Activities The Great Outdoors The Most Important Rule in Scuba Diving: Never Hold Your Breath Share PINTEREST Email Print Loic Lagarde/Moment Mobile/Getty Images The Great Outdoors Scuba Diving & Snorkeling Gear Safety Climbing Skiing Snowboarding Surfing Paddling Fishing Sailing By Natalie Gibb Natalie Gibb Natalie Gibb owns a dive shop in Mexico and is a PADI-certified open water scuba instructor and TDI-certified full cave diving instructor. Learn about our Editorial Process Updated on 04/21/19 If you remember one rule of scuba diving, make it this: Breathe continuously and never hold your breath. During open water certification, a scuba diver is taught that the most important rule in scuba diving is to breathe continuously and to avoid holding his breath underwater. But why is this rule so important? Avoiding a Pulmonary Barotrauma Scuba diving is different from snorkeling or freediving. When a snorkeler or freediver takes a breath from the surface and dives down, the air in his lungs compresses due to the pressure of the water as he descends and expands to its original volume as he returns to the surface. A scuba diver, on the other hand, breathes air compressed to the same pressure as the surrounding water. If he ascends, the air in his lungs expands as the pressure around him decreases. A diver who holds his breath underwater seals off his lungs. If the diver ascends, the air in his lungs will expand but have no way to escape his lungs. Lungs may seem highly flexible (they do expand and contract with each breath) but this is not necessarily the case. At the smallest level, lungs are made of tiny sacs of tissue called alveoli. Alveoli are very, very small and have incredibly thin walls. These walls are easy to rupture, and relatively small changes in depth can cause the air inside them to expand enough to rupture them if the air is prevented from escaping. A depth change of even a few feet can be enough to damage a diver's lungs if he holds his breath underwater. A lung over-pressurization injury is known as a pulmonary barotrauma, and can occur at both microscopic and macroscopic levels if a diver holds his breath and ascends. A pulmonary barotrauma is a dangerous injury because it can force air into a diver's chest cavity or blood stream. Before deciding that breath-holding while scuba diving is acceptable as long as the diver does not ascend, read the next section. Preventing a Loss of Buoyancy A diver's buoyancy is dependent upon a variety of factors, one of them being his lung volume. Student divers experiment with the effects of lung volume on buoyancy during open water certification using exercises such as the fin pivot. A diver who is neutrally buoyant and increases his lung volume by inhaling deeply will find that he slowly starts to rise in the water, because increasing his lung volume increases his buoyancy. Of course, ascending causes the air in the diver's lungs to expand, leading to the risk of lung damage if he holds his breath. The act of holding his breath underwater causes a diver to rise and prevents air from escaping his lungs. Maintaining Breathing Efficiency Finally, even if a diver is so negatively buoyant that holding his breath will not cause him to ascend, it is still a bad idea to hold his breath underwater. When a diver holds his breath, carbon dioxide builds up in his lungs. This causes him to feel starved for air, and he will need several deep exhalations and inhalations to recover. In the best of cases, recovering from a build up of carbon dioxide disrupts a diver's breathing cycle, and may even increase his air consumption. In the worst of cases, the increased density of air underwater may make recovering from a breath hold difficult and lead to hyperventilation. Key Takeaway About the Most Important Rule in Scuba Diving The rule to never hold your breath when scuba diving is important both for dive safety and dive efficiency. A diver who holds his breath underwater will not decrease his air consumption or prolong his dive. He merely increases the concentration of carbon dioxide in his lungs, which makes him feel starved for air. Furthermore, a scuba diver who holds his breath underwater risks a lung over-expansion injury if he ascends, which is likely as breath-holding while scuba diving increases a diver's buoyancy.