Cold-water immersion

Cold-water immersion is the body's response to sudden entry into cold water. It is not one process. Modern research describes four overlapping stages: cold shock, cold incapacitation, hypothermia, and collapse around the time of rescue. A person can drown before they ever become hypothermic.

Cold water has decided the outcome of shipwrecks, evacuations by sea and river crossings throughout history. Only in the twentieth century did researchers, many of them working for navies and coastguards, separate the stages and show that the old assumption, that people in cold water slowly freeze to death, missed most of what happens.

1. Cold shock, in the first seconds to about three minutes. Sudden cooling of the skin triggers an involuntary gasp and rapid, uncontrolled breathing, and the heart works harder. A gasp with the head under water can be fatal on its own.
2. Cold incapacitation, from about five to thirty minutes. The hands, arms and legs cool. Grip, coordination and swimming fail. Near 0 °C this can happen in five to fifteen minutes. A person can be fully conscious and unable to hold a rope or climb into a boat.
3. Hypothermia, often after twenty to thirty minutes. Core body temperature falls below 35 °C.
4. Circum-rescue collapse. Some people deteriorate while being lifted out of the water or in the hours after. Rescue does not guarantee recovery.

These timings are working approximations, not a clock for any individual. Water temperature, clothing, flotation, waves, injury, exhaustion, body size and the circumstances of rescue all change them.

• The Wilhelm Gustloff, 30 January 1945. The sea was near freezing with ice on the surface, and many of the dead died in the water rather than inside the ship. Survivors were pulled out through the night; some did not recover.