Another 45 minutes passed and the alarm again sounded its full fury. This time we opened windows and doors to ventilate the house. Again, we reset the alarm. My mother wasn't convinced that there was a problem. She went up to the furnace and hot water heater, taking deep breaths to find the problem.
Carbon monoxide is in the exhaust that comes from furnaces, hot water heaters, dryers, etc.
The natural gas that fuels these appliances is laced with a detectable substance that you can smell to aid in detection. Do not confuse the two. Car exhaust, although heavy in carbon monoxide, does not smell because of the CO, it smells because of the by-products of gasoline combustion and sometimes oil. I pulled my mother away from the furnace, giving her this lecture. This time we set up fans in strategic places to help the cross ventilation and we reset the alarm.