The Gas Laws are very important to our understanding of decompression sickness, the way they work together in the body enables us to understand how nitrogen works when under pressure and how it effects divers.
Definition: ‘a law stating that the pressure exerted by a mixture of gases in a fixed volume is equal to the sum of the pressures that would be exerted by each gas alone in the same volume.’
Dalton’s Law is also known as Dalton’s Law of Partial Pressures.
As a diver descends the total pressure of the air he is breathing increases, following Boyle’s Law, thus, the partial pressures of the individual gases in the breathing air are increased proportionally. As the diver descends deeper nitrogen dissolves in the blood, and is carried to all the body tissues until a new balance has been reached.
At higher partial pressures the gas in the blood alters the electrical properties in the cerebral nerve cell membranes. This may cause a narcotic anaesthetic, Nitrogen Narcosis or a certain level of toxicity in the body.
During descent the partial pressure increases and so the amount of dissolved oxygen in the body increases. Breathing 100% Oxygen at 18m will cause oxygen toxicity in as little as 30-60 minutes. At 100m the normal 21% oxygen in the tank can become toxic to a diver because the diver is breathing in more oxygen per breath.
For these reasons divers (mostly military or professional, but increasingly sport divers as well) learn to use specialised mixtures which can replace the nitrogen with helium. The percentage is so small and provides a partial pressure which supports life without inducing oxygen toxicity.