I've got one from Kidde and it just does not seem sensitive enough.
As part of my mechanical contractor license, I have to take 8 hours of continuing education per year. The city puts on a convention so everyone can get those hours easily, which is nice. A few years ago, I attended a seminar about carbon monoxide safety. I'm pretty well versed in it, but a tidbit I did not know was that the UL code governing CO detectors actually specifically prevents them from going off below a threshold.
If an alarm is exposed to 400 PPM, it must alarm between 4 and 15 minutes.
If an alarm is exposed to 150 PPM, it must alarm between 10 and 50 minutes.
If an alarm is exposed to 70 PPM, it must alarm between 60 and 240 minutes.
An alarm must NOT sound when exposed to 30ppm continuously for 30 days.
The reason given was that during the development of these alarms and the codes around them, it was found that many nuisance alarms happened below that 30ppm threshold. Cooking, vehicle exhaust from outside, candles, etc. Nuisance alarms are very bad because they train the occupants to ignore the alarm or disconnect it.