2000-05 Oxygen and Diving
By Fred Bove, M.D., Ph.D.
Nearly all of the energy-producing reactions that we are familiar with in our daily lives involve the combination of a fuel with oxygen to produce energy. These include burning gasoline in a car engine, heating with a wood stove or burning fuel oil in a furnace to heat a building.
We also need oxygen to burn the fuels (carbohydrates and fats) in the body, otherwise the body would cease to function in a very short time. And so we must carry an oxygen supply underwater to maintain the body energy processes.
Many non-divers think we carry oxygen in our tanks. Divers use a mixture of an inert gas with oxygen rather than pure oxygen because, as essential as oxygen is for life, too much oxygen is a poison that causes the brain to malfunction and the lungs to be injured. The inert gas is a necessary diluent to keep the oxygen partial pressure at a safe level.
Oxygen can be toxic to the body in two ways. Prolonged breathing of oxygen even on the surface (1 ATA) will eventually cause lung injury and reduce the ability of the lungs to transport oxygen to the blood. Lung oxygen toxicity is not a problem with sport divers because the damage usually takes many hours to occur, and sport divers usually don't spend this long at depth. Even with extreme oxygen partial pressures, lung injury is not common in divers.
The more important toxicity of oxygen for divers is on the brain. Too much oxygen will cause a seizure and puts the diver at risk for drowning.
To understand the toxicity of oxygen, we need to understand the relation between depth, gas mix and oxygen partial pressure. For example, when breathing air (21% oxygen) at 99 feet (4 atmospheres absolute, ATA), the partial pressure of oxygen is 4 x 0.21 = 0.84 ATA. If 50% oxygen is breathed at 99 feet, the partial pressure of oxygen would be 4 x .5 = 2 ATA.
After many years of study and experimentation, the partial pressure limits of oxygen for safe diving have been established, and several standards have been set. Two of the standards are provided by the U.S. Navy and by
the National Oceanographic and Atmospheric Administration (NOAA). The table provides limits for oxygen partial pressure (PO2) and the amount of time a diver can be exposed to the increased partial pressure. Although there are differences between the two standards, both provide similar exposure times in the 1.3 to 1.6 ATA range.
If we accept a safe oxygen limit of 1.4 ATA (recommended in some standards for safety) the maximum depth allowed for air would be: 1.4/0.21 = 6.67 ATA or about 187 feet (subtract one atmosphere for the surface pressure). Most air diving is done at depths shallower than 187 feet, so oxygen toxicity is very unlikely when diving with air.
If you want to use a gas mixture other than air (usually 32% nitrox), you must calculate the partial pressure of oxygen at the depth you expect to reach in order to avoid entering the toxic range of oxygen. If you use a 32% nitrox mixture, with a safety limit of 1.4 ATA, the depth would be 1.4/0.32 = 4.37 ATA or about 112 feet. Based on the NOAA and Navy standards, a diver with 32% nitrox should be able to dive safely to 112 feet as long as the time is kept under 50 minutes.
However, there are other factors involved because divers have experienced oxygen seizures at 1.4 ATA. The reasons are varied. Excess work at depth can reduce the threshold for oxygen seizures, and elevated carbon dioxide levels in the blood will do the same. It is easy to "skip breath" with nitrox because of the increased oxygen partial pressure, but the consequence will be retention of carbon dioxide and risk of an oxygen seizure. Remember, the rate of breathing is driven by the need to remove carbon dioxide, and no matter how much oxygen is in the breathing gas, you need to breath the same amount of any gas to remove carbon dioxide. A tank of gas with increased partial pressure of oxygen will not last longer than a tank of air because the breathing rate must be maintained to eliminate carbon dioxide.
If you plan to use mixed gas scuba, a thorough understanding of oxygen toxicity and the ability to calculate oxygen partial pressure are essential for safe diving. A maximum of 1.3-1.4 ATA is recommended.