Dehydration may be said to occur whenever there is loss of water by an organism to a point below the physiological norm. Animals die when they lose 20-25% of their body water, and disorders arise when the water loss reaches 10%. Dehydration may develop either as a result of excessive water loss (repeated vomiting, diarrhoea, excessive perspiration, extensive burns, increased urine production) or restriction of water intake. Dehydration is aggravated by strenuous activity in cold weather. Cold air is dryer than warm air, so water loss from breathing is greater. Moreover, exercising while heavily dressed can cause excessive sweating.
The consequences of dehydration include fatigue, muscle cramps, reduced apetite, flushed skin, dizziness, dark and strong-smelling urine, and a loss of blood plasma. Blood plasma loss in turn reduces muscular stamina, increases heart rate and body temperature and prolongs recovery time after exercise. Drinking an adequate amount of non-caffeinated liquid before, during and after physical activity prevents and alleviates dehydration. A litre of drink is needed for every 1000 calories expended.
The cabin air moisture in airplanes can fall as low as 8 percent (even a very dry climate on the ground can register a humidity level of 20 percent, a tropical climate about 90 percent); air passengers frequently suffer from signs of dehydration, including dry tight skin, bloodshot eyes, tiredness and headache.