Additionally, in many states there is a requirement to have CO detectors anyway, and it is generally built-in functionality to modern smoke alarms. So it's a double whammy of many homes not having any source of CO plus having built in detectors.
I think you've sort of missed the point I was making. Yes, CO detectors will alarm when CO levels are high enough to kill you.
They will intentionally NOT alarm if there is a low, persistent level of CO that probably has devastating health effects over the long term (think 20ppm in your house all the time).
Some years ago, my furnace had a cracked heat exchanger and, unknown to me, was pushing CO into my living space. I only noticed by luck. I walked by one of my CO alarms, hit the "max level" button and thought to myself that 25ppm is quite high. So I reset it and checked it again the next day. Sure enough, it shot back up.
