Understanding Deep Sea Diving Breathing Mixtures

Deep sea diving involves descending to significant depths where the pressure of the surrounding water is much higher than at the surface. This increased pressure affects the gases that divers breathe, and the standard air mixture (approximately 21% oxygen and 79% nitrogen) used for shallow dives becomes problematic.

Why Air is Not Suitable for Deep Diving

At increased pressures encountered in deep sea diving, the components of air can cause issues:

• Nitrogen Narcosis: Nitrogen, which is an inert gas at surface pressure, becomes narcotic under high pressure. This effect, often described as feeling 'narcotized' or drunk, impairs a diver's judgment, coordination, and ability to react, becoming increasingly severe with depth.
• Oxygen Toxicity: While essential for life, oxygen becomes toxic at high partial pressures. The partial pressure of oxygen increases with depth. Breathing air below certain depths can lead to central nervous system oxygen toxicity, potentially causing symptoms like visual disturbances, twitching, dizziness, nausea, behavioral changes, and seizures, which can be fatal underwater.

Finding a Better Breathing Gas for Deep Dives

To overcome these issues, deep sea divers use special breathing gas mixtures. The goal is to replace or reduce the amount of nitrogen and/or oxygen in the mix relative to air at depth, while still providing enough oxygen for the diver's metabolic needs.

The Role of Helium in Deep Sea Diving

The question asks what gas is mixed with oxygen to help deep-sea divers breathe. The correct answer is helium.

Helium is preferred over nitrogen for deep diving for several reasons:

• Inertness: Like nitrogen, helium is an inert gas, meaning it doesn't chemically react with the body. However, helium is significantly less narcotic than nitrogen at pressure. This greatly reduces the risk of nitrogen narcosis during deep dives.
• Low Density: Helium is much less dense than nitrogen. Breathing a less dense gas mixture at high pressure reduces the work of breathing, making it easier for the diver to ventilate their lungs.
• Lower Solubility: Helium is less soluble in body tissues than nitrogen. This means less helium is absorbed into the body tissues during a dive. Although divers still need to ascend slowly to avoid decompression sickness (the 'bends'), the decompression stops may be shorter or managed differently compared to breathing nitrogen-based mixes at equivalent depths and times.

A common breathing mixture containing oxygen and helium is called Heliox. The proportion of oxygen and helium in Heliox varies depending on the planned depth and duration of the dive.

Why Other Options Are Incorrect

• Chlorine: Chlorine is a highly toxic and corrosive gas. It is used in disinfectants and industrial processes, not for breathing mixtures.
• Nitrogen: As explained, while nitrogen is in air and used for shallow dives, it causes severe nitrogen narcosis and increases the risk of decompression sickness at the pressures encountered in deep diving.
• Ozone: Ozone (O₃) is a highly reactive form of oxygen and a strong oxidizing agent. It is a pollutant at ground level and is toxic to breathe, damaging the lungs. It is not used in breathing mixtures.

Summary of Gases in Diving

Therefore, deep-sea divers carry cylinders of oxygen mixed with helium to safely breathe at high pressures.

Revision Table: Deep Diving Breathing Gases

Additional Information: Other Diving Gas Mixes

Besides Heliox, other gas mixtures are used in diving, especially for technical and commercial deep diving:

• Trimix: A mixture of Oxygen, Helium, and Nitrogen. Trimix is often used for deep dives where a diver wants to benefit from helium's properties (reduced narcosis, easier breathing) but also wants to keep the mixture cost down (helium is expensive) by including some nitrogen, or for thermal reasons (helium conducts heat away from the body faster than nitrogen, which can make divers feel colder). Different Trimix blends are used for different depths and purposes.
• Nitrox: An oxygen-enriched air mixture (more than 21% oxygen, less nitrogen). Nitrox is typically used for recreational diving to shallower or moderate depths to reduce nitrogen absorption and extend no-decompression limits, not for very deep diving where oxygen toxicity becomes the primary limitation.

The choice of breathing mixture depends heavily on the planned maximum depth, dive duration, water temperature, and the specific training and equipment of the diver.