Prolonged swimming underwater and breath holding is a dangerous practice that can lead to drowning. Some aquatic facilities have posted signs prohibiting these activities. Do not allow your grandchildren to engage in these practices, even if there are no signs posted.
In order to understand why breath holding poses a threat, it is necessary to know something about human respiration. A high level of carbon dioxide in the blood is what actually triggers air hunger. Many times swimmers who are going to hold their breath hyperventilate before going under, artificially lowering the level of carbon dioxide in their bloodstream. When they are actually starving for oxygen, their bodies don't know it because their carbon dioxide levels are normal. Oxygen-starved, they may black out and drown before anyone realizes what is happening. This phenomenon is sometimes known as shallow water blackout.
Three other factors make breath holding even more dangerous. First, an unconscious swimmer may make involuntary movements, preventing observers from realizing that the swimmer is in trouble. Second, low levels of oxygen in the blood may trigger the release of endorphins, so the swimmer may experience a sense of euphoria rather than a sense of danger. Third, breath-holders are likely to be experienced, adept swimmers and as such may not be as closely watched by lifeguards as beginning swimmers.
Although the dangers of breath holding have been known for many years, education efforts have been spotty. Training materials from the American Red Cross and the YMCA mention the dangers of breath holding, but many facilities still lack the appropriate signage, and among the public there is very little awareness of the dangers of underwater swimming or breath holding. Grandparents can be a positive force by educating grandchildren and other family members about this practice.