Carbon monoxide in diving

Carbon monoxide in diving Unfortunately, diving incidents, some fatal, are still being caused by carbon monoxide toxicity from contaminated breathing gas, as Gavin Anthony explains

Carbon monoxide (expressed chemically as CO) is a colourless and odourless gas that is highly toxic. Toxicity primarily results from low oxygen levels in the cells, also known as cellular hypoxia. Carbon monoxide has a nominally 300 times greater affinity for binding to haemoglobin in red blood cells than oxygen. This means that the blood is less able to transport oxygen, around the body. The body is further starved of oxygen because, once it is bound to carbon monoxide, the haemoglobin molecule changes shape and it releases oxygen much less readily. At a cellular level there are additional toxic mechanisms, especially those affecting the circulation and the nervous system. As a result, subsequent to the initial hypoxia, longer term complications can occur such as dementia and symptoms like Parkinson’s disease.

CARBON MONOXODE TOXICITY The level of carbon monoxide toxicity is normally expressed in terms of both the concentration of carbon monoxide, as inhaled, and the percentage of haemoglobin carrier sites that are bound with monoxide. An inhaled concentration (at atmospheric pressure) of 200 ppm (parts per million) or 0.02 %, may result in 15-20 % of haemoglobin sites being bound; at this level, clinical symptoms such as headaches, fatigue, nausea and dizziness would start to occur. Active smokers may inhale carbon monoxide levels up to 60 ppm and have 10 % or more of their haemoglobin bound with carbon monoxide. In the UK the current occupational exposure limit (HSE EH 40) for carbon monoxide breathed at the surface (i.e. atmospheric pressure) is 20 ppm.

When considering the toxic effect of gases during diving, it is the partial pressure of the Image courtesy of MMC Diving Services

“An inhaled concentration (at atmospheric pressure) of 200 ppm (parts per million) or 0.02 %, may result in 15-20 % of haemoglobin sites being bound; at this level, clinical symptoms such as headaches, fatigue, nausea and dizziness would start to occur.”

gas, rather than the fraction or percentage that should be considered. An air dive to 50 m (6 bar) will expose the diver to a total gas pressure six times that at the surface, thus breathing a carbon monoxide level of 20 ppm at 50 m could be considered clinically equivalent to breathing 120 ppm at atmospheric pressure (1 bar), a level which would cause severe symptoms. As depth increases so does the inspired partial pressure of oxygen, offering some mitigation against the affinity of carbon monoxide over oxygen to bind to haemoglobin. However, as a diver ascends the partial pressure of oxygen reduces, but the carbon monoxide remains bound to the haemoglobin, so amplifying its toxic effects.

Haemoglobin without any oxygen bound to it is blue in colour (hence cyanosis when someone is hypoxic), it turns red when oxygen is attached. When carbon monoxide is attached instead of oxygen it turns a bright (cherry) red.

CONSIDERATIONS FOR DIVERS Traditionally divers have been taught to look out for symptoms such as cherry red face and lips; in reality this is very rarely seen. The symptoms of headaches, fatigue, dizziness and nausea are more common. In extreme cases carbon monoxide poisoning can cause unconsciousness, severe neurological symptoms, cardiac issues and even death. As

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ever with diving, these symptoms can also occur with other diving illnesses; anyone experiencing unusual symptoms during or post diving should seek medical advice. Fortunately, as with decompression illness, a treatment for carbon monoxide poisoning is to give the diver oxygen to breathe. In certain situations hyperbaric oxygen therapy in a compression chamber may be used to treat carbon monoxide poisoning.

For air diving, it is typical to specify a maximum acceptable level of contaminants at a level considerably less than those acceptable at the surface. The current standard for compressed gas purity is BS EN 12021:2014 and in Table 1 it specifies 5 ppm as the maximum permitted level of carbon monoxide in compressed air. It also states (Para 6.1) that contaminants should be one sixth of the national exposure limit. Prior to 2018 the HSE EH 40 occupational limit (8 h TWA level) for carbon monoxide was 30 ppm. Applying a one sixth logic to the national exposure limit gives a diving adjusted limit of 5 ppm, i.e. no conflict with the Table. However, in 2018 EH40 was revised and the UK 8 h TWA level for carbon monoxide was reduced to 20 ppm. Applying the one sixth logic to the national exposure limit gives a diving adjusted limit of 3.33 ppm, i.e. the one sixth limit now conflicts with the 5 ppm limit in the table.

This conflict is currently being addressed by BSi and it is expected that the UK National foreword to BS EN 12021 will be revised to state ‘If a gas or contaminant level is specified in a Table it should be applied and not one sixth of the national EH40 limit’. Thus if this is applied there will be no change in the operational requirement to provide compressed air containing no more than 5 ppm of carbon monoxide.

MINIMISING THE RISK OF CO CONTAMINATION Carbon monoxide is produced as the result of an inefficient combustion of hydrocarbons; instead of being completely converted to carbon dioxide and water, the combustion also produces carbon monoxide and other chemical species. There are primarily two mechanisms by which an unacceptable level of carbon monoxide may end up compressed into diving cylinders. Probably the best understood is carbon monoxide being drawn into a compressor inlet from an external source such as a motor vehicle exhaust or other fossil fuel combustion e.g. gas heaters. A second, more subtle method is pyrolysis (chemical decomposition by heat), which might occur when a compressor is hot, but not necessarily overheating. Pyrolysis may cause the lubricating oil in the compressor to break down releasing carbon monoxide, or other plastic/organic compounds in the system to decompose creating toxic chemical species. The risk of carbon monoxide poisoning as a result of either of these mechanisms may be reduced by correct compressor operation and maintenance such as: positioning of the inlet away from and upstream of exhaust fumes, use and periodic replacement of the correct lubricating oil, ensuring compressors are adequately cooled and including a carbon monoxide catalyst (such as hopcalite) in the filter system – these catalysts convert the carbon monoxide to carbon dioxide.

The quality of compressed air from a compressor or supply bank may be checked by portable test apparatus (such as colorimetric gas detector tubes) or by taking a sample for detailed laboratory analysis. The requirements and procedures for these analyses are presented in the UK in an Annex of BS EN 12021. Samples for analysis should be taken at least every three months or more frequently if contamination is foreseeable within this period. Also the air from portable compressors should be checked each time they have been moved to a new location. Whilst this is a very useful procedure for monitoring routine maintenance and filter performance, it would not identify an acute event between samples, e.g. a combustion exhaust, unknown to the compressor operator, being moved close to the inlet (such as a vehicle stopped outside a compressor house with its engine running) or a component overheating and producing carbon monoxide by pyrolysis.

PREVENTION Analysis of diving incidents has shown that high levels of carbon monoxide may be found in the diving cylinders, even if the air from the compressor was analysed at the recommended test intervals. All operators should be aware of the risks of a sudden change in supply gas to the diver. To mitigate this, on-line analysis of at least oxygen and carbon monoxide should be undertaken on the gas in the direct supply to the diver. It is also reasonable to apply a parallel logic to all diving compressor and breathing gas supplies, and to continuously check the level of oxygen and major toxic components, in the air being used to fill diving cylinders.

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