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Most divers breathe compressed air whilst diving, and so do not normally need to consider oxygen levels except in relation to extremely deep diving.
However, many divers also breathe an oxygen enriched mixture called "Nitrox" to enable them to dive longer with incurring decompression penalties due to the build up of nitrogen in their tissues.
However, Nitrox carries its own risks. If a diver breathes a mix which is too rich in oxygen for a certain depth, she may black out due to oxygen toxicity.
To ensure that this does not happen, Nitrox divers are trained never to breathe gas mixes where the partial pressure of oxygen is greater than 1.4 ATA (atmospheres absolute).
To calculate the partial pressure of oxygen, one simply multiplies the percentage of oxygen in the gas by the pressure at depth. So at the surface breathing air (which is 21% oxygen at 1 ATA), we are breathing 0.21 ATA. However, if we were breathing a mix of 36% Nitrox at a depth of 99 feet (so, 32% oxygen multiplied by 4 ATA (the absolute pressure at 99 feet), we are breathing oxygen at 1.28 ATA. Still safe, but getting much closer to the limit of 1.4 ATA.
The formula can be reversed to work out the maximum depth at which you can safely dive on a particular mix of Nitrox. The maximum depth (D) can be calculated using this formulae:
D = 33 x ((1.4 / FO2) - 1)
Where FO2 is the fraction of oxygen in the mix (D is calculated in feet, to change the formula for meters, use 10 instead of 33). So for example, if the diver wanted to use 50% Nitrox, then D = 33 x ((1.4 / 0.50) - 1) = 59 feet.
The limit of 1.4 ATA is recommended by NOAA, but it is not an absolute limit. Although the science is not exact, NOAA suggests that a diver can breathe oxygen at 1.4 ATA for 150 minutes (2.5 hours) without blacking out. However, for richer mixtures, that time drops exponentially. A diver can only breathe oxygen at 1.6 ATA in diving conditions for approximately 45 minutes before blacking out.
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