Other gases Breathing gases such as trimix and heliox are used in technical diving to reduce nitrogen narcosis by replacing a portion of nitrogen in the gas mixture with helium, thus reducing the partial pressure of nitrogen at depth. Equivalent air depth is a commonly used way of expressing the narcotic effect of different breathing gases. Standard tables list conversion factors for narcotic effect: for example, neon at a fixed pressure has a narcotic effect equivalent to nitrogen at 0.23 times the pressure, so in principle it should be usable at four times the depth. Some gases have other dangerous effects when breathed at pressure; for example, high-pressure oxygen can lead to oxygen toxicity. Helium is the least intoxicating of the breathing gases, but it can cause high pressure nervous syndrome, a still-mysterious but apparently unrelated phenomenon. Inert gas narcosis is only one factor which influences the choice of gas mixture; the risk of decompression sickness and oxygen toxicity, cost, and other factors are also important.