Filed under: Heat
Walk into any fitness center on base or take note of a group of soldiers training, and you’ll probably notice at least a few people in form-fitting synthetic t-shirts. The sports apparel industry has exploded in popularity over the past decade, with numerous manufacturers now competing to develop, market, and sell the newest pieces of clothing (shirts, shorts, underwear, socks), all geared to keep athletes cool while competing or training in hot environments. Is there any science behind these claims? Does tight-fitting clothing made of “high-tech” materials actually help with heat regulation and enhance athletic performance?
You heat up when you exercise, and sweating is the primary method your body uses to stay cool. Sweat evaporating off your skin is the most important method your body has to cool itself during exercise. High-tech materials are supposed to enhance “wicking”—the delivery of sweat away from the skin surface toward the clothing, which allows for evaporation—and limit the absorption of sweat by the clothing itself. Cotton, by contrast, absorbs moisture, so it’s not considered a good choice for exercise.
To date, there’s no evidence that this high-tech clothing improves thermoregulation when worn during exercise in hot environments. Specifically, researchers found no differences in heart rate or body and skin temperatures when subjects performed repeated 20–30 minute bouts of running outfitted in shorts, sneakers, and either a form-fitting compression or traditional cotton t-shirt. Research has also found that wicking sportswear had no effect on cooling when worn under a bulletproof vest or on a cycling sprint when worn under full ice hockey protective equipment. As of now, the best advice for staying cool during exercise in the heat is to wear lightweight clothing, stay properly hydrated, and listen to your body for signs of potential heat illness. For more information on performing in hot environments, please visit the “Heat” section of HPRC’s Environment domain.
It’s important to get enough water, especially when it’s hot. However, too much water can lead to a dangerous condition known as hyponatremia in which the sodium levels in your blood drop too low. It’s often caused by drinking too much water and is common among military personnel, athletes, and hikers. Significant weight gain (due to fluid retention) during exercise can occur, along with longer finish times for endurance activities. If you have a Body Mass Index (BMI) below 20, you are more likely to develop this condition. For more in-depth information, read HPRC’s InfoReveal on over-hydration.
Sweat is a critical function when you’re performing in hot environments. As your body absorbs heat from the environment, your nervous system activates sweat glands to release sweat. The moisture on your skin then evaporates, taking heat away from your body and cooling you off.
Protective clothing impedes the evaporation of sweat and the heat exchange between you and the environment, a condition known as “evaporative resistance.” This means that the exposed parts of your body will cool off more quickly than the parts that are covered, but they are also more prone to insect bites. Reports from Marines and National Park employees feeling “excessive heat” and a loss of sweating sensations after applying moderate to high amounts of DEET to their skin brought the safety of this insect repellent into question.
In a recent study, researchers found that when 33% DEET lotion is applied according to military protocol, it does not interfere with sweat production or other physiological responses. Nor does it interfere with the evaporation process necessary for cooling to take place. Researchers concluded that 33% DEET can be worn safely during military and occupational activities performed in hot, insect-infested environments. Similar studies have found oil- or alcohol-based repellents may increase core temperature by reducing sweat evaporation rate but do not affect sweat production. The military-approved form of DEET is polymer-based.
You can watch a YouTube video about the science behind the study.
DEET is considered by the EPA to be a toxic pesticide. It should be used with caution and as directed. More information about DEET, its uses, and warnings can be found on the EPA fact sheet. As of 2004, DEET was considered safe for use on children older than two months of age. Specific information on its use and effect on children can be found in the EPA TEACH chemical summary.
Approximately 300 million people around the world have sickle cell trait (SCT), including approximately 9% of African Americans. It is a hereditary condition in which red blood cells are affected, but most people who have it never experience symptoms. (It is important to note that SCT is not the same as sickle cell disease [SCD]. Sickle cell disease [or sickle cell anemia] can lead to other serious clinical risks and can cause severe symptoms. Those with the SCD usually have a shorter lifespan.)
Individuals with SCT usually can participate in normal physical activity and sports, as SCT doesn’t seem to adversely affect performance. In fact, some studies have found that those with SCT excel in short-distance power activities such as sprinting and jumping.
While SCT is largely a benign condition, there have been related complications such as exertional rhabdomyolysis and exercise-related sudden death. They have been found in non-SCT individuals as well, but they occur at higher rates in those with SCT and are a “hot topic” in military, and civilian communities; the National Collegiate Athletic Association even requires screening for all its Division I and II athletes.
It has been suggested that those with SCT may be more prone to sudden death from dehydration, heat illness, and high-intensity exercise; however, these factors and the role of prevention standards, medications, and the use of dietary supplements are still being studied. In both military and civilian SCT populations, collapsing during exercise is most commonly observed during times runs and sprints within the first few weeks of conditioning. SCT Recruits who have difficulty passing the Physical Readiness Test are also at higher risk for collapse. Military leaders should be aware of safe training guidelines and take universal precautions. Effective prevention tactics include heat acclimatization, hydration, gradual physical conditioning, and addressing progressively worse symptoms early on.
All newborns in the United States are screened for both SCT and SCD as part of a public health imperative. Each military branch has its own policies regarding SCT. The Army does not screen for SCT but promotes universal precautions for all soldiers, whereas the Air Force, Navy, and Marines all screen for SCT after accession. Further testing and counseling may be done for those who are positive for SCT. If you are unsure about SCT and exercise, consult with your physician, especially if you are starting a new exercise routine.
Face paint has been used for many decades to blend the appearance of Warfighters’ exposed skin into their environments and protect them from the enemy. The American Chemical Society is taking a new approach to the traditional camouflage face paint by making it from a material that also can provide some protection from the heat wave of roadside bombs, IEDs, and other explosions on the battlefield. Thermal blasts last only a few seconds, but can cook the face, hands, and other exposed skin. The new face paint will protect exposed skin against temperatures reaching around 600 degrees Fahrenheit, for up to 60 seconds. The paint even incorporates the insect repellent DEET in a form that will not catch fire.
This new face paint is still in the testing stages, but already there are plans for a colorless form for use by men and women in other occupations—such as firefighters and other emergency responders—who are at risk of extreme heat exposure.
Heat illness is a hot topic for the military. Did you know there is a spectrum of conditions that fall under the term “heat illness,” some more severe than others? HPRC has great resources on how to prepare for exposure to hot environments and how to prevent heat illness. Read HPRC’s Answer to “What IS heat illness?” for more about what heat illness is and how to identify the signs that you might be developing more serious conditions.
With the summer weather here to stay for a few months, HPRC wants to remind you of the dangers of heat illness and the importance of staying hydrated. This information can relate to any outdoor activity such as exercising, hiking, bike riding, or playing in the park.
HPRC has tips on preventing heat-related illness and various guidelines for avoiding heat injuries. Hydration is an important factor for keeping you and your loved ones happy and healthy. Children need to be careful as well since they seem to have an infinite amount of energy while playing outside. In addition to water, sports drinks can also be beneficial. Keep this information in mind while you are out and about with your friends, family, and pets. Happy Summer!