Under normal, healthy conditions, the body maintains a relatively stable temperature of around 98.6 degrees Fahrenheit (ºF) or 37 degrees Celsius (ºC). If the environment gets too cold or the body's heat production decreases, the core temperature can drop, and hypothermia can develop.
Between 2003 and 2013, over 13,400 people died from hypothermia in the United States, according to the Centers for Disease Control and Prevention (CDC).
What is hypothermia?
Hypothermia can quickly become an emergency.
Body temperature is controlled in a part of the brain called the hypothalamus. The hypothalamus recognizes changes in body temperature and responds to bring it back in line.
The body produces heat during normal metabolic processes, in cells that support vital body functions. Most heat is lost from the skin's surface through convection, conduction, radiation, and evaporation.
If the environment gets colder, the body may need to generate more heat by shivering. The increase in muscle activity promotes heat formation. But, if heat loss is greater than the body's ability to make more heat, the core temperature will fall.
As the temperature falls, the body shunts blood away from the skin to minimize heat lost to the environment. It directs blood flow to the vital organs of the body including the heart, lungs, kidney, and brain. The heart and brain are most sensitive to cold, and electrical activity in these organs slows down in response to cold.
If the body temperature keeps falling, the organs begin to fail, and, eventually, death will occur.
Hypothermia is the opposite of hyperthermia, which is present in heat exhaustion and heat stroke.
As hypothermia sets in, the ability to think and move, and therefore take preventive action, starts to decline.
Symptoms of mild hypothermia include:
- hunger and nausea
- increased breathing
- difficulty speaking
- lack of coordination
- increase in heart rate
Symptoms of moderate to severe hypothermia include:
- shivering, but importantly, as hypothermia worsens, shivering stops
- worsening coordination difficulties
- slurred speech
- significant confusion
- apathy or lack of concern
- weak pulse
- shallow, slow breathing
The body starts to slow as the temperature drops. Aside from feeling cold and shivering, hypothermia affects thinking and reasoning. As a result, it may go unnoticed.
The person is at risk of lying down, falling asleep, and dying. In some cases, the individual will paradoxically remove their clothes just before this occurs.
Infants lose body heat more easily than adults, and they cannot shiver to keep warm.
They may have:
- bright red skin
- cold skin
- very low energy
- a weak cry
Infants should not sleep in a cold room, even with extra blankets, as there is a risk of smothering. The CDC suggests making alternative arrangements if a warm space cannot be maintained for an infant to sleep in.
Treatment depends on the degree of hypothermia. This can range from noninvasive, passive external warming, to active external rewarming, to active core rewarming.
Passive external rewarming uses a person's own heat-generating ability. It involves wearing properly insulated, dry clothing and moving to a warm environment.
Active external rewarming consists of applying warming devices externally, such as hot water bottles or warmed forced air. In cold environments, this may be done by placing a hot water bottle in both armpits.
Active core rewarming uses warmed, intravenous fluids to irrigate body cavities, including the thorax, peritoneum, stomach, or bladder. Other options are to inhale warm, humidified air, or to apply extracorporeal rewarming by using a heart-lung machine.
Anyone with symptoms of hypothermia needs immediate medical assistance.
Until medical help arrives, the following can help:
- moving the person to a warm, dry place, if possible, or sheltering them from the elements
- removing wet clothing, cutting items away if necessary
- covering the whole body and head with blankets, leaving only the face clear
- putting the individual on a blanket to insulate them from the ground
- monitoring breathing, and carrying out CPR if breathing stops
- provide skin-to-skin contact, if possible, by removing clothing and wrapping yourself and the individual in the blanket to transfer heat
- providing warm drinks, if the individual is conscious, but no alcohol or caffeine
Direct heat, such as heat lamps or hot water, must not be used as this can damage the skin. Worse, it may cause irregular heartbeats and, potentially, a cardiac arrest.
Rubbing or massaging should be avoided, as any jarring movement could lead to a cardiac arrest.
Hypothermia generally progresses in three stages: Mild, moderate, and severe.
Mild symptoms might include feelings of hunger and nausea, giving way to apathy as the core body temperature drops. Confusion, lethargy, slurred speech, loss of consciousness, and coma can follow.
The decrease in brain function happens as the body temperature drops, so, the colder the body, the less effectively the brain will function. Brain function stops at a core temperature of 68ºF or 20ºC.
Abnormal electrical rhythms may affect the heart as hypothermia progresses through its stages. Ventricular fibrillation, a disorganized rhythm in which the heart is unable to pump, may occur at core temperatures below 82.4ºF or 28ºC. This is a type of cardiac arrest.
A phenomenon known as "paradoxical undressing" can happen, where the person removes their clothes, despite the cold. This can happen during later stages of hypothermia, as the person becomes disoriented, confused, and combative. As they remove their clothing, the rate of heat loss increases. It can be fatal.
Hypothermia is a devastating and potentially avoidable condition. Education and preparedness are the cornerstones of prevention.
People who work or sleep outside have a higher risk of hypothermia.
People are at higher risk if they:
- work outdoors in cold weather
- practice snowsports, watersports, and other types of outdoor activities
- are at home during winter weather, especially older people
- are stranded in a vehicle in severe winter conditions
- sleep rough
- have other medical conditions
- use alcohol or illicit drugs
To prepare for hypothermia indoors, the CDC recommends creating a winter survival kit, with nonperishable food, blankets, a first aid kit, water, and necessary medications.
Other measures, such as home insulation, are important, especially for older adults.
Anyone who becomes stranded in a motor vehicle should move everything they need into the vehicle, and run the car for 10 minutes every hour, making sure the exhaust pipe is not covered by snow, and keeping the window open a crack to prevent a buildup of fumes.
When preparing for the outdoors, it is important to check weather conditions in advance.
Multiple layers of clothing should be worn, with the innermost layers made of wool, silk, or polypropylene, because these materials retain heat better than cotton.
Layered clothing traps multiple layers of air, minimizing heat loss. Wearing a hat or heavy scarf on the head, even indoors, also helps to reduce heat loss.
People who expect to be in the cold for a long time should avoid alcohol. It is also important to eat enough, as additional fat under the skin can protect against cold during a winter weather spell.
Overexertion should be minimized, as this can lead to a combination of exhaustion and sweat-drenched clothing, and this can cause the body to lose excess heat.
A person who begins to experience or show signs of mild hypothermia should retreat to a warmer place immediately to prevent progression to a life-threatening condition.
It is worth remembering that hypothermia can happen in summer too. Excessively cool air conditioning or water-based activities pose a risk especially for infants and older people, who may not be able to express how they feel.
The NIH suggests keeping room temperatures at 68ºF or above, or 20ºC, and closing off rooms that are not in use.
Taking a person's temperature with a thermometer can show whether they are experiencing hypothermia.
If the temperature appears to be below 96ºF or 35.5ºC, or if an oral thermometer is not showing the temperature, a rectal thermometer can give a more exact reading.
If the thermometer still does not show the temperature, or if it is below 96ºF or 35.5ºC, urgent medical attention is needed.
The only way to tell accurately if a person has hypothermia is to use a special thermometer that can read temperatures below 94ºF or 34ºC. Most hospitals have these thermometers.
Hypothermia is more common in winter, but temperatures do not have to be excessively cold for it to develop.
Spending too long in water, even at 79ºF, can lead to hypothermia.
Heat is lost more quickly in water than on land. Water temperatures that would be comfortable as outdoor air temperatures can lead to hypothermia.
A water temperature of 50ºF, or 10ºC, may lead to death within an hour, and water temperatures hovering at freezing can lead to death in as little as 15 minutes.
Water at 79ºF or 26ºC will, after prolonged exposure, lead to hypothermia.
Indoors hypothermia can happen during summer due to air conditioning or ice baths.
Older people who develop hypothermia indoors may have subtle symptoms and approach their doctor with vague complaints of mental or motor skill deterioration.
More deaths result from indoor than from outdoor hypothermia, probably because indoor hypothermia tends to affect older people, and the diagnosis comes at a late stage.
Other causes of hypothermia include metabolic disorders that are linked to a lower basal metabolic rate. This means that less heat is being generated internally. Dysfunction of the thyroid, adrenal, or pituitary glands may be an underlying cause.