Carbon monoxide is a colorless, odorless gas produced whenever fuel burns. A healthy gas furnace makes CO during normal combustion, but the heat exchanger and flue keep it sealed away from your air and vent it outside. Furnace-related CO leaks into the home come from two main failures: a cracked or corroded heat exchanger, and a blocked, disconnected, or back-drafting flue. This is why a furnace problem can be a safety emergency rather than a comfort complaint.
The stakes are not abstract. The CDC attributes more than 400 unintentional, non-fire CO deaths and more than 100,000 emergency-department visits in the US each year. Because the gas is imperceptible, the early symptoms — headache, dizziness, nausea, fatigue — are easy to mistake for the flu, and they ease when you leave the house and return when you come back.
The defense is a working CO alarm, which is a different device from a smoke detector. Look for one listed to the UL 2034 standard, installed outside each sleeping area; the standard sets how fast an alarm must respond at a given concentration — roughly 60 to 240 minutes at 70 ppm and just 4 to 15 minutes at 400 ppm. CO sensors wear out, so most units carry an end-of-life signal and should be replaced about every five to seven years per the manufacturer. Pair an alarm with an annual furnace inspection that includes a heat-exchanger check and a combustion analysis, and you've covered both the detection and the source.