Why do deep-sea divers breathe a mixture of helium and oxygen?

Deep-sea divers must contend with a phenomenon known as gas narcosis, often colorfully termed the 'martini' effect. This condition occurs when divers breathe compressed air at substantial depths. The increased pressure causes gases like nitrogen to dissolve more readily in the bloodstream and fatty tissues, leading to a sensation similar to intoxication. This effect increases with depth, potentially resulting in a dangerous impairment of judgment, dexterity, and coordination.

Although the exact mechanisms are not entirely understood, gas narcosis is thought to alter the functioning of nerve cells in the brain. Divers often describe the onset of narcosis as a feeling of euphoria or confusion, making it challenging to carry out even simple tasks. To minimize this risk, special gas mixtures are employed, which is where knowledge of chemistry and physics comes into play to create a safer diving experience.

Nitrogen narcosis is a specific type of gas narcosis that occurs when divers using air or nitrogen-rich mixes go beyond certain depths. Known to affect divers at depths beyond 30 meters, or about 100 feet, nitrogen narcosis can lead to a compromised diving experience and pose significant risks.

Due to nitrogen's narcotic effect under pressure, a diver's neurological functions can be compromised. The depth at which narcosis impacts a diver can vary based on individual susceptibility, diving conditions, and the diver's experience and training. To combat nitrogen narcosis, the use of an alternative gas, such as helium, in a diver's breathing mix is a common solution. Helium's smaller atomic size and lower solubility in bodily fluids reduce the narcotic effect, allowing divers to descend safely to greater depths.

When it comes to decompression sickness (DCS), commonly known as 'the bends,' understanding the basics of human physiology and chemistry is essential for safe diving. DCS occurs when a diver ascends to the surface too quickly, causing gases that have dissolved in the body's tissues under pressure to form bubbles as the pressure decreases.

These bubbles can cause a range of symptoms from skin rashes and joint pain to more severe issues like paralysis or even death if not treated promptly with recompression therapy. Helium, with its lower solubility in comparison to nitrogen, leaves the body more quickly during the ascent, reducing the risk of DCS. Divers must follow carefully calculated decompression schedules to avoid this dangerous condition, allowing gas to be released from the body at a rate that prevents bubble formation.

A specially crafted mixture of helium and oxygen, known as heliox, strikes a balance between reducing the risks of gas narcosis and decompression sickness while providing the necessary oxygen for metabolism. Divers can't breathe pure oxygen at depth because of its toxicity, which increases with pressure and can cause convulsions and other symptoms in what's called oxygen toxicity syndrome.

The ideal diving gas mix addresses this by diluting oxygen with helium, thus ensuring the oxygen concentration remains at safe levels. The proportion of helium to oxygen will vary depending on the depth of the dive. Planning and executing dives with heliox require an in-depth understanding of dive tables and decompression schedules to maintain safety throughout the expedition. The physics of pressure and the chemistry of gases are key forces at play in this aspect of diving.