Analyze your gas! - soon possible on a broader scale

Last year, I tried to draw attention in the dekoblog to disturbing reports of carbon monoxide (CO) in breathing gas. For example, an Italian technical diver in Sharm el-Sheikh discovered helium contaminated with carbon monoxide (CO) in the diluent before a CCR dive. The industry is now addressing this problem. Halcyon will soon launch the first multi-gas analyser that can measure not only oxygen and helium, but also detect carbon monoxide.

Halcyon Trisense Symbios

It is suspected that the case in Sharm el-Sheikh involved contaminated imported gas from Russia, where low quality standards apply. However, this is speculation. What is clear, though, is that the concentration measured during the dive, at 88 ppm (parts per million, 0.0088 vol.%), could have had potentially catastrophic consequences.

The concentration of CO in a gas or the environment is expressed in ppm (parts per million). ‘Clean’ air contains 0.1 ppm CO. The normal value in a home is 0.5 to 5 ppm (e.g. wood burning; 5 ppm = 0.0005 vol.%). The maximum permissible workplace concentration is 30 ppm (max. 42 hours/week). The European standard EN12021 stipulates that breathing gas must not contain more than 5 ppm CO.

CO is invisible, odourless and is oxidised to CO2 in the atmosphere. CO can contaminate breathing gas in the following ways (list not complete):

• Absorption of exhaust gases from incomplete combustion of hydrocarbons

• Oil pyrolysis: CO is produced in the final stage at temperatures > 200°C from oil in contaminated compressors

• Cracking: complex hydrocarbon molecules from oil residues in diving cylinders are broken down into smaller, simpler molecules via chemical processes under heat and/or pressure

• Smoke from cigarettes and cigars

• Inferior distillation of helium during extraction from natural gas

The production of nitrox, for example using the membrane process, further increases the number of CO molecules in the gas. The production of 40 per cent nitrox, for example, increases the CO concentration by up to three times.

CO has the following effects on the body:

• CO displaces oxygen from haemoglobin (Hb), its transport protein in erythrocytes (red blood cells), by binding to it with an affinity 240 times stronger than that of O2. This reduces the O2 transport capacity of the blood.

• CO alters Hb in such a way that it is less able to release O2 in the tissues.

• CO impairs O2 utilisation by interfering with key proteins involved in cellular respiration. This effect is particularly strong in the heart, whose function is impaired as a result.

• CO produces harmful waste products from O2 that can directly damage proteins and cells, especially neuronal cells.

CO intoxication leads to internal suffocation by hindering O2 transport, O2 delivery to the tissues and O2 utilisation in vital organs (e.g. the heart).

In addition, it causes long-term damage to the nervous system due to reactive oxygen species.

The severity of CO poisoning depends on the CO and oxygen concentration in the respiratory gas, the duration of exposure and the ventilation rate (minute volume, MV). The higher these are, the greater the percentage of Hb occupied by CO (%CO-Hb). This percentage correlates approximately with the symptoms (Table 1).

ppm	%CO-Hb	effects on the body
<5	1	none
10	1.8	none
30	5	none
60-150	10-20	headache, shortness of breath
150-300	20-30	vertigo, nause, impairment of coordination
300-650	30-50	vomiting, dizziness, loss of consciousness
650-1000	50-65	coma, death if untreated
>1000	>65	death

table 1: AMV 20 l/’, 1 h exposition

DAN recommends a maximum of 5 ppm CO in breathing gas. Why not 10 ppm, which is still safe? Because this value means that there is probably a source of CO in the compressor area that needs to be investigated. Or because the origin of the gas supplied needs to be checked. 

People who have been poisoned by carbon monoxide do not turn blue, but remain rosy-cheeked.

The diagnosis must be made on site, e.g. in the case of fires, based on the circumstances. It is important to note that people suffering from CO poisoning do not turn blue as they would in the case of O2 deficiency, but remain rosy-cheeked. In the emergency room, the %CO-Hb can be determined relatively easily in the blood. If there is even the slightest suspicion, high doses of O2 must be administered at the scene of the accident. In cases of severe CO poisoning, the treatment of choice is hyperbaric oxygen therapy (hyperbaric chamber treatment), which displaces CO from Hb. This reduces the half-life of CO-Hb from 4-6 hours when breathing air to approximately 1.5 hours at 1 bar O₂ and to 30 minutes at approximately 2.5 bar O₂.

Emergency treatment: high-dose oxygen or hyperbaric chamber treatment

The problem with diving is that Dalton's law also applies to CO. This means that as the ambient pressure increases, the pressure of CO also increases proportionally.

If, for example, the breathing gas is contaminated with a CO concentration of 30 ppm and is used for diving to a depth of 30 m (4 bar ambient pressure), the amount of CO reaching the Hb also increases fourfold. This would correspond to a CO concentration of 120 ppm at the water surface with a resulting CO-Hb of 17%, which leads to symptoms.

Due to Dalton's law, the effect of CO concentration multiplies during the dive.

The good news is that the increased partial pressure of oxygen during diving counteracts this. The increased amount of dissolved O2 can support the supply to the tissue, while CO-Hb transports less O2. This is why divers can remain symptom-free at depth.

During ascent, however, the partial pressure of oxygen and thus the amount of dissolved O2 decreases. This does not apply to CO-Hb, however, as it is a chemical bond. It can be assumed that divers who have breathed CO at depth are therefore at the highest risk of losing consciousness when surfacing, as they then lose the protection provided by the increased partial pressure of O2 at depth. However, the toxic limits of CO at depth and their modification by different ambient and O2 partial pressures have not been investigated.

When breathing CO, the risk of unconsciousness is greatest at the end of the dive due to the falling partial pressure of O2.

Smoking one pack of cigarettes a day leads to a CO-Hb level of approx. 3–6%. This baseline value must be added to the CO poisoning level. Smokers therefore have a worse prognosis than non-smokers in the event of CO poisoning.

Taking the 88 ppm CO mentioned at the beginning and assuming that a moderately demanding technical dive to 60 m had been carried out, this would have corresponded to an equivalence of approximately 600 ppm CO at the surface at a depth of 60 m (7 x 88 ppm). Even if we assume that the high O2 setpoint of the rebreather (approx. 1.2 bar PO2) would have counteracted this, it is easy to see that the dive could have ended in disaster.

So will we also have to measure the CO content in the breathing gas in future? Until now, even high-quality gas analysers for technical diving have not been able to detect carbon monoxide. However, Halcyon is launching the first multigas analyser that can also measure CO next spring.

The use of a CO analyser is highly recommended for divers travelling around the world.

The report mentioned at the beginning clearly shows that it makes sense to test your breathing gas yourself – especially if the source or processing of the gas is not known beyond doubt. Anyone diving in regions where gas quality may be poor – and this is apparently already the case in popular diving destinations in Egypt – would be well advised to rule out CO contamination. The new analyser from Halcyon makes checking this a lot simpler.