Carbon monoxide is invisible, odourless and undetectable to the human senses. It is the leading cause of death by poisoning worldwide.
Long-term exposure at low levels causes many health problems, physical and mental, often confused with other conditions. It increases the likelihood of heart attack, and can permanently damage short term memory.
Carbon monoxide monitors detect when the gas has reached levels where there is a risk of death, but do not detect lower level exposures which can damage health over time. The best protection is to be aware of the symptoms of CO poisoning, and the circumstances where it is most likely to happen.
The following two articles are very helpful.