An air embolism can happen if a diver runs out of air and holds their breath while coming up to the surface.

This can cause damage to the lungs, called pulmonary barotrauma, which allows air bubbles to enter the blood.

An air embolism can also happen when a diver surfaces too quickly. This is commonly known as the bends or decompression sickness.

These are explained in more detail below.

Pulmonary barotrauma

Pulmonary barotrauma is injury to the lungs caused by a change in pressure.

It happens when divers run out of air and, in a panic, hold their breath while rapidly swimming to the surface. As they come up and the pressure around them decreases, the air in their lungs expands. The expanding air causes the lungs to overinflate, which can rupture the alveoli (tiny air sacs in the lungs). Air can escape from the lungs and enter the blood vessels, which can cause an air embolism.

An air embolism can happen in as little as one metre of water, if the diver is breathing compressed air and holds their breath while rapidly surfacing.

Decompression sickness

Decompression sickness happens when nitrogen that is dissolved in the blood under high pressure forms bubbles when a diver comes up to the surface.

When deep underwater, divers breathe compressed air containing nitrogen gas. This accumulates in the diver's blood and tissues. When the diver comes up and sea pressure decreases, the nitrogen forms bubbles in the tissues and blood as it cannot be breathed out immediately. These bubbles may lead to an air embolism.

The following factors increase the risk of decompression sickness:

• some heart defects
• cold water
• dehydration 
• fatigue (tiredness)
• increased depth of water (due to higher pressure)
• increased time spent in deep water
• obesity 
• older age
• coming up to the surface rapidly
• repeated dives on the same day
• flying immediately after diving

Other causes

An air embolism from other causes is very rare. It can occasionally occur in the following situations:

• surgery to the blood vessels, or large blood transfusions – if a large quantity of air is mistakenly injected into the vessels (although doctors are trained to remove excess air from a syringe before giving injections)
• operations including caesarean sections or open-heart surgery, if air becomes trapped inside the body,
• an injury in which the chest is crushed, such as in a car crash
• removal of a catheter (a thin, flexible tube that is inserted into the body)
• oral sex during pregnancy (there have been a few reported cases of an air embolism occurring when air blown into the vagina has got into the enlarged blood vessels surrounding the pregnant woman's vagina)