If I sleep with my window slightly open, I’m safe from CO poisoning. (Wrong!)
CO (carbon monoxide) is a colorless, odorless, tasteless gas. At room temperature it’s slightly lighter than air. If it’s in your bedroom, you’ve got problems, whether you have a window slightly open or not.
Rule Number 1. If the CO detector alarms, evacuate the home!
Just because you have a window open does NOT mean that carbon monoxide will head for the window and leave your bedroom. It far more likely that CO will linger in the room, even though fresh-air might be coming in the window. The fresh air will help dilute the CO, at least in the room with the window, but it won’t do much for the rest of the house.
If the air from your bedroom happens to be leaving via the open window because of prevailing winds, then the open window is actually working against you! It’s pulling air from other areas of the house into your bedroom and out the open window. If the kitchen or lower level of your home has begun to load with carbon monoxide, the open window may actually accelerate CO migration into your bedroom.
This is why it makes sense to put a carbon monoxide detector in the hallway outside your bedroom, or in the bedroom itself. (Don’t use a lot of hair spray or other aerosols around the CO detector. Once the detector is fouled, it is useless.)