In article .com
John wrote:
snip
The spot cards don't give an audible alarm. Unless you happen to notice
them, they're of limited use.
CO alarms will work if an appliance develops a fault or it's air supply
becomes unexpectedly restricted by whatever means. They are also being
used to detect fires I'm told. I would always favour a smoke alarm for
this though.

I suspect that in the large majority of cases where a domestic appliance
produces dangerous amounts of CO it is as a result of prolonged and
gradual deterioration, rather than sudden failure. In this situation
the spot cards should be adequate.