It appears that in the UK there is no specific government or gas industry guidance that describes the recommended actions which should be taken in the event of a carbon monoxide alarm sounding.
It is left to manufacturers to provide guidance and this can be wanting at best. For example, some typical advice from a manufacturer can be seen at the end of this page. The user is not told to evacuate the property until instruction No.4. I suggest this should be instruction No.1 and that can be seen in my suggested list of actions on the home page of this web site.
Gas Safe suggested to me that the response to a carbon monoxide alarm can be the same as when smelling leaking gas. I disagree for the very simple reason that there is no “odour cue” to help us estimate how exposed we have become.
And if an alarm sounds there is also no means of assessing the degree of exposure. If the alarm is faulty then it may not sound until carbon monoxide levels are very high, so it is safest to assume the worst.
The vast majority of people to whom I have spoken did not know that carbon monoxide combines chemically with the blood and is not readily displaced just by entering fresh air. CO blocks the receptors in the blood that normally attach to oxygen. If the blood cannot take up oxygen then the victim effectively suffocates. Moving into the fresh air does not displace the CO and so the receptors remain blocked – the victim continues to suffocate.
The treatment includes wearing an oxygen mask with 100% oxygen under pressure to forcibly displace the CO from the blood. Hence, leaving the property and seeking medical advice should be one of the very first actions in my estimation.
Any statement about time eg “Waiting five minutes before re-entering the property” has no technical merit and indeed could prove fatal advice. There is no reason for believing that carbon monoxide concentrations are safe after five minutes or even five hours. It may not even be safe for you to move - an exposed person merely moving can induce collapse.
If a gas appliance has become faulty and is producing carbon monoxide, then re-entry at any time without specialist equipment and training is not safe and risks death.
The above details are the results of my research but it appears there is a need for clarification by authoritative organisations.
From The NHS web site*
After breathing in carbon monoxide
It enters your bloodstream and mixes with haemoglobin (the part of red blood cells that carry oxygen around your body), to form carboxyhaemoglobin.
When this happens, the blood is no longer able to carry oxygen, and this lack of oxygen causes the body’s cells and tissue to die.
Treating carbon monoxide poisoning
You will need oxygen therapy treatment in hospital if you have been exposed to a high level of carbon monoxide, or have symptoms that suggest exposure.
Oxygen therapy involves breathing in 100% oxygen through a tight-fitting mask (normal air contains about 21% oxygen). Breathing in concentrated oxygen enables your body to quickly replace carboxyhaemoglobin.
<Carbon Monoxide alarm sounds>
1. Open doors and windows.
2. Turn off all fuel appliances where possible.
3. The alarm can be silenced by pushing the button.
4. Evacuate the properly leaving doors and windows open.
5. Get medical help immediately.
6. Ring the gas emergency number.
7. Do not re-enter the property until the emergency services give the ‘All Clear’.
8. Ensure that fuel-burning appliances are checked before re-lighting.