Dalton's Law of Partial Pressures states that for a mixture of gases in a container, the total pressure is equal to the sum of the pressures of each gas. Where P₁ is the partial pressure of gas 1, P₂ is the partial pressure of gas 2, and so on...

The gas that a diver breathes must be maintained at a partial pressure of 0.20 atm of oxygen. More oxygen could poison the diver, and less would lead to suffocation. The total pressure, however, must be equal to the external pressure to avoid collapsing the lungs. So, a special valve is used to equalize the pressure inside the divers lungs with the external pressure by adding helium gas. The valve also maintains oxygen levels by using Dalton's Law.