Do scuba divers breath pure oxygen?

Believe it or not, we can die from too little oxygen AND too much oxygen.

This concept involves partial pressures.

If we first consider using standard air (i.e. 20% Oxygen and 80% Nitrogen):

(1 bar = 1 kilogram of pressure per square centimetre)

At the surface - Total Pressure of air = 1 bar

- Pressure of Oxygen = 0.2 bar

- Pressure of Nitrogen = 0.8 bar

At 10m - Total Pressure of air = 2 bar

- Pressure of Oxygen = 0.4 bar

- Pressure of Nitrogen = 1.6 bar

At 20m - Total Pressure of air = 3 bar

- Pressure of Oxygen = 0.6 bar

- Pressure of Nitrogen = 2.4 bar

At 30m - Total Pressure of air = 4 bar

- Pressure of Oxygen = 0.8 bar

- Pressure of Nitrogen = 3.2 bar

At 40m - Total Pressure of air = 5 bar

- Pressure of Oxygen = 1.0 bar

- Pressure of Nitrogen = 4.0 bar

At 50m - Total Pressure of air = 6 bar

- Pressure of Oxygen = 1.2 bar

- Pressure of Nitrogen = 4.8 bar

At 60m - Total Pressure of air = 7 bar

- Pressure of Oxygen = 1.4 bar

- Pressure of Nitrogen = 5.6 bar

At 70m - Total Pressure of air = 8 bar

- Pressure of Oxygen = 1.6 bar

- Pressure of Nitrogen = 6.4 bar

It is at the partial pressure of 1.6 bar (which occurs at 70m where oxygen becomes toxic. That is why most dive organisations recommend 50m as a maximum for recreational diving.

However, if pure oxygen is used:

At surface - Pressure of Oxygen = 1 bar

At 10m - Pressure of oxygen = 2 bar

The oxygen has already become toxic!

Hope that answers your question. The concept is called "partial pressures" and "oxygen toxicity" if you want to research more on a search engine.