Why is breathing pure oxygen at 1 atmosphere toxic, but breathing pure oxygen at 1/3rd atmosphere safe for astronauts?

Question

Why is breathing pure oxygen at 1 atmosphere toxic, but breathing pure oxygen at 1/3rd atmosphere safe for astronauts?

In: Biology

Answer 1

Partial Pressure of Oxygen on Earth is at about 1/5th of an atmosphere.

At an ELI5 perspective, Oxygen is highly reactive — it oxidizes things and reacts with various tissues in your body. Your body is naturally able to handle that at “normal” concentrations for people (i.e. what’s in the air we breathe day-to-day). But, when you increase that concentration, your body can no longer handle it.