What is the primary hazard of switching to another gas?

Switching to another gas during a dive, particularly if done below its maximum operating depth, can indeed pose a significant risk of central nervous system (CNS) oxygen toxicity. Oxygen has a partial pressure limit that varies with depth; when divers exceed this limit while breathing oxygen-rich mixtures at depths greater than what they are designed for, it can lead to toxic effects on the CNS. Symptoms of CNS oxygen toxicity can include visual disturbances, muscle twitching, seizures, and even loss of consciousness, which can have serious consequences while underwater.

Understanding the depths associated with different gas mixes is crucial for divers. Using a gas that is not suitable for the current depth not only increases the risk of oxygen toxicity but also can lead to disorientation and impaired decision-making, which are critical in the already complex environment of diving.