Joules, per gram).
During phase changes, the temperature of the melting material doesn’t change, e.g., applying 79.7 calories to one gram of 32oF ice makes one gram of 32oF water. Changing the temperature of solid or liquid water without changing its physical state requires much less heat. The amount of heat required warm ice by 1.0oC (1.8oF), a value known as the “specific heat” of ice, is much smaller than the heat of fusion (it takes just 0.5 calories to raise one gram of ice by 1.8oF), and the specific heat of liquid water is just 1 calorie per gram. For example, the amount of heat needed to turn one gram of 32oF ice into one gram of 32oF water would, if applied to already liquid water, increase the water temperature to nearly 79.7oC (about 144oF).
Why does this matter? For PCM cooling vests, only minor cooling is provided as ice warms to the melting temperature or by additional warming of cold water after all ice has melted. Nearly all heat absorption by PCM cooling vests occurs during phase change at melting. For practical purposes, this means systems using PCM should be replaced when completely melted for maximum cooling benefit.
Qore Performance ice-based system has another distinct advantage over many other personal cooling products: rehydration. As ice melts, the wearer has a supply of cold drinking water, but easy access to water for rehydration is only part of the benefit. Since personal cooling devices help keep core temperature from increasing, sweating rates are reduced. The wearer dehydrates slower and needs to consume less water in a positive reinforcement cycle.
Dehydration and Personal Cooling
The remainder of this article focuses on research into sweat rates when wearing personal cooling devices. In many studies, wearing a PCM vest reduces sweat rates by over 20%. Beyond simply avoiding dehydration, less sweating reduces cardiovascular strain by, among other things, helping maintain blood volume and reducing the need to redirect blood flow from organs and muscles to the skin surface for thermoregulation.
Studying the Effects of Cooling Vests
Human studies on the effectiveness of personal cooling vests typically involve a study population subjected to a physically strenuous test in a controlled, hot environment with fixed temperature and humidity (theoretical studies based on temperature and the heat of fusion, or studies using thermal manikins, are also common).
Physical task protocols vary among studies. Walking on a treadmill is common, although levels of intensity differ between studies. Some tests incorporate breaks between rounds of effort to simulate work-rest cycles. Stationary bikes are also commonly used, and some tests designed to mimic real-world work (e.g., firefighting) include lifting boxes or repeatedly stepping up on an elevated block. A core temperature safety threshold of 102.2oF (39oC) is commonly used to prematurely stop testing to avoid heat stroke, a distinct possibility in the more intense study protocols, especially during control testing without cooling vests.
In addition to performing physical tasks, tests often include some type of occupational clothing. These range from light athletic shirts and shorts, to standard military or law enforcement uniforms and, at the extreme end of the spectrum, firefighting equipment or chemical, biological, and radiological suits. Testing under various levels of clothing allows researchers to study the effects of cooling vests under simulated real-world conditions and as a proxy for wearing protective gear that severely restrict heat loss.
To measure the effect of cooling vests on dehydration rates, most studies weigh each individual before and after each test; any reduction in body weight is assumed to occur because of sweat loss. In test that allow subjects to consume water, the amount of water intake is considered.
Study Results
During physical effort, sweat rates of 1.5 liters per hour (L/hr) are common, and rates can exceed 2.0 L/hr during extreme effort in hot environments. Studies on PCM cooling vests commonly find sweat rate reductions on the order of 20-30%. One article summarizing several studies found PCM cooling vests reduced sweat rates by an average of 0.24 liters per hour; for context, this amount of sweat “saved” by wearing a PCM cooling vest is about a quarter of a 1-quart canteen, or about half the volume of an ICEFLASK, every hour.
Individual studies have considered PCM cooling vests under military-specific occupational clothing. One of the most thermally stressful scenarios is working under chemical protective suits; researchers note “working in such [equipment] can only be tolerated for short durations without supplementary cooling”. Addressing this challenge, subjects testing PCM cooling systems often wear chemical suits and similar equipment that severely restrict heat loss. One such study found a reduction in sweat rate of 0.2 L/hr, in line with values provided in the previously mentioned review article but noteworthy given the intense physical tests performed (in this particular study, all test subjects reached endurance limits or were stopped for safety reasons before the three-hour test protocol ended).
Perhaps more practically, a U.S. Navy study of subjects wearing firefighting equipment found wearing a cooling vest results in significantly lower water intake. The test included 40 minutes of treadmill exercise in full firefighting equipment, including breathing apparatus. When wearing a cooling vest, subjects drank 27% less water compared with control trials without cooling vests (on average, 0.352 liters less, or about 40% the volume of a canteen). Sweat rates averaged 20% lower when wearing cooling vests. A similar Canadian study of cooling vests under firefighting equipment found a 26% decrease in sweat rates and a 0.53 liter reduction in water intake during a test protocol where subjects stepped onto and off a box every 5 seconds for 45 minutes, followed by a 45-minute rest period wearing the equipment.
Greater reductions in sweat rates appear during lighter working efforts over longer periods. One study, again using Navy personnel but wearing only utility work uniform, included a 6-hour treadmill walk at various high temperature and humidity combinations. Sweat rate reductions of 49% were observed in the “coolest” (111oF) test, and, notably, all 14 subjects completed the test protocol when wearing cooling vests, while only 5 completed the control test without a vest. At higher temperatures (one test was conducted at 134oF), sweat rate reduction wearing a vest was measured at 38%.
Cooling vests can also reduce sweat rates even if only worn for pre-cooling and during recovery. Pre-cooling involves reducing body core temperature before exercise, providing more “room” for the body to store heat before performance is degraded. Since sweating is thought to be partly triggered when internal temperature reaches a set threshold, pre-cooling means a longer period of exercise before heavy sweating begins. (Pre-cooling as a field of study usually focuses on athletes seeking a competitive edge.) For example, pre-cooling the body from a normal 98.6oF to around 97oF provides an additional 1.6oF of temperature range available for internal heat storage. A study considering PCM cooling vests worn during stretching, warmup, and recovery, but not during a 30-minute run, found subjects sweated 10-23% less and were able to hold a near-maximum (95% of VO2 max) run effort slightly longer.
Limits of cooling vests
There are limits on cooling vest performance. In the most extreme studies, cooling vests (and specifically PCM cooling vests) have not been found highly effective. These studies tend to have two things in common- extremely difficult physical tasks that generate large amounts of metabolic heat and occupational clothing that severely restricts heat transfer away from the body. Simply put, in extreme situations, it is possible to overwhelm the cooling power of a PCM vest.
One great example of PCM cooling vest limits is a 1991 Canadian Defence and Civil Institute of Environmental Medicine report. The test protocol involved a treadmill walk at steep incline followed by rounds of lifting and moving weighted boxes for an hour while wearing a chemical suit. While some reduction to sweat rate was measured, the study authors concluded cooling vests in such an extreme scenario were ineffective, stating they were “probably not able to remove enough heat from the body at the work rates used in this experiment, especially when combined with the increased heat entrapment provided by the chemical protective suit”. While surely accurate, an hour of intense physical effort in hot conditions while wearing a chemical suit must be about the most extreme thermal protocol possible!
Other studies conducted under extreme conditions have found slightly more favorable results. In another study, PCM cooling vests were worn under nuclear, biological, and chemical suits during a 2-hour treadmill test at moderate incline in 103oF temperature. Cooling vests were found to increase performance, albeit to a modest degree. On average, test subjects couldn’t sustain the effort for the entire two hours even if they wore cooling vests, but test subjects wearing vests were able to continue about 12 minutes longer.
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About the author: Dr. Erik Patton holds a PhD from Duke University where he conducted research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120oF) to Arctic conditions in central Alaska (-42oF).
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