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Dehydration, heat and cold

Dehydration, heat and cold

As we all know, the human body is mostly water. Our need for water is only second to our need for oxygen. While a person can survive for weeks without food, without water death usually occurs within three days.

In temperate zones, average daily fluid loss by an adult is one and a half to two and a half litres as urine, one to two litres as sweat, and one half to one litre as vapour while breathing. In hot and humid conditions, perspiration increases rapidly, and at high altitude vapour loss can exceed five litres per day.

In one case, researchers determined that distance runners lost in excess of three litres per hour of sweat during a race in Rio de Janeiro with a mid-30C temperature and high humidity. Such conditions regularly occur during the summer in the southern US and Europe. Even in the Rockies, Coast Mountains and Cascades of western North America, summer temperatures can exceed 30C.

Carrying a heavy pack up hundreds of vertical metres over 15 kilometres of trail can result in massive fluid loss. Yet, all dangers posed by pathogens or chemical pollutants pale in comparison to dehydration, at least in the short term. Severe fluid loss can cause death in a day or less, in both hot and cold weather, particularly at high altitude. Dark or yellow urine, or volume under a half litre per day, are clear indications of dehydration. The best indicator of proper hydration is about two litres of light yellow or clear urine per day.

Heat Illnesses

In hot weather, without proper hydration, mild dehydration can advance rapidly to heat cramps, heat exhaustion and eventually heat stroke.

Heat cramps are painful knotting or spasms, usually in the abdominal and calf muscles, caused by a depletion of salt and fluid.

Heat exhaustion occurs when body fluid levels are low enough that blood drops in volume and thickens. Blood flow to internal organs, including the brain, decreases as the body directs more blood to the skin in an effort to give off heat and reduce body temperature.

Without immediate treatment, heat exhaustion will lead to heat stroke. At this stage, the body cannot cool itself, sweating stops and body temperature rises causing the brain and internal organs to start failing. The body then slips into a coma and death follows.

Heat Exhaustion

Heat exhaustion is characterized by a slightly elevated body temperature, cool skin that is moist and pale or red, headache, nausea, dizziness, weakness and exhaustion.

A person with symptoms must get into the shade, remove sweat-soaked clothes, pour cool water on the skin and drink fluids. These steps can reverse early symptoms of heat exhaustion. Fanning can increase evaporation and further cool the body.

A conscious casualty may experience vomiting and nausea, which can be minimized by drinking small amounts of water every 15 minutes for several hours. The ill person must stop all activity for the day and rest in a cool place while continuing to take in fluids under the guidance of a companion.

Heat Stroke

Untreated heat exhaustion will progress to heat stroke.

Heat stroke is characterized by high body temperature, skin that is red, hot, and dry, strange or irritable behaviour, eventual loss of consciousness, a weak rapid pulse, and quick shallow breathing.

In the wilderness, beyond the reach of emergency medical services, treatment is difficult and time is critical.

Cool the casualty’s body in any manner possible including wetting with cool water and fanning. However, do not submerge the body in cool water because breathing may stop or the casualty might go into cardiac arrest.

While cooling the casualty, monitor his breathing and heart rate and be prepared to initiate rescue breathing or CPR.

A conscious casualty must drink fluids. In advanced heat stroke, when the casualty is unconscious, death will follow without intravenous replenishment. Only trained medical personnel with the proper equipment should start intravenous hydration.

In most backcountry situations, a casualty must be transported to a hospital within one or two days or death is inevitable.

Preventing Heat Illnesses

The best treatments for heat emergencies are preventative.

Avoid exerting yourself during the heat of the day, slow down as the temperature rises, take breaks in the shade, wear a light-coloured hat and most importantly drink plenty of fluids including water, soup, fruit juices, and sport drinks.

Alcoholic and caffeinated beverages should be avoided because they contain diuretics that promote fluid loss.

While hiking, backpacking, or climbing in warm weather drink a half to one litre of water per hour of activity. In hot weather, when carrying a heavy load, or at altitudes above 4,000 metres drink one litre or more per hour.

If you or a companion experience symptoms of overheating such as heavy sweating and cramping muscles immediately move into the shade, cool off and begin drinking liquids. Stretching and massage can relieve heat-cramped muscles. If cramps vanish and you observe no further signs of illness activity can resume with frequent breaks and regular hydration.

Dehydration and Cold Illnesses

In cold weather, or at altitudes above 4,000 metres, dehydration can occur quickly because of rapid water loss into dry air during breathing.

In cold conditions, dehydration can lead to depression, impaired judgement, physical weakness and lethargy, all inherently dangerous in especially when route finding, in bad weather or while climbing.

Dehydration contributes to hypothermia by reducing blood volume. The smaller flow inhibits heat production from exercise and increases the possibility of frostbite. In severe cases, thickened blood can clot in the body and cause pulmonary embolism, which can kill instantly.


A person who is dehydrated in cold weather, or at high altitude, will likely have a reduced sense of thirst and must be monitored to ensure proper hydration, particularly if they display symptoms of impaired judgement or lethargy.

Preventing dehydration requires a calculated effort to drink at least two litres of water per day in low effort activities such as ice-fishing and up to six litres or more per day when cross-country skiing or climbing at high altitude.

Effective Drinking

Dehydration and related heat and cold illnesses come on rapidly with minimal warning so preventative hydration is absolutely essential. To the contrary, illnesses caused by waterborne pathogens usually require days or weeks of incubation time. In the case of short wilderness adventures, most people are at home long before symptoms start and even then illnesses are rarely life threatening.

Because dehydration and associated illnesses can kill within hours you should rarely hesitate to drink untreated water as a preventive measure. The exceptions are seawater and urine, which actually promote water loss from the body and are worse than not drinking at all or water that is obviously contaminated with chemicals.

Try to drink regularly as you hike, climb, or paddle. Water bottles or bags, which are carried in your pack and allow easy drinking through tubes are excellent for ensuring proper hydration. If you cannot afford to carry the extra weight of water, such as during a rapid summit bid, try guzzling large quantities at every water source and only carry a litre with you.

If you are camping, drinking large quantities before bed is an effective way to rehydrate and prepare for the next day. When you need to get up early for a long hike or summit attempt the bonus of bedtime guzzling is that your bladder works as a water alarm! With some practice, you can learn to how mauch water to dirnk to wake up at a specific time.