The gas inhaled by divers is a mixture of several gases that comprises it. Air is composed of approximately 78 percent nitrogen, 21 percent oxygen and one percent trace gases, such as argon, carbon dioxide and water vapor. For our purposes, we will refer to air as being 21 percent oxygen and 79 percent nitrogen.
A diver that breathes air or Nitrox will always be breathing the two main component gases oxygen and nitrogen. What changes is the percentage of the two gases within the breathing mixture.
Dalton’s Law states that the total pressure exerted by a mixture of gases is the sum of the partial pressure of each gas in the mixture.
Partial Pressure is the pressure each individual gas within a mixture exerts.
For example, using air as the subject gas: at sea level, we inhale air at one atmosphere of pressure, which is equal to 1 atm / 14.7 psi. Oxygen makes up 21% of air, or more precisely, 0.21×1 atm = 0.21 atm (or 0.21×14.7 psi = 3.09 psi) and nitrogen makes up the other 79%, 0.79×1 atm = 0.79 atm (or 0.79×14.7 psi = 11.61 psi).
A scuba regulator delivers gas from the cylinder at a pressure equal to the ambient pressure. This means that at depth, a diver inhales air that is at the same pressure the diver is subjected to; therefore, a diver at 33 feet / 10 meters will be under twice the amount of pressure, and will breath gas from the cylinder at twice the amount of pressure. While the percentages of oxygen and nitrogen in the air contained in the cylinder remain the same, the amount the diver inhales is twice as much.
Again, using air as an example, the diver inhales 21% oxygen, at 10 meters / 33 feet, or twice the pressure: 0.21×2 atm = 0.42 atm (or 0.21×29.4 psi = 6.18 psi). The partial pressure of nitrogen at this depth is also twice as much: 0.79×2 atm = 1.58 atm (or 0.79×29.4 psi = 23.22 psi). Combined, the gases form a total pressure of 2 atm / 29.4 psi, which is exactly the total ambient pressure exerted at 10 meters / 33 feet.
Based on Dalton’s Law, we can say that in practice, the partial pressure of a gas within a mixture would be the pressure of that gas if it alone occupied the volume of the mixture.
This concept forms the basis for understanding what the effects are of breathing compressed gas while under water (increased ambient pressure).