Which gas is known not to cause narcosis at typical recreational diving depths?

Helium is the gas known not to cause narcosis at typical recreational diving depths. This characteristic primarily stems from the physical properties of helium. Inert and non-toxic, helium is not metabolized by the body and does not produce narcotic effects like other gases, such as nitrogen and oxygen, when subjected to increased pressure underwater.

At greater depths, divers often experience nitrogen narcosis, which occurs due to the effects of nitrogen on the central nervous system under high pressures. Oxygen, while vital for breathing, can also be toxic at high pressures, especially beyond 1.6 ATA (atmospheres absolute), where it can lead to central nervous system toxicity. Carbon dioxide, while also a gas involved in respiration, can lead to hypercapnia when accumulated, particularly under increased workloads or inadequate ventilation, but is not directly associated with narcosis.

Due to these properties, helium is frequently utilized in mixed gas diving to mitigate the risks associated with narcosis and oxygen toxicity, allowing divers to operate safely at greater depths. This makes it especially advantageous in deep or technical diving scenarios.