Helium - oxygen mixture is used by deep sea divers in preference to nitrogen - oxygen mixture. Why?
Think about blood being mostly water. Now look at the solubility of He vs Nitrogen at 10C
http://www.engineeringtoolbox.com/gases-solubility-water-d_1148.html
And as one comes up from the depths, pressure is reduced, and the excess gas comes out of solution and can block blood vessels (esp in the brain).
Which is the most desirable gas to be dissolved in the blood?
Why isn't pure oxygen used in the breathing mix? Think on that.
a quick google search on helium diving will answer your question
nitrogen narcosis and the bends.
While under great pressure, the most abundant gas in our atmosphere (over 79%) of nitrogen slows down the mental processes of man.
His condition may be similar to alcohol intoxication.
The use of a helium-oxygen mixture, commonly referred to as heliox, is preferred by deep-sea divers over a nitrogen-oxygen mixture, known as nitrox, for several reasons. Let's discuss why:
1. Decreased narcotic effects: Nitrogen, when breathed under high pressure, can cause a condition known as nitrogen narcosis or "raptures of the deep." This condition impairs cognitive function, similar to being under the influence of alcohol or drugs. Helium, on the other hand, has a much lower narcotic effect, making it safer for deep-sea diving.
2. Reduced decompression sickness: Nitrogen, which is a major component of air, can lead to decompression sickness (also known as "the bends"). This occurs when divers ascend too quickly, causing the dissolved nitrogen in their tissues to form gas bubbles. These bubbles can block blood vessels, leading to severe and potentially life-threatening consequences. Helium has a lower rate of diffusion into tissues, reducing the likelihood of decompression sickness.
3. Improved communication: Helium's buoyant properties make it advantageous for divers to communicate effectively underwater. Compared to nitrogen, it causes the divers' voices to have a higher pitch, making it easier to understand each other in the sometimes noisy and challenging underwater environments.
To summarize, a helium-oxygen mixture is preferred over a nitrogen-oxygen mixture for deep-sea diving due to the reduced narcotic effects of helium, the decreased risk of decompression sickness, and the improved communication capabilities it provides.