Filed under: Heat
Can you train in the heat to improve your performance at altitude? The answer is “sort of.” “Cross acclimation” or “cross tolerance” is the idea that exposing yourself to one environmental condition can help you adapt to another one as long as they have certain things in common.
As it turns out, this is the case for heat and hypoxia (low oxygen). This is important because athletes and service members can be exposed to altitude without prior or sufficient acclimatization. Altitude sickness can cause several problems, especially decreased performance. But some evidence shows that this method of training in hot conditions to prepare for altitude can actually work.
If you climb to the top of a mountain, there’s less air and less pressure. And you’re getting less oxygen with each breath. This can be simulated at sea level (in special labs) where pressures are normal, but the amount of oxygen in the air is reduced (fake altitude).
However, there’s a bit of a catch. Training in the heat under artificial low-oxygen conditions—normobaric hypoxia or “fake altitude”—involves normal pressure, which is different from “real altitude” or hypobaric hypoxia, which involves reduced oxygen at low pressure. The difference is in the pressure.
So, do these two environments cause the same types of physiological changes? There are several other factors involved in real-altitude acclimatization that might not be accounted for at fake altitude, so the jury’s still out.
Training in the heat might prepare you for performance at altitude—to a point. Ideally, if you’re going to be at altitude, try to acclimatize yourself as much as you can.
Skin cancer is the most common form of cancer in the U.S., but with proper precautions you can decrease your risk considerably. The sun releases invisible ultraviolet (UV) radiation, which can cause cataracts (clouding of the eye lens) and skin cancers. An estimated 63,000 new cases and 9,000 reported deaths from melanoma—the deadliest form of skin cancer—occur each year.
UV rays also cause tanning and sunburns—and can damage your skin after only 15 minutes of exposure. They weaken the skin’s elasticity, causing wrinkling, rash, and freckles too.
Remember that you can get sun damage on sunny and cloudy days. UV rays penetrate clouds, exposing you to 80% of the sun’s harmful effects. The good news is that you can take steps to protect yourself from UV rays, while enjoying the outdoors.
- Limit your time in the sun. Seek shade and try to avoid sun exposure during midday (10 a.m. to 2 p.m.) when the rays are strongest. And avoid suntanning and burning.
- Cover up. Wear protective clothing, including hats, long-sleeved shirts, and pants when going outdoors. Remember that protection decreases when clothes are wet.
- Apply sunscreen. Use water-resistant sunscreen with Sun Protection Factor (SPF) 15 or higher. Apply and let it absorb 15–30 minutes before heading outdoors. Use lip balm with SPF 30 or higher to protect your lips too. Reapply every 2 hours or after swimming, sweating, or toweling off.
- Protect your eyes. Wear sunglasses to cover the skin around your eyes and help prevent eye damage. When choosing sunglasses, check the label to make sure they block 100% of UV rays.
Exercising outside on hot summer days when the heat and humidity seem unbearable may be more harmful than helpful. Pollutants in the air (such as carbon monoxide, particulate matter, and ozone) can inflame your respiratory system more than usual, because on hot days you’re more likely to breathe faster and deeper and through your mouth (bypassing your nose’s natural filtration system).
However, the risks associated with not exercising at all are far greater than the risks of exercising outdoors. It may just take a little more planning on days and in conditions when pollution is bad. When planning outdoor exercise activities, follow these tips to limit your exposure to pollutants.
- Avoid exercising in areas of heavy traffic, such as along highways and during rush hour.
- During summer, exercise earlier in the morning, when ozone levels and temperatures are not as high.
- Check the domestic or international air-quality ratings to determine if it’s safe to exercise outside. Limit your time outside on Code Red and Code Orange days. Environmental conditions on these days are not healthy, especially for children, the elderly, and people with existing respiratory conditions.
- Exercise indoors when the air quality indicates high ozone and particulate levels.
- Before any demanding physical activity, limit your carbon monoxide exposure by avoiding smoky areas and long car rides in congested traffic.
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.