What should a diver do if they experience CO2 toxicity?

When a diver experiences CO2 toxicity, the appropriate response is to ventilate the helmet. This action helps to reduce the concentration of carbon dioxide in the breathing gas. CO2 toxicity, also known as hypercapnia, occurs when there is an excess of carbon dioxide in the bloodstream, often as a result of inadequate breathing or increased metabolic CO2 production.

Ventilating the helmet allows fresh, oxygen-rich gas to enter while expelling CO2 that has accumulated due to the diver's breathing. This is particularly important in environments where CO2 can build up quickly, such as while utilizing closed-circuit rebreather systems where the CO2 scrubber may not be functioning effectively or where a diver is working hard and producing more CO2.

Increasing work effort is not advisable; it can exacerbate CO2 retention and worsen the situation. Deepening the dive would similarly increase the partial pressure of CO2 and could make the situation more hazardous. Continuing to breathe normally, without addressing the buildup of CO2, fails to mitigate the toxic effects and does not provide a solution to the underlying issue. Thus, ventilating the helmet is the best approach to alleviate CO2 toxicity.