Hypothermia and Running

Contributed by Dr. Steven T. Devor – Director of Performance Physiology for MIT and OhioHealth, and Associate Professor of Exercise Physiology, Department of Human Sciences, and Department of Physiology and Cell Biology, The Ohio State University

In central Ohio the first weeks of early spring in late March and the end of autumn at the beginning of December are generally transitional times when it comes to our weather, and the potential for rainy, windy, and cold conditions are high. Completing a training run or racing in these sorts of environmental conditions can not only profoundly influence your performance, but also carries the potential for significant health risks. And one of the most medically dangerous conditions we must all be aware of in cold, windy, and wet conditions is hypothermia, which is generally defined as low body temperature.  

Although most of us would be surprised to learn this, the fact is hypothermia can occur in somewhat warm environmental conditions. Indeed, hypothermia occurs as often in the summer as it does in winter, and the persistent misunderstanding that hypothermia is exclusively related to colder weather often results in runners being ill prepared and susceptible to serious health problems.

When you become hypothermic your ability to rationally reason through a situation and make appropriate decisions becomes impaired, which frequently results in even greater risks to your health. Thus the old axiom that “the first casualty of hypothermia is good judgment" rings particularly true. Making non-prudent decisions when running can be very dangerous when there is vehicle traffic in the area or when trying to navigate technical and difficult off road trails. Clinical research indicates that between 20% and 50% of the deaths due to hypothermia involve the victim removing their wet clothes even though they are cold, a symptom of poor judgment directly related to severe hypothermia.

The clear risk factors for hypothermia in runners include:

1. Air Temperature. This is perhaps the most obvious factor of them all, but it can also be one of the most deceiving. Hypothermia can and does occur at temperatures as high as 50 degrees Fahrenheit, and can even occur at warmer temperatures when other additional environmental conditions are correct;
2. Precipitation. Wet snow, sleet, or rain will all make you wet, but steady heavy rain will immediately drop the temperature of your skin. And being caught in an extremely heavy rain shower rain can rapidly trigger hypothermia due to the potential for a quick drop in overall body temperature;
3. Evaporation. When water evaporates, it cools your body making the temperature that your body perceives much colder than what the real temperature actually is. Water and sweat soaked running clothes also lose their ability to insulate, and cotton is particularly poor in this regard. You should always choose high quality technical fabrics for your running attire. Wear technical clothing that can be easily layered and adjusted by opening zippers. Yes, well made sophisticated technical running clothing is much more expensive than cotton workout clothing. But it has a much longer lifespan and is critical to perform at your best and protect your health;
4. Wind speed. The faster the wind is blowing the more rapid evaporative cooling occurs and subsequently the heat loss from your body. And if you are also wet from rain, sweating, or both, the evaporative cooling will be accelerated. Even when running in conditions where the breeze is calm there is still air movement across your body that results in evaporation;
5. Slower running paces. The high physical exertion of an activity like running results in your body generating a good deal of heat. However, runners whose paces are naturally slower do not generate as much internal metabolic heat as faster runners. So when training or racing distances become longer the time spent running increases, and if the environmental conditions are cold, wet, and windy the likelihood for hypothermia increases;
6. Physical depletion. If your energy is low or you are beginning to feel exhausted during a run your body will begin to struggle to keep itself warm. Accordingly the longer a training run or race the greater risk there is for initiation of hypothermia; and,
7. Sunset. Hypothermia risk rises when the heat of the sun is lost, as the environmental temperature drops the fastest at dusk. This can be a particular concern during an ultra-marathon event as many participants are still in the throes of their race when the sun sets.

When someone in your running group begins to suffer from hypothermia their initial symptoms are likely going to be slurred or impaired speech and a loss of coordination. When out running in cold, wet, and windy conditions be aware of how your partners are speaking and how their running form looks. It is not uncommon for one person in a group to be fine while another is approaching the initiation of hypothermia. The response to environmental conditions is different for everyone, and some individuals are simply more susceptible to hypothermia.

Medical experts agree that shivering is one of the first signs of hypothermia that most people notice. When you are shivering during a run, but are able to concentrate and make it stop, your core temperature has already dropped to approximately 96 – 97 degrees Fahrenheit, which is the definition of mild hypothermia (98.6 degrees Fahrenheit is normal body temperature). At this point your coordination begins to become impaired, and this can make technical trail running more difficult. Also at this point your decision making skills will start to deteriorate and therefore the potential for other injuries rises. Seeking warmth at this stage is advised, as more severe levels of hypothermia will come on quickly.

When shivering is unable to be stopped, even with a focused and deliberate effort, you are likely suffering from moderate hypothermia. At this point your body temperature has dropped down to between 91 and 95 degrees Fahrenheit. Individuals often appear to have a pale look to them and their lips, outer ears, and fingers may even develop a bluish tinge. Due to a loss of coordination your partners running form will likely become impaired. Seeking medical attention at this stage is very important, and attempting to find warm and dry conditions immediately is advised. At this point the situation is dangerous.

One the best defenses you have against hypothermia is dressing appropriately when going out for a run, and layering your running clothing is key. Wearing several light layers of clothing helps to insulate your body due to the air that is held between each layer. Layering is most important for your upper body.

Your base layer should be made from a snug fitting wicking fabric in order to draw sweat away from your skin. Technical wicking fabrics are always preferred over cotton when it comes to athletic clothing. Also be aware of seams in base layer clothing that may be in places where chafing could occur. The second layer of clothing should be long sleeved and also be made from lightweight technical wicking fabrics. When the temperature is not severe, this long sleeved shirt may serve as your outer layer. When the temperature dictates, a water resistant and windproof jacket that allows moisture to escape should be the outer layer. This garment should be somewhat looser fitting. Finally, a hat and gloves or mittens are absolutely essential clothing pieces, as these areas of your body loose large amounts of heat in cold, wet, and windy conditions.  

For your lower body I advise running tights or lightweight training pants made of water resistant material. For the coldest winter month training runs a slightly thicker tight is potentially necessary. Your legs do not sweat nearly as much as your upper body, so one layer is almost always sufficient for an outdoor activity like running. If you should elect to wear shorts, once again chose wicking technical fabrics that will transfer sweat away from your skin and that will and prevent chafing.

With regard to your feet, the amount of thought and technology that has been directed at the development of running socks over the last ten years has created some very useful products. Keeping your feet warm and dry in the colder weather is very important. My advice is to choose socks that are constructed of a wicking material to keep sweat away from your skin. Additionally, look for a sock that has a reinforced heel and toe region and one with a toe box that has a hand sewn closure to reduce seam size to aid in the prevention of blisters.

In summary, running and racing in cold, windy, and wet conditions can pose health challenges and is always more physically and mentally demanding. In order to minimize the risks to your health, and be able to enjoy your run or race, wear appropriate cold weather technical clothing and adjust your expectations consistent with the weather conditions. If you should develop symptoms of hypothermia, inform others in your training group, seek medical attention immediately, and change into warm and dry clothing as soon as possible.