In many locations there are no mechanical codes, nor are contractors required to be licensed. Fire departments and utilities often are checking for the presence of CO and do not check venting and appliances for downdrafting. They are often called after windows and doors are opened and the carbon monoxide dispersed, resulting in no readings at all. Some medical personal are not aware of the possible extent of the problem and may misdiagnose what on the surface looks like a simple case of the flu. Carbon monoxide kills by replacing oxygen in the blood.
High levels kill in a short period.
Moderate levels, because CO is a cumulative poison, can cause misdiagnosed health problems or even death. Low levels over a long period can cause health problems, especially for patients at risk. Exposure can result in long lasting or permanent effects including brain damage. After exposure, merely removing the victims from the area is not enough. Some form of medical attention is required.