How can I protect myself from carbon monoxide poisoning when I dive?

It was a beautiful Caribbean day and the water conditions were excellent, but Anna was feeling confused. Water had started to seep into her mask and, although she knew very well how to clear it, somehow she was unable to get the water out.

Sensing the onset of anxiety, she started to ascend. Concerned, the divemaster followed her up, signaling to Pauline, the other diver in his charge, that she should wait there on the seabed and he would come back.

On the surface, Anna removed her regulator, adjusted her mask, gathered her thoughts and decided she had been foolish. So, although she still did not feel well, when the divemaster swam over, she gave him the thumbs down signal, indicating that she wanted to continue with the dive.

They descended again, but when they arrived on the bottom, Pauline was nowhere to be found. Very soon, however, Anna began feeling sick and disorientated again and the divemaster too started having problems.

"I felt dizzy,” he said afterwards. “I don't remember much, but I know my eyes closed at some point and during our ascent I got a bad pain in my chest. It was terrible."

Once they reached the surface, they signalled for the boat to come and get them and raised the alert. A search was carried out for Pauline but she was never found.

An official enquiry was ordered but, by that time, there was no way of identifying which tanks Anna and the divemaster had used and blood tests of the two surviving divers revealed nothing abnormal. The reason the investigators wanted to examine the tanks and the divers’ blood was that they suspected that the culprit behind the accident was carbon monoxide gas.

What is carbon monoxide?

Carbon monoxide is a completely invisible, tasteless and odourless gas that is formed when fuels such as gas, oil, coal and wood do not burn fully. For example, car engines generate carbon monoxide, as do kerosene heaters and wood stoves. It is very poisonous and can be fatal when inhaled in a closed space.

Why is this?

The reasons why carbon monoxide is so poisonous are poorly understood. It is well known that it bonds with the haemoglobin in our bloodstream much better than oxygen, so when carbon monoxide is present, less oxygen is delivered to our body tissues. However, at mild to moderate levels this can be compensated for by an increase in blood flow so that, although our blood contains less oxygen, we can still get enough to continue to function normally. However, when even a little carbon monoxide is present inside a scuba cylinder and we go diving, as we go deeper the partial pressure of the gas increases and its effect on us becomes greater. Carbon monoxide poisoning affects brain function and our respiratory system in particular. Common symptoms include headaches, fatigue, nausea, dizziness and confusion.

How can it get into a scuba cylinder?

Carbon monoxide can get into a scuba cylinder while the cylinder is being filled if exhaust fumes from an engine nearby are polluting the air entering the compressor. On a liveaboard, fumes from the boat engine could be diverted by the wind into the intake or, on land, the carbon monoxide could come from a car parked with its engine running close to the dive shop’s compressor room. The source might even be the exhaust outlet from the compressor engine itself if it is broken or poorly located.

How will you know?

Without deploying a little technology, you can’t know. De-Ox and other companies make analysers (see image below), which can be used to detect the presence of carbon monoxide in a scuba cylinder. They work the same way as the analysers that you use to measure the oxygen content in your tank before a nitrox dive. You place the analyser next to your tank valve outlet and allow a little air to run over the sensor. If the analyser detects the presence of carbon monoxide it will tell you. Similar devices are available that dive centres and filling stations can add to compressor systems, which will warn them if they have a carbon monoxide problem. Top dive centres will post certificates like the one below, showing that the air they provide divers has been tested professionally and found to be within safe parameters.

Otherwise, the first indication will be that you start to feel unwell during the dive. If this happens then assume the worst, gather your dive team together and abort the dive to live to dive another day.

Remember the golden rule: one up – all up! Never leave a diver behind.

Learn more about this in Scuba Confidential - An Insider's Guide to Becoming a Better Diver