What should you do if the diver shows signs of CNS oxygen toxicity at a 30-20 fsw water stop?

When a diver exhibits signs of Central Nervous System (CNS) oxygen toxicity at a specific water stop, the first and most appropriate response is to shift to air. CNS oxygen toxicity can occur when a diver is exposed to high partial pressures of oxygen, leading to symptoms such as visual disturbances, muscle twitching, or convulsions.

By switching to air, the partial pressure of oxygen that the diver is exposed to is significantly reduced, minimizing the risk of severe adverse effects associated with CNS toxicity. This action helps in stabilizing the diver's condition and allows for safe management of the situation while potentially preparing for a proper ascent or further treatment if necessary.

Continuing on oxygen at this stage would exacerbate the situation, increasing the risk of further CNS impairment. Similarly, surfacing immediately without addressing the situation could be dangerous, as it does not provide immediate relief from the symptoms. Descending to a lesser depth is also not advisable because it does not alleviate the exposure to high levels of oxygen that could be causing toxicity. Therefore, transitioning to air is a critical and effective intervention in managing CNS oxygen toxicity during diving.