The frequently quoted warning against holding one's breath on scuba is a gross oversimplification of the actual hazard. The purpose of the admonition is to ensure that inexperienced divers do not accidentally hold their breath while surfacing, as the expansion of gas in the lungs could over-expand the lung air spaces and rupture the alveoli and their capillaries, allowing lung gases to get into the pulmonary return circulation, the pleura, or the interstitial areas near the injury, where it could cause dangerous medical conditions. Holding the breath at constant depth for short periods with a normal lung volume is generally harmless, providing there is sufficient ventilation on average to prevent carbon dioxide buildup, and is done as a standard practice by underwater photographers to avoid startling their subjects. Holding the breath during descent can eventually cause lung squeeze, and may allow the diver to miss warning signs of a gas supply malfunction until it is too late to remedy.