Because the amount of oxygen stored in the human body is not much, after exceeding a certain limit, people may begin to lose consciousness. The reason why people can't breathe is not lack of oxygen, but the accumulation of carbon dioxide in the blood. When the pressure of carbon dioxide in the blood is high enough, the human body's desire for breathing will become very urgent. This is because the respiratory center in the brain is triggered by carbon dioxide, forcing the human body to open its mouth to breathe.
By the swimming pool, children often take deep breaths. Every breath is rapid and deep. For a few minutes, they thought they inhaled a lot of oxygen, but in fact they were expelling carbon dioxide from their bodies. Children may be able to stay underwater for another 30 seconds, but it is dangerous because they actually have no extra oxygen supply. Because the oxygen partial pressure in the brain is too low, it may lead to brain dysfunction, so people begin to suffocate or even drown in the pool. The human body lacks strong receptors to sense hypoxia in the body. In fact, lack of oxygen can make people feel happy and excited temporarily, which is one of the reasons why people get high when they go to the mountains for the first time.
Before the invention of oxygen masks, early pilots engaged in air combat were also affected by temporary euphoria. These pilots became more and more excited when they flew the plane through the dense air defense fire net and flew over the target at an altitude of more than 6000 meters, and then they began to make mistakes in judgment and lost consciousness, which eventually led to the plane crash and ended in a crash.