Enriched Air Diving
Enriched air, containing over 21% oxygen (commonly 32% or 36%), extends dive time by increasing no-decompression limits, but it reduces the maximum safe diving depth.
The International Standard Atmosphere (ISA) is a model of the pressure exerted by the Earth's atmosphere. At sea level, the pressure of a standard atmosphere is:
Instead of hPa, divers measure pressure in atmospheres. One atmosphere equals 1013.25 hPa. At 30 metres underwater, you'd feel 4 atmospheres of pressure: 3 from the water and 1 from the air above.
Air pressure is made up of about 21% oxygen and 79% nitrogen. This means the oxygen itself exerts 0.21 atmospheres of pressure, and the nitrogen exerts 0.79 atmospheres at the surface. When you dive to 20 meters, the total pressure increases to three atmospheres. Because of this, the individual pressures of the gases also triple: oxygen becomes 0.63 atmospheres, and nitrogen becomes 2.37 atmospheres.
Using enriched air increases the amount of oxygen and reduces the amount of nitrogen we breathe:
| Mixture | Oxygen | Nitrogen |
|---|---|---|
| Air | 21% | 79% |
| EANx32 | 32% | 68% |
| EANx36 | 36% | 64% |
We evolved to breathe air at 1 atmosphere of pressure. Breathing at higher pressures can cause decompression sickness, oxygen toxicity, and nitrogen narcosis.
Decompression Sickness
When diving, nitrogen from the air we breathe dissolves into our body until it reaches the same pressure as the air. Because nitrogen is inert, this buildup isn't harmful while underwater. However, during ascent, the pressure decreases, and the dissolved nitrogen needs to be released to regain equilibrium. If this release happens too quickly, nitrogen bubbles can form in the body, leading to decompression sickness.
At 10 meters deep, where the pressure is 2 atmospheres, the nitrogen partial pressure doubles to 1.58 atmospheres. This means your body absorbs twice as much nitrogen as at the surface, once it reaches equilibrium. By using breathing gas mixtures with less nitrogen, we reduce the amount absorbed. Dive computers and specialized dive tables calculate no-decompression limits based on this reduced nitrogen absorption.
Unlike nitrogen, oxygen doesn't cause decompression sickness because our bodies actively use it. Our physiology regulates the partial pressure of oxygen delivered to our cells. This system functions effectively within the partial pressure ranges experienced during recreational diving. As a result, only inert gases contribute to decompression sickness.
Oxygen Toxicity
Too much oxygen can lead to pulmonary toxicity or central nervous toxicity.
Pulmonary Toxicity
Breathing oxygen at high partial pressures for extended periods can irritate the lungs, causing a burning sensation and cough. The time required for these symptoms to develop is significantly longer than recreational dive durations.
Central Nervous System Toxicity
Central Nervous System (CNS) toxicity can have a sudden and unpredictable onset, leading to seizures and unconsciousness. A diver experiencing CNS toxicity underwater would almost certainly drown. There is ongoing discussion about safe oxygen partial pressure limits, with conservative estimates at 1.4 atmospheres and a generally accepted upper limit of 1.6 atmospheres.
Using standard air (21% oxygen), to reach a 1.4 atmospheres oxygen partial pressure, you would have to dive to 6.6 atmospheres (56.6 metres). This is beyond recreational diving limits. Breathing pure oxygen would result in reaching the 1.4 atmospheres danger zone at just 4 metres deep. This is why breathing gas mixtures must always include inert gases to reduce the oxygen concentration.
The safe maximum depth must be calculated for every dive using exact percentage of oxygen in your mixture. The maximum depth in atmospheres can be calculated by dividing the 1.4 atmosphere limit by the partial pressure of oxygen.
Nitrogen Narcosis
Nitrogen narcosis can cause impaired judgment and intoxication-like symptoms, leading to risky behaviors such as disregarding decompression stops or depth limits. These effects rapidly subside upon ascending to a shallower depth. Most divers don't experience noticeable narcosis shallower than 30 metres.
Air dives are more susceptible to nitrogen narcosis. Using an enriched air mixture, with a higher oxygen and lower nitrogen content, like EANx32, reduces the risk. For example, at 10 metres (2 atmospheres), the partial pressure of nitrogen on EANx32 is 1.36 atmospheres, compared to 1.58 atmospheres on air.
Pre-dive Checks
Oxygen content will vary between cylinders due to production differences. You must check your cylinder's oxygen percentage with an oxygen meter before diving. Input the measured percentage into your dive computer for accurate no-decompression and maximum depth calculations.
This information should be written on your cylinder to show that the checks have been completed:
