This information is from the book I wrote for home buyers a couple of years back. This was from the Chapter “What falls through the cracks”
Carbon monoxide detectors may not be required in many states, but I recommend you install them prior to moving in. Consult your fire department for suggested (or required) locations. Don’t put one right next to the heating system. Replace all units over five years of age. BOX/HIGHLIGHT
IMPORTANT! Carbon monoxide detectors are intended to prevent people from dying due to high CO levels. Most are designed to go off if the levels rise above 30 PPM (parts per million) for a specified time. What they are not designed to do is detect the lower levels that will seriously affect the health of the home’s occupants and possibly leave them with chronic respiratory or other health problems. Why is this? The reason stems from the fact that, due to temperature inversions that occasionally occur, the CO levels the outside air can, for a short time (a week or so?) can exceed 20 ppm. When this happened in Chicago some years back many of the early generation CO detectors went off and the fire department spent long days responding to calls from homeowners who thought they were being poisoned from internally generated CO. Out of necessity, the carbon monoxide detectors now sold to the public will sound alarms only when they detect levels that are higher than possible background levels of CO.