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)

An air embolism is an air bubble that becomes trapped in a blood vessel and blocks it. It is the leading cause of death among divers.

An air embolism can happen when a scuba diver holds their breath while they quickly surface.

When an air bubble travels along an artery, it moves through a system of blood vessels that gradually become narrower. At some point, the bubble will block a small artery and cut off the blood supply to a particular area of the body.

Read more about what causes an air embolism.

How serious is it?

The seriousness of the blockage depends on which part of the body the artery supplies blood to. For example, an air embolism in:

• the arteries to the brain – causes immediate loss of consciousness and may lead to convulsions (fits), a stroke or heart attack
• the coronary arteries (which lead to the heart) – may cause a heart attack
• a blood vessel to the lungs (pulmonary embolism) – may cause chest pain and shortness of breath

If the embolism stops blood getting to the brain, tissue in the brain will be starved of oxygen and die. This can cause permanent brain damage.

Warning signs

Divers should always be carefully monitored by their colleagues and supervisors so that if an air embolism occurs, it can be immediately identified and treated.

Warning signs of an air embolism may include:

• low blood pressure 
• irregular heartbeat
• extreme fatigue (tiredness) or lack of strength
• disorientation
• a faint blue tone to the skin caused by a lack of oxygen in the blood
• irregular breathing
• a lack of oxygen to the body tissues

The following symptoms of air embolism usually appear as soon as the diver reaches the surface:

• dizziness 
• blurred vision
• bloody froth from the mouth
• paralysis or weakness
• convulsions (fits)
• unconsciousness
• no breathing 
• cardiac arrest (the heart stops)

If a scuba diver loses consciousness within 10 minutes of surfacing, they probably have an air embolism and should be treated immediately.

How is it treated?

If a diver develops an air embolism the only effective treatment is immediate recompression treatment in a hyperbaric chamber.

The diver is given oxygen and laid horizontally until they reach the hyperbaric chamber.

Recompression treatment involves lying in a hyperbaric chamber, usually for several hours, and breathing a mixture of air and pure oxygen under high pressure. The treatment is effective up to 48 hours after diving. The high pressure will restore normal bloodflow and oxygen to the body’s tissues and reduce the size of the air bubbles in the body.

After recompression, pressure is reduced gradually to allow the gases to leave the body without causing harm.

Air embolisms in surgery

As healthcare standards in the UK are high, air embolisms caused by surgery are rare.

In hospitals and health centres, care should be taken to prevent air bubbles from entering the bloodstream. Before injections, air should be removed from syringes and surgery should be closely monitored to ensure that air bubbles do not form in blood vessels.

Catheters or other tubes inserted into the body should be removed using a technique that minimises the possibility of air embolism.

If someone is thought to be at risk of an air embolism during surgery their blood pressure may be monitored. For example, surgery that takes place in a sitting position carries a very small risk of an air embolism.

If an air embolism is suspected during surgery, the surgeon will: