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Shallow water blackout (SWB) is an unexpected loss of consciousness caused by a lack of oxygen during a breath-holding dive. It can occur in an individual who is free diving without the use of scuba gear, or even someone who is swimming underwater laps in a pool. Most often, SWB occurs because the diver has hyperventilated before diving. This can deactivate the body's natural alarm system, which triggers the need to breathe before oxygen levels drop too low and the diver passes out.
Many divers and swimmers practice a technique called intentional hyperventilation, which temporarily suppresses the urge to breathe. This allows the diver to stay under water for a longer period of time. This technique works because the urge to breathe comes from rising carbon dioxide levels in the body, not from low oxygen levels. Hyperventilating removes more carbon dioxide from the body, so the urge isn't triggered as quickly. When the diver doesn't feel that urge to breathe, however, it is far more likely that he will stay underwater for too long and suffer a shallow water blackout.
Although some experts say that some intentional hyperventilation before a dive may be acceptable as long as only three to four breaths are taken, many others say that the risk of excessive hyperventilation is too high. In fact, evidence suggests that hyperventilating before a dive does not extend diving time significantly. Instead, experts recommend that swimmers and divers practice relaxing before diving, breathing normally and allowing the body to calibrate itself naturally. An excited diver can start breathing quickly without realizing it, so it's a good idea for new divers especially to take a few minutes to calm down before diving.
It is also important for divers rest a minute or two between breath-holding dives so the body can restore its oxygen level. Excessive exercise under water should be avoided. When the need to breathe becomes urgent, the diver should get access to air as soon as possible. A diver who experiences symptoms of fatigue or dizziness should stop the dive immediately to avoid a shallow water blackout.
Divers should remember to never dive alone; people should always dive with a partner who can assist in the case of an emergency situation. A partner can help to protect an unconscious diver experiencing a shallow water blackout from drowning and death. Before diving, the diver and the partner should go over the proper emergency procedure in case of an SWB, which is to get your partner to the surface as quickly as possible. Once at the water's surface, the partner should hold the unconscious diver's head above water to prevent drowning.
When I was little, I could stay underwater just for seven seconds; it didn't matter what I did.
As adult, my endurance went up to about 1'20", and is increasing time by time since I started to do physical activities (I'm still rather unfit because I just started last month).
Honestly, if you don't hyperventilate, time of breath holding doesn't change a lot (I can imagine unless you don't hyperventilate for one minute, I never tried that), and you feel better the whole time until you reach the signals you need to breathe again.
I stopped to do this practice when I was 8, because I've read somewhere that it was useless.