What is CO?
Carbon monoxide (CO) is a colourless, odourless, and tasteless gas produced by the incomplete combustion of carbon-based fuels. It cannot be detected by human senses under any circumstances — no smell, no taste, no visible signs. For divers, this matters enormously, because the breathing gas in a scuba cylinder is filled under pressure from a compressor, and if that compressor introduces CO into the fill, there is no way to detect it without an instrument.
Understanding what CO is, how it gets into your cylinder, and what it does to your body underwater is one of the most important pieces of knowledge any diver can have.
The Science Behind CO
Carbon monoxide forms when carbon-based fuels — petrol, diesel, oil, or lubricants — burn without sufficient oxygen. The result is a molecule with one carbon atom bonded to one oxygen atom (CO), rather than the two oxygen atoms found in the harmless CO₂ produced by complete combustion.
The problem for divers is what CO does inside the body. When inhaled, CO binds to haemoglobin — the protein in red blood cells that carries oxygen — with approximately 200 times the affinity of oxygen itself. Once CO occupies a haemoglobin binding site, oxygen cannot. The result is a progressive reduction in your blood’s oxygen-carrying capacity, leading to hypoxia, even while you continue breathing normally. Your body is starving of oxygen while feeling no urgency to breathe harder, because the sensation of breathlessness is triggered by CO₂ buildup — not oxygen deficit.
How CO Gets Into Scuba Cylinders
Unlike household CO exposure — which typically comes from faulty appliances or poor ventilation — CO contamination in diving breathing gas almost always originates at the compressor. There are three main pathways:
Exhaust intake contamination is the most common cause. If a compressor’s air intake is positioned close to a vehicle exhaust, a generator, or even the compressor’s own engine exhaust, combustion gases including CO can be drawn directly into the compression stream and concentrated into the cylinder fill.
Oil combustion inside the compressor can occur when a compressor overheats or when lubricating oil degrades and burns in the compression stages. This produces CO internally, regardless of what the intake air quality is.
Inadequate filtration means that even low levels of ambient CO in the surrounding air — present in any busy marina, boat yard, or urban dive shop — can accumulate in the fill if the compressor’s filtration cartridges are exhausted, overdue for replacement, or not rated for the operating conditions.
All three causes share one characteristic: they produce no smell, no taste, and no visible change to the gas. A contaminated fill looks, smells, and tastes identical to a clean one.
Why Depth Makes It Worse
A CO concentration that might produce only mild symptoms at the surface becomes significantly more dangerous at depth, due to the effect of increased ambient pressure on partial pressures.
At 10 metres of seawater (2 bar absolute), the partial pressure of every gas in your breathing mix doubles. At 30 metres (4 bar absolute), it quadruples. This means a CO concentration that reads as 10 PPM at the surface is physiologically equivalent to 40 PPM at 30 metres — well above the EN 12021 limit of 5 PPM for compressed breathing air used in diving.
The practical consequence is that a fill which passes a surface smell-check — even if that were reliable, which it is not — could still deliver a meaningfully elevated CO dose at recreational diving depths.
Recognised Limits for CO in Diving Air
The primary international standard governing the quality of compressed breathing gas for diving is EN 12021, which sets a maximum CO limit of 5 PPM for compressed air intended for breathing apparatus. The equivalent North American standard, CGA Grade E, sets the same 5 PPM limit.
These thresholds exist because even sub-symptomatic CO exposure during a dive can impair judgment, reduce reaction time, and accumulate across multiple dives in a day — making what feels like mild fatigue after diving a potential indicator of low-level CO exposure rather than simple exertion.
Dive shops and compressor operators who follow best practice test their fills regularly against EN 12021 using laboratory analysis. However, laboratory testing is periodic — typically monthly or quarterly — and cannot detect contamination events that occur between tests, such as an intake obstruction, a compressor fault, or a change in the surrounding environment.
Symptoms of CO Poisoning Underwater
The symptoms of CO poisoning are particularly dangerous for divers because they can easily be attributed to other causes:
Early symptoms — headache, nausea, mild dizziness, unusual fatigue — are frequently dismissed as seasickness, dehydration, or the exertion of the dive itself.
Progressive symptoms — confusion, loss of coordination, impaired judgement — can be mistaken for nitrogen narcosis, especially at depth.
Severe symptoms — loss of consciousness, incapacitation — leave the diver unable to manage their own ascent or signal their buddy.
The combination of symptom misattribution and the physiological effects of depth make CO one of the most difficult in-water emergencies to recognise and respond to correctly. By the time symptoms are severe enough to be unmistakeable, the diver may already be incapacitated.
Pre-Dive Gas Checking
Because CO contamination is undetectable by the human senses and because laboratory testing cannot provide point-of-use assurance, pre-dive gas checking with an instrument has become an increasingly recognised practice among informed divers and dive operators.
Portable gas analysers such as the Go-NOGo Gas Tester provide indicative CO readings at the point of use — giving the diver a go/no-go indication based on the EN 12021 threshold before they enter the water. This does not replace periodic laboratory analysis of the compressor’s output, but it does provide a fast, practical check that identifies contamination events that may have occurred since the last formal test.
The Go-NOGo Gas Tester detects CO at concentrations as low as 1 PPM, provides a clear pass/fail indication referenced to the EN 12021 limit, and delivers a result in under 30 seconds. It is an indicative instrument — readings should be used to inform the diver’s go/no-go decision, which remains the diver’s own responsibility.
What to Do If CO Poisoning Is Suspected
If you or your buddy experience symptoms consistent with CO poisoning during or after a dive — headache, nausea, unusual confusion, or disproportionate fatigue — treat it as a medical situation immediately:
- End the dive — signal your buddy and begin a controlled ascent following normal safety stop procedures
- Administer 100% oxygen at the surface as soon as possible — this is the primary first-line treatment and accelerates CO elimination from the blood
- Seek immediate medical attention — CO poisoning can have delayed effects and symptoms can recur or worsen after initial improvement; professional medical evaluation is essential even if the diver appears to recover
- Do not re-use the cylinder — set it aside for testing; if multiple divers are affected, all cylinders from the same fill session should be quarantined and the fill station notified immediately
- Contact DAN (Divers Alert Network) — DAN operates a 24-hour emergency line and can advise on hyperbaric treatment if required