Writing as toxicologist who specializes in CO poisoning and thinks it is the cause of CFS,
there are many ways for people to get high CO levels.
You can inhale high CO from external sources (called exogenous),
and/or make high CO internally (called endogenous),
or just make a normal amount but excrete less, which will cause your total CO to build up.
We all make endoCO 24/7 from before birth to after death via the breakdown of heme proteins by heme oxygenase enzymes (HO-1 and HO-2 in humans), which all living things have some form of (plants, animals, bacteria).
HO-1 is known as Universal Stress Enzyme because so many different stressors boost its activity, including infections of all kinds (esp. viruses, which don't have any CO of their own but boost ours), heat, light, odors, sounds, EMF, radiation, exercise, and mental stressors. The same pathway also produces heat, so whenever you have a fever, you have high CO.
Normal healthy CO and body temp should go up and down together, from low on waking to high in the afternoon. But during a CO hangover, after CO poisoning ends, HO-1 and body temp may be chronically low, causing waking body temp <97F, while CO in tissues and organs remains high and only very slowly comes down.
You can measure CO in your breath with any portable professional CO detector that displays accurately from 1ppm, not a home CO alarm that only displays 0 until over 30ppm.
These cost a few hundred dollars, however, so if you can't afford that or want to try this before you buy one, visit any fire department and ask if you can borrow one of theirs for a minute while you blow on it.
Normal exhaled CO is <5ppm. They may also have a pulse CO oximeter that can estimate arterial SpCO(tm) through your skin, which should be close to arterial COHb.
But breath analysis is more accurate if you can do it. And by my patented method, breath can distinguish levels of CO coming from lungs, arteries, veins and the average of all tissues beyond lungs (which shows you where CO is highest).
If you think you might have exo or endo CO poisoning, please check out this CO awareness poster I designed for clinic exam rooms. It shows 16 common CO sources and 32 common CO conditions (each hyperlinked to a collection of references on Pubmed if you want to learn more about their CO connections).
www.tinyurl.com/COposter2022
The more of these CO conditions anyone has, the more likely they have CO poisoning
If you do, the good news is that there are many treatment options, including many non-pharma and some free.
But please note that I (heretically) advise against using oxygen therapy for CO poisoning. Because high oxygen is one of those stressors that causes people to make more HO and CO. Oxygen therapy also drives whatever CO people have in their arteries into their tissues and organs, where CO is much more harmful than in blood.
