Dalton’s Law and Henry’s Law have explained that every gas contained in a mixture that is breathed under increased atmospheric pressure will have a proportionately higher partial pressure and the amount of gas that dissolves into our blood is directly proportional to the partial pressure of each gas in the mixture.
As we’ve discussed, the nitrogen contained in air may cause nitrogen narcosis when breathed at increased atmospheric pressure. Breathing oxygen at increased partial pressure may also have potential negative physiological effects.
Prolonged exposure to elevated partial pressures of oxygen has a toxic effect on the lungs and can damage the nervous system. This is referred to as oxygen toxicity and its onset is indicated by the symptoms of hyperoxia.
Variable exposure to oxygen partial pressures of 0.6 to 1.6 atm for anywhere from hours to days will result in pulmonary oxygen toxicity. Symptoms usually manifest as coughing, chest pain and shortness of breath, also known as the Lorraine/Smith effect.
Breathing oxygen with a partial pressure of 1.6 atm or higher may result in central nervous system oxygen toxicity, or the Paul Bert effect, in which muscle twitching in the face and hands may occur, and ultimately lead to full body convulsions. Tolerance to high partial pressures of oxygen varies from diver to diver, and may be different for each diver on any given day.
It is highly unlikely that a diver will experience oxygen toxicity during a dive breathing air or recreational Nitrox mixes, as long as divers stay within the recommended limits: the Oxygen Partial Pressure of any gas mixture breathed during a dive shall not exceed 1.40 atm.