Understanding Oxygen Exposure Limits: CNS & MOD Explained

Dan Johnson
22 hours ago
2 min read

When diving with Enriched Air Nitrox, we extend our bottom time — but we also increase our exposure to oxygen. Understanding oxygen limits isn’t just theory from the Nitrox course — it’s critical for safe dive planning.

Let’s break it down.

What Is CNS Oxygen Toxicity?

CNS (Central Nervous System) oxygen toxicity occurs when a diver is exposed to elevated partial pressures of oxygen (PO₂) for too long.

The primary concern?A sudden convulsion underwater.

This is why we respect oxygen exposure limits and plan our dives carefully.

Recreational nitrox diving typically limits PO₂ to:

1.4 ATA during the working phase of the dive
1.6 ATA as an absolute contingency limit

Exceeding these limits increases the risk of oxygen toxicity.

What Is MOD and Why Does It Matter?

MOD (Maximum Operating Depth) is directly linked to oxygen exposure.

MOD tells us the deepest depth we can safely dive with a specific nitrox mix before exceeding our chosen PO₂ limit.

For example:

EAN32 at 1.4 PO₂ → MOD ≈ 33 meters
EAN36 at 1.4 PO₂ → MOD ≈ 28 meters

The higher the oxygen percentage, the shallower your MOD.

That’s the trade-off:More oxygen = longer no-deco time But also = shallower depth limit

How CNS % Works

Dive computers track oxygen exposure as a percentage.

CNS % accumulates over the dive and resets slowly over time.If you reach 100%, you’ve reached your recommended oxygen exposure limit for that period.

Important:

• Multiple dives in one day increase cumulative exposure• Short high-PO₂ exposures can spike CNS quickly• Proper planning keeps you well within safe limits