A human being can survive an average of three to five days without the intake of water. The issues presented by the need for water dictate that unnecessary water loss by perspiration be avoided in survival situations. The need for water increases with exercise. Since the human body is composed of up to 89% water, it should be no surprise that water is higher on the list than fire or food. Ideally, a person should drink about a gallon of water per day. Many lost persons perish due to dehydration, and/or the debilitating effects of water-born pathogens from untreated water.
A typical person will lose minimally two to maximally four litres of water per day under ordinary conditions, and more in hot, dry, or cold weather. Four to six litres of water or other liquids are generally required each day in the wilderness to avoid dehydration and to keep the body functioning properly. The U.S. Army survival manual does not recommend drinking water only when thirsty, as this leads to underhydrating. Instead, water should be drunk at regular intervals. Other groups recommend rationing water through "water discipline".
A lack of water causes dehydration, which may result in lethargy, headaches, dizziness, confusion, and eventually death. Even mild dehydration reduces endurance and impairs concentration, which is dangerous in a survival situation where clear thinking is essential. Dark yellow or brown urine is a diagnostic indicator of dehydration. To avoid dehydration, a high priority is typically assigned to locating a supply of drinking water and making provision to render that water as safe as possible.
Recent thinking is that boiling or commercial filters are significantly safer than use of chemicals, with the exception of chlorine dioxide.