The Risks Of Ascent

The greatest danger to the free diver during an ascent is from pulmonary barotraumas, the consequence of which may be fatal.

A free diver not exhaling during an ascent or making a fast ascent will cause their lung volume to expand rapidly. If the ascent is too fast then the musculoskeletal cage may exceed its expansion limits. Once this situation is reached, injuries such as pulmonary barotrauma, pneumothorax, and rupture of the pulmonary vein causing an air embolism may occur. The most common of these injuries is pulmonary barotraumas, sometimes referred to as pulmonary overpressure syndrome.

For an equivalent change in depth the risk of barotrauma is greatest near the surface, a fact explained by Boyle’s law. A breath-holding free diver rising from 33 feet to the surface experiences a change in ambient pressure from two to one atmospheres. If the lungs fully expand within the chest cavity the lung volume will double. By contrast, a 33-foot rise from 99 to 66 feet depth (i.e., from 4 to 3 atmospheres) would maximally increase a free diver’s lung volume only 33 percent, posing less risk of a barotrauma.

Using the same example, a breath-hold ascent from 33 feet would result in the lung volume doubling, almost guaranteeing barotrauma if breath were held at or near the diver’s total lung capacity. If the lungs could not vent expanding air they would be subjected to a distending pressure of nine times the barotrauma threshold! Experiments in dogs undergoing rapid ascent in a decompression chamber demonstrated that the lungs withstand much higher pressures (before barotrauma occurs) if ‘over stretching’ of the thoracic cavity is prevented.

 Although both over-stretching of lung tissue and the pressure of expanding air are factors that may predispose the free diver to lung trauma, pressure seems to be the major problem. The pressure-difference across the lungs (from inside to outside) that is the threshold for experimental barotrauma is about 80 mm Hg, a change that may occur with a breath-hold ascent from only four feet! At this pressure the alveoli are prone to tearing and may vent air into the interstitial space. From here, the air may take one of three routes. If it travels to between the lungs or around the lungs a pneumorathorax will result (discussed later), and if it escapes into the bloodstream an air embolism will occur. Neither is a pleasant experience.

The nature of barotrauma may range from mild, to a complete collapse of the lung. In some cases the air may escape into the pulmonary venous system where it will move through the heart and into the arteries. When this occurs, bubbles form in the blood, causing an Arterial Gas Embolism (AGE). AGE is a major cause of death in conventional diving and the initiating cause (pulmonary barotrauma) often goes undetected. Experimental evidence demonstrates that intra-tracheal pressures of about 10 mm Hg are all that is needed for AGE to occur. Distention of the alveoli leads to rupture, alveolar leakage of gas, and leakage of the gas into the blood. As the free diver takes their first breath after surfacing, the extra-alveolar gas enters the ruptured blood vessels, and may migrate to the left side of the heart. Here it is distributed systemically as emboli sent to areas determined by buoyancy. AGE may arise from gas bubbles in the pulmonary capillaries, pulmonary veins to the left side of the heart (resulting in a rare coronary artery embolism) or internal carotid and vertebro-basilar arteries to the brain (causing a cerebral artery embolism – blockage) with the clinical picture of a stroke. As bubbles pass over the endothelium, there are direct cellular effects (within 1-2 minutes) causing local swelling, downstream coagulopathy and occasionally hemorrhages. There is also an immediate increased permeability of the blood-brain barrier, loss of cerebral auto-regulation, and a rise in the systemic blood pressure. In order to ensure immediate and effective response, it is important for free divers to recognize the symptoms of AGE and its associated problems. Some of these are listed below.


  • Bloody froth from mouth or nose (hemoptysis)
  • Disorientation
  • Chest pain
  • Paralysis or weakness
  • Dizziness
  • Blurred vision
  • Unconsciousness
  • Convulsions
  • Stopped breathing
  • ir bubbles in the retinal vessels of the eye
  • Death

If any of the above signs and symptoms are observed, certain steps should be taken as outlined below:

  • CPR, if required
  • Open airway, prevent aspiration, and intubate if trained person available
  • Give O2, remove only to open airway or if convulsions occur.
  • Place in horizontal, neutral position
  • Restrain convulsing person loosely and resume O2 as soon as airway is open.
  • Protect from excessive cold, heat, water or fumes.
  • Transport to ER for evaluation in preparation for removal to the nearest recompression chamber.Air evacuation should be at sea level pressure


Air that escapes into the lung during a pulmonary barotrauma event may also enter the space outside of the lung, an airtight area called the pleural space. When air moves from the lungs into this area, the injury is termed a Pneumothorax. Pneumothorax as a result of barotrauma is relatively rare in free diving but it can be serious when it occurs. One possible result is a Tension Pneumothorax. This condition occurs when the hole that the air exited through acts as a one-way valve. Since the pleural space is airtight, the expansion of the air upon ascent places increasing pressure on the heart and the other lung. As the pressure on the heart increases, the heart becomes less able to function efficiently and can ultimately become unable to function at all. Additionally, pressure on the uninjured lung may cause it to collapse. 

If this occurs, the free diver’s lungs will be unable to exchange gas and may suffer a range of symptoms as listed below. 


  • Severe pain.
  • Reduction of breathing capability.
  • Coughing of blood.

Treatment of such an injury normally requires a chest puncture to release the trapped air. In some cases, recompression is also necessary.

Many of the above injuries discussed in this article are associated with conventional scuba diving, and are rarely observed during free diving activities. However, an awareness of the symptoms and causes of each will ensure that safety during free diving practice and competition is enhanced and risks minimized.