What action is taken if a recompression chamber is not available and a diver experiences CNS toxicity at 20 fsw?

When dealing with CNS (central nervous system) toxicity during a dive, the primary action revolves around reducing the exposure to the toxic environment, which in this case is high partial pressure of oxygen. When a recompression chamber is not available, the best course of action is to adjust the diver's environment to mitigate the effects of the toxicity.

Shifting to a shallower depth and ventilating is a critical response because by ascending to a shallower depth, the partial pressure of oxygen in the surrounding environment decreases. This helps alleviate the symptoms of CNS toxicity as it reduces the amount of oxygen that is exerting pressure on the CNS. Additionally, facilitating ventilation supports the removal of any excess oxygen that may already be present in the diver's bloodstream, further reducing toxic effects.

In contrast, the other options do not effectively address the immediate risk of CNS toxicity. Proceeding directly to the surface without ensuring adequate ascent rates and safety stops can lead to decompression sickness. Waiting for symptoms to resolve ignores the acute nature of CNS toxicity, which can lead to serious neurological damage if not promptly addressed. Continuing with the original oxygen schedule would maintain the same levels of CNS exposure, potentially worsening the situation.

Thus, adjusting the depth and ensuring proper ventilation is the safest and most effective