pyotr filipivich writes:

Now, the next question is: how sensitive do you want your
detector? 100ppm? 50ppm? the least amount detectable?

The atmospheric fraction of CO is 0.1PPM, so you don't want
your detector to be too sensitive (e.g. to the PPB).

The lethal concentration leading to a 50% death rate is 4000PPM
over 30 minutes, or 5000PPM over 5 minutes.

The permissible exposure limit is 50PPM.

The recommended exposure limit ie 35PPM.

Immediate danger at 1200PPM.