Partial Pressure

In a mixture of gases, each gas contributes to the total pressure of the mixture. This contribution is the partial pressure. The partial pressure is the pressure of the gas if the gas were in the same volume and temperature by itself. Dalton's law states the total pressure of a mixture of ideal gases is the sum of the partial pressure of each individual gas.
While the usual symbol for pressure is P or p, partial pressure is indicated by a subscript (e.g., P1 or p1).

Partial pressure is the measure of the thermodynamic activity of gas molecules. The gasses diffuse and react based on their partial pressures and not concentrations in a gaseous mixture. Partial pressure is important in the fields of chemistry, physics, and biology. Blood levels of oxygen and carbon dioxide are determined by measuring their partial pressure.