What is the primary method to prevent CNS oxygen toxicity during a dive?

• A. Stay within oxygen exposure limits
• B. Increase oxygen partial pressure to accelerate the dive
• C. Ignore exposure limits and extend the dive
• D. Always breathe 100 percent oxygen during the dive

Answer: (A)

The main idea is that CNS oxygen toxicity is driven by how high the partial pressure of oxygen is in the breathing gas. The safest, most reliable way to prevent it is to plan and conduct the dive so you stay within established oxygen exposure limits. In practice, this means choosing gas mixes and depth/time so the inspired O2 pressure stays below the recommended ceiling (for example, commonly about 1.4 atm during the active portion of the dive, with a brief allowance up to around 1.6 atm if needed). On a closed-circuit dive, your computer or gas planning keeps you within these limits, prompting adjustments like gas-switching or changing depth.

Pushing the PO2 higher to go faster, ignoring exposure limits, or breathing 100 percent oxygen for extended periods all raise the risk of CNS toxicity, potentially causing seizures or unconsciousness without warning. Staying within oxygen exposure limits is the foundational method to prevent CNS oxygen toxicity.