It can be said that oxygen is a poison which humans and other organisms just have adapted to tolerate to some extent by developing antioxidants, which work to prevent the damage from free radicals.

In fact, a long long ago there was a time when Earth's atmosphere didn't contain oxygen at all and everyone was happy. Then appeared organisms with the capability to produce oxygen and the oxygen content of the atmosphere started to rise. This is called the oxygen holocaust. Many, many species died and became extinct as the oxygen level rose before they could adapt.

When the oxygen level had finally stabilized, the remaining organisms had developed protection against oxygen and it later became a vital gas for a large part of all life on Earth.

Meanwhile, humans have inherited the resistance to oxygen poisoning, but in larger partial pressures that do not occur naturally the oxygen still becomes toxic. The limits used to be higher, but nowadays most organizations place the upper limit to 1.4 bar of ppO2 when working and to 1.6 bar in rest.

These limits are very easy to violate by breathing the wrong mix underwater. Technical divers usually carry a decompression gas with high oxygen content in addition to their bottom gas. Accidentally breathing the decompression gas (which typically has a 50 to 100 percent oxygen content) when deep underwater will put the diver into convulsions after a few breaths. This (breathing the wrong gas) is in fact the single most common cause of death in technical diving.

"Hey, I have been given 100% oxygen in hospital! What could possibly be poisonous about oxygen?!" I hear you holler, after reading the title of this write-up.

And you are correct; oxygen at 100% concentration isn't particularly bad for you in moderate quantities.

However, there is a way that you can breathe oxygen at concentrations that are more than 100%? How? By subjecting your body to pressure, and breathing oxygen at that increased pressure. This could happen in one of several ways, but the most common ways are by breathing compressed gas in a hyperbaric chamber, or whilst scuba diving.

Symptoms of Oxygen Toxicity

Divers tend to remember the symptoms of Oxygen Toxicity with the VENT - IDC mnemonic.

V - Visual narrowing (tunnel vision)

E - Ears (ringing in your ears)

N - Nausea

T - Twitches, especial facial twitches

I - Irritability and personality changes

D - Dizziness

C - Convulsions

The 'IDC' part of this mnemonic is memorable for divers because the instructor course in dive-world is known as the Instructor Development Course - or the IDC. For new divers, the mnemonic is changed to C - VENT - ID, which puts 'convulsons' as the very first symptom. Cute, but if you don't catch oxygen toxicity before you start convulsing, you're going to be facing a pretty big problem - convulsing under water tends to lead to drowning, which, in turn, tends to cause stress to your dive buddy. Very rude indeed.

Partial pressures explained

Oxygen only becomes dangerous to us at certain partial pressures. A partial pressure of a gas is the percentage of that gas multiplied by the pressure it is breathed at. So, for 100% oxygen at normal atmospheric pressure, you're breathing a partial pressure of O2 (PPO2) of 1.0.

There is some debate about at which level oxygen toxicity sets in; like with many other physiological toxins, it's hard to say how high a partial pressure would affect me (a 30-year-old reasonably fit male), compared to, say, a 70 year old slightly over-weight man with circulation problems, or a 20-year-old olympic athlete.

In diving, the accepted 'safe' limit whilst in motion (i.e swimming around) is a partial pressure of 1.4. Whilst doing decompression stops, the 'safe' limit is a PPO2 of 1.6.

Based on this, we can calculate the depth at which we can breathe 100% oxygen. At 4 meters depth, we are at a pressure of 1.4 bar - which equals a partial pressure of 1.4 whilst breathing 100% oxygen. In other words: If you were breathing pure oxygen at depths beyond 4 meters, you'd be putting yourself at risk.

Using the same mathematics, we can calculate how deep we can dive on air (21% oxygen). At 56m / 184ft, we are under 6.6 bar of pressure. Multiply by 0.21, and we get a partial pressure of 1.386 - or close enough to 1.4 to be 'safe'.

Diving on other air mixes

Of course, divers don't just dive on 21% oxygen (and rarely dive on 100% oxygen) - we also use Nitrox, which uses air mixes to reduce the nitrogen content in our blood to avoid decompression sickness. Typical mixes include 32%, 36%, and 40% oxygen, with maximum operating depths of 32m/105ft, 28m/92ft, and 24m/79ft, respectively.

A full table of depths and oxygen percentages is below.

             21%   22%   26%   28%   30%   32%   34%   36%   38%   40%   50%   60%   70%   80%   90%  100%
0.5m / 2ft  0.22  0.23  0.27  0.29  0.32  0.34  0.36  0.38  0.40  0.42  0.53  0.63  0.74  0.84  0.95  1.05
1m / 3ft    0.23  0.24  0.29  0.31  0.33  0.35  0.37  0.40  0.42  0.44  0.55  0.66  0.77  0.88  0.99  1.10
2m / 7ft    0.25  0.26  0.31  0.34  0.36  0.38  0.41  0.43  0.46  0.48  0.60  0.72  0.84  0.96  1.08  1.20
4m / 13ft   0.29  0.31  0.36  0.39  0.42  0.45  0.48  0.50  0.53  0.56  0.70  0.84  0.98  1.12  1.26  1.40
6m / 20ft   0.34  0.35  0.42  0.45  0.48  0.51  0.54  0.58  0.61  0.64  0.80  0.96  1.12  1.28  1.44  1.60
8m / 26ft   0.38  0.40  0.47  0.50  0.54  0.58  0.61  0.65  0.68  0.72  0.90  1.08  1.26  1.44  1.62  1.80
10m / 33ft  0.42  0.44  0.52  0.56  0.60  0.64  0.68  0.72  0.76  0.80  1.00  1.20  1.40  1.60  1.80  2.00
12m / 39ft  0.46  0.48  0.57  0.62  0.66  0.70  0.75  0.79  0.84  0.88  1.10  1.32  1.54  1.76  1.98  2.20
14m / 46ft  0.50  0.53  0.62  0.67  0.72  0.77  0.82  0.86  0.91  0.96  1.20  1.44  1.68  1.92  2.16  2.40
16m / 52ft  0.55  0.57  0.68  0.73  0.78  0.83  0.88  0.94  0.99  1.04  1.30  1.56  1.82  2.08  2.34  2.60
18m / 59ft  0.59  0.62  0.73  0.78  0.84  0.90  0.95  1.01  1.06  1.12  1.40  1.68  1.96  2.24  2.52  2.80
20m / 66ft  0.63  0.66  0.78  0.84  0.90  0.96  1.02  1.08  1.14  1.20  1.50  1.80  2.10  2.40  2.70  3.00
22m / 72ft  0.67  0.70  0.83  0.90  0.96  1.02  1.09  1.15  1.22  1.28  1.60  1.92  2.24  2.56  2.88  3.20
24m / 79ft  0.71  0.75  0.88  0.95  1.02  1.09  1.16  1.22  1.29  1.36  1.70  2.04  2.38  2.72  3.06  3.40
26m / 85ft  0.76  0.79  0.94  1.01  1.08  1.15  1.22  1.30  1.37  1.44  1.80  2.16  2.52  2.88  3.24  3.60
28m / 92ft  0.80  0.84  0.99  1.06  1.14  1.22  1.29  1.37  1.44  1.52  1.90  2.28  2.66  3.04  3.42  3.80
30m / 98ft  0.84  0.88  1.04  1.12  1.20  1.28  1.36  1.44  1.52  1.60  2.00  2.40  2.80  3.20  3.60  4.00
32m / 105f  0.88  0.92  1.09  1.18  1.26  1.34  1.43  1.51  1.60  1.68  2.10  2.52  2.94  3.36  3.78  4.20
34m / 112f  0.92  0.97  1.14  1.23  1.32  1.41  1.50  1.58  1.67  1.76  2.20  2.64  3.08  3.52  3.96  4.40
36m / 118f  0.97  1.01  1.20  1.29  1.38  1.47  1.56  1.66  1.75  1.84  2.30  2.76  3.22  3.68  4.14  4.60
38m / 125f  1.01  1.06  1.25  1.34  1.44  1.54  1.63  1.73  1.82  1.92  2.40  2.88  3.36  3.84  4.32  4.80
40m / 131f  1.05  1.10  1.30  1.40  1.50  1.60  1.70  1.80  1.90  2.00  2.50  3.00  3.50  4.00  4.50  5.00
42m / 138f  1.09  1.14  1.35  1.46  1.56  1.66  1.77  1.87  1.98  2.08  2.60  3.12  3.64  4.16  4.68  5.20
44m / 144f  1.13  1.19  1.40  1.51  1.62  1.73  1.84  1.94  2.05  2.16  2.70  3.24  3.78  4.32  4.86  5.40
46m / 151f  1.18  1.23  1.46  1.57  1.68  1.79  1.90  2.02  2.13  2.24  2.80  3.36  3.92  4.48  5.04  5.60
48m / 157f  1.22  1.28  1.51  1.62  1.74  1.86  1.97  2.09  2.20  2.32  2.90  3.48  4.06  4.64  5.22  5.80
50m / 164f  1.26  1.32  1.56  1.68  1.80  1.92  2.04  2.16  2.28  2.40  3.00  3.60  4.20  4.80  5.40  6.00
52m / 171f  1.30  1.36  1.61  1.74  1.86  1.98  2.11  2.23  2.36  2.48  3.10  3.72  4.34  4.96  5.58  6.20
54m / 177f  1.34  1.41  1.66  1.79  1.92  2.05  2.18  2.30  2.43  2.56  3.20  3.84  4.48  5.12  5.76  6.40
56m / 184f  1.39  1.45  1.72  1.85  1.98  2.11  2.24  2.38  2.51  2.64  3.30  3.96  4.62  5.28  5.94  6.60
58m / 190f  1.43  1.50  1.77  1.90  2.04  2.18  2.31  2.45  2.58  2.72  3.40  4.08  4.76  5.44  6.12  6.80
60m / 197f  1.47  1.54  1.82  1.96  2.10  2.24  2.38  2.52  2.66  2.80  3.50  4.20  4.90  5.60  6.30  7.00
62m / 203f  1.51  1.58  1.87  2.02  2.16  2.30  2.45  2.59  2.74  2.88  3.60  4.32  5.04  5.76  6.48  7.20
64m / 210f  1.55  1.63  1.92  2.07  2.22  2.37  2.52  2.66  2.81  2.96  3.70  4.44  5.18  5.92  6.66  7.40
66m / 217f  1.60  1.67  1.98  2.13  2.28  2.43  2.58  2.74  2.89  3.04  3.80  4.56  5.32  6.08  6.84  7.60
68m / 223f  1.64  1.72  2.03  2.18  2.34  2.50  2.65  2.81  2.96  3.12  3.90  4.68  5.46  6.24  7.02  7.80
70m / 230f  1.68  1.76  2.08  2.24  2.40  2.56  2.72  2.88  3.04  3.20  4.00  4.80  5.60  6.40  7.20  8.00

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