# What does partial pressure of oxygen mean?

Both you and I are currently breathing air at approx. $1 \cdot a t m$ pressure. In terms of partial pressures, this is $0.21 \cdot a t m$ $\text{dioxygen}$, and $0.79 \cdot a t m$ $\text{dinitrogen}$. The total pressure is the sum of the partial pressures.