CO/CO₂ in Breathing Gas: Sources of Contamination and How to Detect It

A Real Story: When Something Felt Wrong

Before we get into theory, here’s a real situation that shaped my approach to gas safety.

Years ago, before I even had a driver’s license, I was diving at Attersee with an older, experienced buddy. On our second dive, something changed. He started feeling unwell underwater—nausea, weakness, a growing sense that something wasn’t right.

This wasn’t someone who complained lightly.

Back on the surface, he described symptoms that reminded me of carbon monoxide exposure: nausea, fatigue, and a strange, worsening discomfort that had already started during the dive. I checked what I could—I even looked at his fingernail beds, expecting the “cherry red” sign I had learned about—but I wasn’t confident in what I saw.

I insisted we go to a hospital.

He refused.

He was the only one who could drive, so we headed home. During the journey, his condition worsened—especially when we were stuck behind a smoking tractor or idling at a border crossing (this was before Schengen). Eventually, he slept it off.

At the time, it felt like a close call—but not a confirmed one.

Only later did we get the answer.

We had the tank analyzed. The results were clear: the air was contaminated due to a poorly maintained compressor. It contained elevated levels of carbon monoxide—and a significant amount of oil contamination. Residual oil was even found settled at the bottom of the cylinder.

That moment changed how I think about gas quality.

Because the real danger wasn’t just what happened—it was how easy it was to dismiss it.

CO vs CO₂: Two Different Problems, One Dangerous Outcome

Carbon Monoxide (CO)

Binds to hemoglobin ~200× more strongly than oxygen Reduces your blood’s ability to carry oxygen Leads to:

• Headache *Dizziness
• Confusion
• Loss of consciousness

At depth, the effects accelerate due to increased partial pressure.

Carbon Dioxide (CO₂)

Caused by inadequate removal (e.g., scrubber failure in CCR) Leads to hypercapnia Symptoms:

• Air hunger
• Anxiety/panic
• Impaired decision-making

CO₂ is particularly dangerous because it feels like stress or exertion, so divers often ignore it.

Why Depth Makes It Worse

At depth:

• Partial pressures increase
• Your body absorbs gases more rapidly
• Symptoms escalate faster—and recovery is slower

Where Contamination Comes From (Real-World Scenarios)

Common CO Sources

Compressor intake near:

• Exhaust fumes (cars, boats, generators)
• Fires or burning materials
• Poor compressor maintenance *Overheating compressors

Common CO₂ Sources

• CCR scrubber issues:
• Incorrect packing

• Exhausted sorb

• Excessive work of breathing (Gas density, pressure, CCR design)

• Skip breathing

Why It’s Hard to Spot Underwater

Symptoms can mimic:

• Fatigue
• Seasickness
• Nitrogen narcosis
• Stress or dehydration

“Divers often rationalize early symptoms instead of reacting to them. That delay is where risk escalates.” — Divesoft safety team

The Rule: Don’t Power Through It

If something feels off:

Don’t “wait and see” Don’t “push through one more dive”

Abort early. Always.

How to Detect Contamination (Before It’s Too Late)

Late Detection (Warning Signs)

• Sudden fatigue
• Nausea during dive
• Confusion or poor coordination
• Unusual breathing discomfort

Early Detection (What Actually Works)

Tools

• CO analyzers for tanks
• Inline CO monitors at fill stations
• Quality gas analyzers (ECHO)

Habits

• Always analyze your gas—not just O₂/He
• Be aware of compressor environment
• Track patterns (same fill station? same symptoms?)

Essential CCR Safety Rules (Gas Integrity Focus)

• Treat air quality as non-negotiable input
• Always perform a proper pre-breathe
• Maintain strict scrubber discipline:
• Consistent packing
• Track usage time
• Define team abort criteria before diving
• Always have bailout ready and accessible

The Inspection / Check Procedure (Save This)

1. Before Choosing a Fill Station

Check compressor location (away from exhaust/fumes) Ask about:

• Maintenance schedule
• Filtration system
• Look for certification (e.g., EN 12021 compliance)

2. During Filling

Observe surroundings:

• Any running engines nearby?
• Smell of exhaust?
• Avoid filling during:
• Heavy traffic nearby
• Poor ventilation conditions

3. After Filling / Before the Dive

• Analyze O₂ and He
• If available, test for CO
• Trust your instincts—if something feels off, don’t use the gas

4. CCR-Specific Pre-Dive

• Perform full pre-breathe
• Verify scrubber packing and duration
• Confirm loop integrity
• Review bailout plan with team

5. If Something Feels Wrong

Hard stop rules:

• Abort the dive immediately
• Do not rationalize symptoms
• Quarantine suspect gas
• Re-test before any further use

Tools & Resources

DAN (Divers Alert Network) – gas safety guidelines
EN 12021 standard – defines acceptable air quality limits
Divesoft analyzers and planning tools – integrate checks into your workflow

Consider building your own repeatable checklist and saving it on your phone.

Why This Matters

Gas contamination incidents are rare—but when they happen, they escalate fast.

And as the story at the beginning shows, the biggest risk is often not recognizing the problem early enough.

Final Thought

• You can’t see CO.
• You can’t reliably smell it.
• And underwater, you might not interpret the symptoms correctly.

But you can control your preparation.

Build habits. Use tools. Trust your instincts. Abort early.

That’s what keeps dives safe.