The individual aspirates water into the lungs, after an episode of breath holding until the victim cannot hold the breath any longer. On inspiration, much of the water is probably swallowed, but a proportion is inhaled. This inhaled water blocks the airways, only a proportion of it getting as far as the alveoli. The result is hypoxia, which after a short period results in hypoxic cardiac arrest and hence death. It only requires 10 mL of inhaled water per kilogram of body weight to be fatal.

Fresh water in the alveoli is absorbed, resulting in haemolysis of red blood cells and haemodilution. Sea water, being hypertonic, causes withdrawal of water from the blood and no haemolysis.

No (or very little) water actually enters the lungs. This may be because of laryngeal spasm but is more likely to be because of primary cardiac arrest due to stimulation of the vagus nerve by cold water. It is this mechanism that causes the death of people who drop into cold water. It is likely that this is the cause of death in ‘spray drowning’ – deaths that occur in people on the surface of rough water. Vagal sensitivity is increased by hypothermia which occurs rapidly in cold-water immersion.

‘Dry’ drowning accounts for between 10% and 25% of drowning deaths.

In the UK, drowning is the fourth leading cause of death in men under the age of 35 years and the second leading cause of death in children, with 40% of all drowning deaths occurring in children under the age of 5 years.