Activities The Great Outdoors What's in a Scuba Diving Tank? Share PINTEREST Email Print ralucahphotography.ro/Getty Images The Great Outdoors Scuba Diving & Snorkeling Gear Skills Safety Hiking Climbing Skiing Snowboarding Surfing Paddling Fishing Sailing Learn More By 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. our editorial process Natalie Gibb Updated April 30, 2019 Diving with pure oxygen can kill a diver even at shallow depths. Recreational scuba tanks are filled with compressed, purified air. This air contains about 20.9% oxygen. Several risks are associated with the use of pure oxygen in diving. Oxygen Toxicity The confusion as to what is in a scuba tank is easy to understand because most people know that we require oxygen to survive. However, our bodies can only handle certain quantities of oxygen. Diving with pure oxygen deeper than 20 feet can cause a person to absorb more oxygen than his system can safely handle, leading to central nervous system (CNS) oxygen toxicity. CNS oxygen toxicity causes a diver to go into convulsions (among other things). All that is needed to stop the convulsions is for the diver to ascend to a depth shallower than 20 feet. Unfortunately, a convulsing diver will be unable to retain a regulator in their mouth, let alone control their depth. Usually, divers experiencing CNS oxygen toxicity drown. High Percentages of Oxygen Require Special Gear and Training The use of pure oxygen (or mixes of oxygen greater than 40%) requires special equipment. Oxygen is a great catalyst and can cause the ordinary lubricants and materials used in recreational scuba diving to explode or burst into flame. Before touching tanks filled with pure oxygen, divers should be familiar with special procedures such as opening tank valves of pure oxygen cylinders very, very slowly. Without going into exhausting detail, there is a significant amount of knowledge and training required to use oxygen safely. Pure Oxygen Is Used in Technical Diving Knowing that pure oxygen can be dangerous, it is easy to assume that you will be unlikely to encounter pure oxygen on a dive boat. Think again. Pure and high percentage mixes of oxygen (such as nitrox or trimix) are used by trained technical and recreational divers to extend bottom times and to speed decompression. On the surface, pure oxygen is recommended first aid for the majority of diving injuries. A recreational diver is likely to run across pure oxygen on a dive boat at some point in his diving career. If a diver remembers the risks of pure oxygen: central nervous system oxygen toxicity, explosions, and fires, it is easy to remember what is in a recreational scuba tank: air, pure and simple.