Cold immersion foot, also called trench foot, is damage to blood vessels, nerves and tissue in the foot caused by prolonged exposure to cold, wet conditions. The tissue does not freeze. It is a non-freezing cold injury (NFCI); the German names are Immersionsfuß and Grabenfuß. Numbness, disturbed walking and later severe pain can prevent isolated personnel from continuing to evade capture and looking after themselves.
How cold and wet conditions damage the foot
The foot does not have to remain submerged continuously. Wet socks and footwear can keep it cold and damp for a long time. Blood vessels constrict to limit heat loss. With prolonged cooling, the small vessels and their control mechanisms are damaged, affecting the blood supply to nerves. Cold can also damage nerves directly. At the same time, wet skin softens and becomes more vulnerable to friction and pressure. Tight footwear further compromises circulation, particularly as the foot swells.
Temperatures above freezing are sufficient. How quickly injury develops depends on cooling, duration, immobility, exhaustion and pressure from equipment, among other factors. Repeated cold exposures without adequate recovery can increase the damage. A set number of hours with wet feet therefore cannot serve as permission to keep walking. A warm body core does not rule out a local cold injury to the foot.
Waxy-looking, pale, firm or hard tissue that is still frozen after exposure to freezing conditions suggests frostbite. After thawing, colour and numbness alone cannot reliably distinguish the injuries. A cold, wet injury can subsequently freeze, so both injuries can coexist. Decisions about rewarming therefore depend on the full cold exposure history and the state of the tissue. Measures for non-frozen immersion foot do not suit every cold injury.
Numbness during exposure, pain afterwards
During cold exposure, the foot may become increasingly numb and feel like a block of wood. Feedback about toe position and contact with the ground deteriorates; steps become uncertain even with little or no pain. The skin may look pale, yellowish or mottled, but initially it may also be red or appear largely normal. Colour changes are often harder to see in darker skin. Waiting for pain or a striking colour change misses important early warning signs.
After leaving the cold, the foot may initially remain cold and insensate. Later it can become red, hot and swollen, with burning or throbbing pain. Even light touch may hurt while some areas remain numb. This increased blood flow does not confirm that the small vessels and nerves are functioning normally again. Heat and allowing the foot to hang down can aggravate pain. Marked swelling, pressure areas and blisters also make footwear harder to put back on.
A personal check brings together the preceding cold, wet exposure, changes in sensation and the ability to walk. At a protected rest point, inspect the sole, toes and areas pressed by footwear gently, and use light touch. If touch feels unusually faint, numbness or tingling returns, or stepping becomes unreliable, stop putting weight on the affected foot. Do not use needle pricks, hot objects or further cold as tests. The British NFCI Field Assessment Tool organises these findings by exposure history, appearance, sensation and response to warming. These features help identify suspected injury; a brief self-check cannot clear an injured person for another march.
Take weight off the foot and protect it
First limit further cold, wet exposure and pressure. An actually reachable place that is drier and sheltered from wind offers more than a rest in water or on cold ground. An available mat, pack or folded clothing can separate the resting body and feet from the ground. If hypothermia is present as well, protecting and rewarming the whole body takes priority; confusion, marked clumsiness and declining consciousness are already threatening the ability to provide self-care.
Loosen laces and other constricting items. Carefully remove wet footwear and socks if the foot can then be protected from renewed cold and wet exposure. Gently remove adhering dirt, blot the skin dry with available clean material and pay particular attention between the toes. Use dry replacements only if they are actually available and leave room for the swollen foot. Without them, at least drain water from the footwear and wring out socks after removing them. This reduces wetness but restores neither a dry foot nor its ability to bear weight. Keep the feet protected from wind, precipitation and cold ground; prolonged exposure of bare feet outdoors can worsen cooling.
Cold immersion foot is rewarmed passively in a protected environment at room temperature. Air drying is useful only if the foot does not cool again. Warm water, forced hot air and heating pads are unsuitable for this injury: rapid local warming can increase pain and swelling, and a numb foot detects a burn too late. Do not rub or massage it, and do not cool it with ice. If a suitably temperate place is unavailable, the immediate personal measures are to limit further heat loss and protect the injured tissue.
At rest, elevate the feet without pressing on injured areas. A light, loose covering that is as clean as possible protects broken skin; available sterile wound dressings are suitable. Do not apply tight bandages or force extra layers into footwear that is already tight. Leave blisters intact. Oil, grease or warming ointments do not treat the vascular and nerve injury. Maintain drying and pressure relief instead of putting the protected foot back into the same cold, wet conditions.
Further walking can cost the remaining ability to move
With swollen or numb feet, every step subjects weakened tissue to pressure and friction. Pressure injuries may not be felt in time. Blisters, open wounds and local tissue death can result, with infection as a further complication. Unreliable footing also increases the risk of falling. Absence of pain therefore does not justify testing the foot through prolonged walking. Movement exercises intended to prevent cold injury are not a treatment for a foot that is already injured.
If concealment and rest are possible at the current location, take the weight off the feet. If immediate danger forces movement, use the shortest route that actually leads out of that danger or to a better protected place. Footwear must protect against the terrain without constricting the swelling; available padding helps only if there is enough room for it. Walking barefoot over rough or cold ground adds other injuries. At the next safe interruption, reassess sensation, swelling and pressure areas. Increasing numbness, unreliable steps or a foot that can no longer be accommodated without constriction limit the distance that remains possible.
After apparent improvement, nerve pain, cold sensitivity, excessive sweating or altered sensation can persist for a long time. A foot may look normal yet rapidly develop symptoms when exposed to cold again. Marked swelling, open wounds, new dark areas of skin or worsening symptoms after the first few days require medical assessment when access actually becomes possible. They may indicate additional pressure injury or infection. Personal protective measures limit further pressure and friction; they cannot treat dead tissue or a spreading infection.
Immersion injuries in warm water
Warm water immersion foot and tropical immersion foot are other foot injuries caused by prolonged wet exposure. In warm water immersion foot, severe waterlogging of the skin predominates: soles become white, wrinkled and painful on weight bearing. In the more severe tropical immersion foot, the tops of the feet and ankles may become red, swollen and tender. Footwear then presses on swollen tissue. These warm injuries are not simply cold trench foot occurring at a higher air temperature.
Here too, the feet need to leave the continuously wet environment, dry and rest in an elevated position; further friction and pressure worsen the skin injury. Fungal or bacterial infections can develop. Distinguishing the conditions therefore requires the actual cold or warm exposure history, the skin areas affected and any altered sensation. White, waterlogged soles alone do not establish cold immersion foot. Warm injuries often resolve after sustained dry rest, whereas cold immersion foot can leave prolonged nerve symptoms and cold sensitivity.