Carbon monoxide is an odorless, colorless, tasteless gas
that is a byproduct of incomplete combustion of just about anything
that burns.
You could be standing in a room filled with carbon monoxide and never know
it. If the carbon monoxide level is extremely high, you could fall unconscious in minutes, and be dead within
an hour.
CNN produced a short, but good video on carbon monoxide
poisoning.
Heat exchanger cracks
... by themselves do NOT cause carbon monoxide
poisoning, however they are dangerous. This video illustrates the effects of large heat exchanger cracks on
the burner flame. This situation is dangerous for a number of reasons. First, there is a potential for
fire. Second, the wiring inside the furnace will be burned/melted. Third, some amounts of carbon
monoxide will be released into the area around the furnace.