Learn about shallow water blackout, its risks, and essential safety measures for freedivers to prevent hypoxia-related incidents.
Freediving Safety
Loss of consciousness due to hypoxia during breath-hold diving often occurs in the last few meters of ascent or immediately after reaching the surface. A trained safety buddy, proper weighting, and avoidance of hyperventilation can save lives.
Written by: Capt. Alpertunga Anıker
Freediving and spearfishing can have a very narrow safety margin when proper training and discipline are lacking. As the breath-hold duration increases, oxygen reserves decrease; pushing the diver's limits, improper respiratory preparation, or diving alone can turn a few seconds of unconsciousness into a fatal drowning incident.
Shallow water blackout can develop without a noticeable sense of panic, pain, or strong breathlessness. It is not sufficient for a person to rely solely on their own feelings to recognize the danger.
What is shallow water blackout?
‘Shallow water blackout’ is a hypoxic loss of consciousness that occurs when the oxygen reaching the brain during breath-holding drops to a level insufficient to sustain consciousness. It is most commonly observed in the last meters of ascent during deep vertical dives or in the first seconds after surfacing; however, it can also occur in a pool or at a fixed shallow depth following hyperventilation.
In loss of consciousness, the person cannot maintain an airway. If their face remains underwater, the airway comes into contact with water, and the drowning process may begin. Therefore, the primary duty of the safety buddy is to keep the mouth and nose out of the water, ensure positive buoyancy, and promptly bring the victim to safety.
Overweighting can lead to the unconscious diver sinking rapidly. The weighting system must be properly adjusted, the diver should maintain positive buoyancy at the surface, and the belt should be easily released in an emergency with a single motion.
Why does loss of consciousness often occur near the surface?
While the absolute ambient pressure at sea level is approximately 1 ATA, it increases to about 2 ATA at a depth of 10 meters in seawater. According to Boyle's law, the gas volume in the lungs roughly halves, and the partial pressure of oxygen in the alveoli temporarily rises. However, as long as the diver holds their breath, oxygen consumption continues.
As the diver ascends, ambient pressure decreases, lung gas expands, and the partial pressure of oxygen can drop rapidly. This decrease becomes pronounced in the last 10 meters, where the relative pressure change is greatest. A conscious diver at depth may lose consciousness in the last meters or immediately after surfacing.
How does hyperventilation increase risk?
The primary stimulus for the urge to breathe is the rising level of carbon dioxide in the blood. Taking rapid, forceful, or repeated deep breaths before diving decreases carbon dioxide levels in the blood; conversely, it does not increase the body's oxygen reserves to the same extent.
The result is dangerous: while the urge to breathe is delayed, oxygen consumption continues. The diver may feel comfortable and extend their time at the bottom; however, they can lose consciousness before a strong urge arises. Hyperventilation does not increase safe dive time; it merely delays the body's warning mechanism.
How can the risk of shallow water blackout be reduced?
Never dive alone. Every dive should be monitored one-on-one by a safety buddy who is trained in freediving rescue and can continuously observe the diver.
Implement the ‘one dives, one watches’ system. The safety buddy should be close enough to respond in time during ascent. A lookout on the boat or a general pool lifeguard cannot replace the trained buddy in the water.
Maintain surface monitoring. Loss of motor control or consciousness can occur immediately after reaching the surface. The diver should be closely monitored for the first 30 seconds.
Do not hyperventilate. Avoid rapid, forceful, or repeated deep breaths before diving that would lower carbon dioxide levels.
Do not push your limits. Past durations or depths achieved do not guarantee safety. Effort, cold, current, stress, fatigue, and previous dives can alter the safety margin.
Use proper weighting. Maintain positive buoyancy at the surface and ensure the weighting system can be quickly released in an emergency.
Obtain appropriate training and an emergency plan. Freediving, rescue, basic life support, and oxygen first aid training are practical skills that cannot be replaced by checklists.
Assess your health status. If there are heart, lung, or circulatory diseases, medication use, or a history of unexplained fainting, seek a fitness-to-dive opinion from a physician experienced in diving medicine.
What should be done if a diver loses consciousness?
The primary goal of initial intervention is to safely bring the victim to the surface, protect the airway from water, and initiate basic life support that includes ventilation if normal breathing is absent.
First, ensure your own safety. Do not enter dangerous water if you are not trained; use a flotation device and call for professional help.
Remove the face from the water and ensure positive buoyancy. Keep the mouth and nose above water; remove the mask and snorkel. If it hinders the rescue, safely release the weighting system as much as possible.
Quickly alert on the surface. A trained freediving buddy may apply the ‘blow–tap–talk’ method taught by their organization for a few seconds. This process does not replace respiratory assessment or basic life support.
Call 112 and bring the victim to shore or the boat. Do not perform chest compressions in the water. A trained lifeguard may provide five rescue breaths during evacuation if conditions are safe and an effective flotation device is available; this should not delay the evacuation.
Evaluate normal breathing on solid ground for no more than 10 seconds. Slow, irregular, or gasping breaths are not normal. Bystanders should not waste time looking for a pulse.
If normal breathing is absent, begin basic life support. A trained rescuer applies cycles of five initial rescue breaths followed by 30 chest compressions and two breaths. The compression rate for adults should be 100–120 per minute, with a depth of 5–6 cm, allowing the chest to fully recoil after each compression. If an AED is available, quickly dry the chest, attach the device, and follow its prompts. If the rescuer cannot provide breaths, continuous chest compressions are better than no intervention; however, ventilation is particularly important in drowning-related cardiac arrest.
If normal breathing is present, do not stop monitoring. If there is no suspicion of trauma, place the victim in the recovery position, keep them warm, and continuously observe their breathing. Oxygen should only be administered by a trained person with appropriate equipment; it should not delay calling 112, evacuation, or basic life support.
The return of consciousness does not mean the danger has passed.
In the event of any loss of consciousness in the water, the dive should be terminated for that day, and the individual should not return to the water for at least 24 hours. If loss of consciousness is prolonged, there is suspicion of water aspiration, or any of the following symptoms occur, 112 should be called, and an emergency medical evaluation should be conducted:
Persistent or worsening cough,
Shortness of breath or rapid breathing,
Wheezing or cyanosis,
Unusual drowsiness or confusion,
Recurrent loss of consciousness or changes in behavior.
Even if a person quickly and completely returns to normal, they should have the cause of the incident evaluated and seek advice from a health professional for a safe return to diving. The narrative of ‘secondary’ or ‘delayed drowning,’ which is claimed to suddenly appear days later without symptoms, is not consistent with current medical terminology; the essential thing is to monitor existing symptoms and seek timely help when necessary.
Experience in freediving is valuable; however, hypoxia does not respect experience. The foundation of safety is not chasing personal records but knowing your limits and staying on the safe side of those limits.
Never dive alone while holding your breath. The person next to you should not just be a spectator but a trained safety buddy who can protect your airway and bring you out of the water if necessary.
Sources and Further Reading
Divers Alert Network, ‘Hypoxia in Breath-Hold Diving,’ May 1, 2016.
Divers Alert Network, ‘Freediving Safety,’ May 1, 2019.
Divers Alert Network, ‘Shallow-Water Blackout,’ August 28, 2023.
Resuscitation Council UK, 2025 First Aid Guidelines, October 27, 2025.
Resuscitation Council UK, 2025 Adult Basic Life Support Guidelines, October 27, 2025.
American Red Cross, ‘Debunking Dry or Delayed/Secondary Drowning,’ August 12, 2024.
Source: SeaNews Türkiye
