At what depth is oxygen toxicity harmful to a scuba diver?

Answer 1

The quick answer is that a diver breathing air should descend no deeper than about 218 feet of seawater due to concerns of oxygen toxcity.

Here's the longer answer:

Oxygen can become toxic to a diver as the partial pressure of the gas increases with increasing depth. The point at which this occurs varies based on the metabolism of the individual diver and time at depth. A general rule of thumb is 1.6 atm PPO2. The NOAA Diving Manual quotes the following maximum times at various oxygen partial pressures:

1.6 45 minutes

1.5 120 minutes

1.4 150 minutes

1.3 180 minutes

1.2 210 minutes

It is important to understand that these are not fixed limits. An individual diver's reaction to high pressures of oxygen varies greatly. Most mixed gas rebreathers are set to maintain 1.2 to 1.4 PPO2, allowing for this variability.

To understand what this means to a diver in terms of maximum depth, we need to understand how depth affects the partial pressure of oxygen. The partial pressure of any gas is the total pressure the gas is under, times the fractional percentage of the gas. Each foot of seawater increases the total pressure of the breathing gas by .030303 atm. This means that the total pressure at 33 feet is 2 atm (1 at the surface plus one of water). Total pressure at 66 feet is 2 atm, 99 feet is 3 atm, and so on.

Since air is about 21% oxygen, the PPO2 of air is .21 at the surface, where the total pressure is 1 atm. At a depth of 33 feet of seawater, the total pressure is 2 atm, making the PPO2 (2 X .21) or .42. Accordingly a PPO2 of 1.6 is reached at about 218 feet of sea water. This depth, 218 fsw, is generally considered the maximum safe depth for a diver breathing air.

The same formula can be used for other breathing gases. A diver breathing pure oxygen reaches 1.6 PPO2 at about 20 feet of seawater. A diver breathing Nitrox32, containing 32% oxygen, reaches 1.6 PPO2 at 132 feet.

Note that much of this is moot for most recreational divers. Recreational dive training agencies set a maximum depth of 40 meters (130 feet) for a variety of reasons including avoidance of decompression illness.