Sentence:  Is there a compromise between too much oxygen and too much inert gas? The answer is the use of enriched air <WSD>nitrox</WSD> (enriched air) mixtures in the SCUBA tank. The two most commonly used mixtures are 32 percent and 36 percent oxygen mixtures. [...] [T]he increased oxygen percentages make the diver using <WSD>nitrox</WSD> mixtures more subject to oxygen seizures. With <WSD>nitrox</WSD> mixtures, the relative increase in oxygen percentage is over three times the relative decrease in nitrogen percentage. 
Meaning of the word marked with <WSD> tags:  A mixture of nitrogen and oxygen, the nitrogen content being lower than what is normally present in air, which is used in place of air as a breathing gas.