# How do you find the partial pressure of oxygen?

We are currently breathing air at $1$ $a t m$ pressure. Given the normal composition of air, 78% dinitrogen, 21% dioxygen, 1% other gases, we can say quite categorically that the partial pressure of dioxygen in the atmosphere is approx. $0.2 \cdot a t m$, and the partial pressure of dinitrogen is approx. $0.8 \cdot a t m$. The total pressure is of course the sum of the individual partial pressures.