Squonk.
I would not use those cards, yes they do work. But look at it this way......
You smoke/fire alarm that you have installed in your house, makes an audible warning should you be in danger, yet, you can smell, taste, and see smoke!
Carbon monoxide, is an ODORLESS, INVISIBLE, TASTELESS GAS,
which is lethal in very small quantities
Laying in bed asleep the card turning black is not going to help you!!
Anyone fitting a CO alarm should make sure it is to standard (EN50291)
IMHO these should be mandatory for all houses (in Canada the CO & smoke alarm are intergrated)