Which factor is crucial in selecting the optimal gas mix for a dive?

Selecting the optimal gas mix for a dive is fundamentally linked to the specified depth of the dive. Depth is crucial because it directly influences the partial pressures of gases in a mixture, which can impact how long a diver can safely spend at a given depth while minimizing the risks associated with nitrogen narcosis and oxygen toxicity.

Deeper dives subject divers to higher pressures, thus increasing the amount of gas absorbed by the body tissues. Therefore, when planning a dive, it is essential to choose a gas mixture that will effectively manage these pressures, particularly by limiting the proportion of nitrogen to reduce the risk of decompression sickness. This is why choosing the correct gas mix according to the dive's depth is paramount for both safety and dive duration considerations.

Factors like the diver's weight or water temperature can affect individual responses to diving, but the specific gas mix is primarily determined by the depth of the dive to ensure safe decompression.