How we lose hydration in the frozen North

ice chiselThe difficulties in getting a glass of water

When operating in winter conditions in the boreal forest our first consideration is often given to dealing with the extreme cold that we can encounter through lighting a fire or seeking shelter. This of course is accurate and an issue that must indeed be addressed in order to ensure survival and allow for effective functioning of the extremities that enable us to carry out the necessary tasks. There is however another all important resource that we need to consider when in low temperatures, water! As in warmer climates and other times of the year our water intake needs to be maintained to ward off the implications of dehydration. The prospect of becoming dehydrated is one that many people may not consider when out in -15°C but there are factors at play that make this a very real threat. At temperatures below -5°C the air becomes dry, the majority of moisture being locked up in snow and ice dramatically lowering the relative humidity. As a result the body needs to humidify the air before it passes into the lungs. This causes us to loose substantial quantities of water simply through breathing. This process is directly related to relative humidity at varying temperatures, at -10°C saturated air contains only 2.4g of water/m³ compared to 17g /m³ at 20°C.

In simplistic terms this puts a much greater demand on the body’s water reserves when breathing cold air compared to warm air. In the case of the temperature examples above there is a 7 times decrease in water/m³ in the air which results in a greater demand on the bodies own water supply. In sub-zero temperatures our bodies need to warm and humidify inhaled air before it reaches the lungs in order for the gaseous exchange to function. For this to be effected there are a set processes that the body undertakes with every breath. Firstly our the air needs to be warmed which is achieved by the mass of blood vessels that coat the inside of the nose and throat. In a healthy adult 75% of the warming should be carried out at this point in this upper respiratory track; the oral and nasal passage down to the larynx towards the back of the throat. These same membranes also release moisture into the air to humidify it on its way down to the lower respiratory track. This process of warming during inhalation can lead to the loss of 0.3 litres through evaporation each day.

As the air passes down into the lungs and into the alveoli the air humidity needs to attain 100% to allow oxygen to move into the blood, so subsequently as we exhale we lose substantially more water. The amounts that we lose is proportional to the air temperature, which effects the humidity levels as well as the activity that we are undertaking. In cold weather we can experience a loss of water during exhaling at the rate of 20ml per hour at rest. When you compare this to higher lung excretion in the warm weather the water loss can be as little as 7ml/h. Although these figures are slightly out of context for daily use they serve to illustrate the differences we will be experiencing in various climates.

DSC02713In these conditions and under these circumstances any increase in perspiration will lead to increased lowering of your core temperature. Regulating layers is the key!

Water lost through perspiration this is something that we aim to avoid at all costs as it has significant implications in cold environments in regards to a cooling effect on the body once we stop exercising. The cooling effect on the skin as sweat evaporates, transferring excess heat away from our core is a key evolutionary trait of humans and although desirable in warm environments is not something you want in cold temperatures. There is also a second negative effect that can occur when the sweat  freezes within our clothing. Our aim is to try to avoid excessive perspiration for its negative impacts but on the flip side it is a natural process which our bodies undertake. The ideal situation is for the body to maintain a 70% humidity level next to the skin, in cold, dry environments this can lead to substantial moisture as moisture is balance is disrupted. The effects of these processes and how we strive to over come them will be covered in future blogs on clothing choice and management.

axe through iceAlternative methods of water collection

One final effect of the cold on our personal water management is the effect the cold has on the level that we urinate. Due to the high level of heat that can be lost through our skin in low temperatures the body reacts by reducing the amount of blood which travels to the extremities from the core. This process, know as vasoconstriction, causes a rise in blood pressure as the same volume of blood is forced through a smaller space. The response of the kidneys to this is to remove excess fluid from the blood causing us to increase the amount we urinate.  The effects of vasoconstriction are much reduced through effective regulation of your temperature during activity and rest. You will notice that the effects of this only become apparent once your extremities start to become cooled.

The net effect of the various ways we loose moisture in sub-zero conditions will generally lead to increased water requirements so we need to have to collection, filtration and purification of adequate water supplies at the for front of our minds  There are a broad range of techniques available to collect water from snow and ice and each have their merit where conditions are appropriate. For our recent trip to Sweden,  Adam and me experienced mostly dry cold conditions that favoured ice or water collection over snow due to this lacking the ability to form together in a mass. This had the advantage for us in regards to the higher yields obtained from melting ice over snow but presented the challenge of how to get through the 5-6 inches of ice on the lake to access the water or collect the ice?

Hopefully the notes above help to explain some of the reasons why it is important to carefully manage your hydration levels in the cold weather to the same extent as in the heat. Along with the effects on our bodies should we fail to do so. In the next blog I will look at various ways in which that we collected water throughout our trip, illustrate some of the varying ways that problems can be solved with different tools and equipment.



Danny Hodgson


– How much water is lost during breathing? Zieliński J1, Przybylski JPneumonol Alergol Pol. 2012;80(4):339-42.

Cold-induced responses in the upper respiratory tract, Jari J. Latvala, Kari E. Reijula, Philip S. Clifford & Hannu E. Rintamaki, Arct Med Res 1995; 54: 4-9

– Outdoor Action Guide to Hypothermia and Cold weather Injuries, Rick Curtis, Princeton University, Outdoor Action Programme, 1995

– Respiratory Humidification Basics,, WILAmed GmbH

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