Please see https://www.itij.com/latest/news/carbon-monoxide-detectors-travel
We recommend that you put CO alarms to EN 50291 in your hand luggage. Please be careful not to pack so tightly that the the test button is squeezed. However, if the carbon monoxide (CO) alarm sounds fully in the aircraft, it could be a fumes event in the ‘plane so explain this to the crew and suggest oxygen masks are used.
Please don’t think carbon monoxide only happens abroad. Carbon monoxide can happen to anyone, anywhere at any time.
Please also be aware that a carbon monoxide alarm will only sound at relatively high levels of CO.
A CO alarm is NOT a monitor.