What should the diver do if experiencing symptoms of oxygen toxicity during an air stop?

In the context of dealing with symptoms of oxygen toxicity during an air stop, switching to air immediately is the most appropriate action. Oxygen toxicity can occur when divers breathe higher partial pressures of oxygen, usually when using enriched air or during deep dives. Symptoms can include visual disturbances, auditory changes, seizures, and other neurological effects.

When a diver begins to experience these symptoms, the safest response is to reduce the partial pressure of oxygen they are breathing, which can be done effectively by switching to air. Air contains a lower concentration of oxygen compared to pure oxygen or higher mixes, thereby mitigating the risk of toxicity. Continuing to breathe oxygen could exacerbate the symptoms and increase the risk of a serious incident.

Surfacing immediately might not be safe, as it poses the risk of decompression sickness, and remaining at the current depth without action does not address the immediate risk of oxygen toxicity. Therefore, switching to air provides a quick and effective way to reduce the danger of toxicity symptoms while ensuring the diver remains safe.