Why Doesn't Everyone Dive Enriched Air

Problems with Oxygen Toxicity

We started out this discussion by saying that the PADI Open Water Diver Manual warns beginners not to use cylinders marked enriched air unless they are properly trained. The reason for this warning has to do with a little problem known as oxygen toxicity.

Believe it or not, oxygen, which we need to stay alive, can cause a diver to convulse when pressure increases on descent. This generally happens when the partial pressures of oxygen exceed 1.6 atmospheres. Although the convulsions themselves are not life-threatening, the convulsions often cause a diver to lose the regulator, aspirate water, and drown. To prevent this from happening, an enriched air diver calculates how deep a blend of enriched can be used, called the maximum operating depth (MOD), and marks this on the enriched air tank.

Enriched air dive computers will calculate the maximum operating depth for the diver. The diver only needs to enter the following into the computer:

  • The enriched air blend (for example, 36%)
  • The partial pressures of oxygen the diver wishes to set (normally 1.4)
  • Optionally: the type of water (Saltwater or Freshwater)

In the example above, if the diver were diving in saltwater, the maximum operating depth would be 95 feet.

When breathing normal air on a dive, we would have to descend to depths below 200 feet before oxygen toxicity becomes a problem. However, using enriched air or oxygen on a dive can cause us to reach depths that are potentially dangerous well within traditional recreational depth limits. For this reason divers must be properly trained to use enriched air to avoid a life-threatening accident.

The example above further highlights why divers making very deep dives will not be able to use enriched air at depth. If the diver were to descend to 130 feet, the maximum operating depth would be exceeded and there would be a risk of oxygen toxicity.