It has been known for decades that CO poisoning can produce lasting health harm, mainly through it's destructive effects on the central nervous system. Some studies found that 25-40% of People died during acute exposure, while 15-40% of the survivors suffered immediate or delayed neuropsychological deficit.
Now, an emerging body of evidence suggests that longer exposures to lower levels of CO, ie. chronic CO poisoning, are capable of producing a myriad of debilitating residual effects that may continue for days, weeks, months and even years.
A CO detector should be installed in EVERY home, especially those with ANY fuel burning furnace or space heater. This is mandatory (code) in many states at the time of this writing (2007), but it is a very inexpensive way to be insured against the tragedy of CO poisoning.
uncombusted CO is a source of fuel burning inefficiency.
Note that coal stoves, oil burners, gas stoves and furnaces are probably more likely to leak CO without your knowing, since wood and pellet units would usually emit a smoky smell long before dangerous CO levels were found. However, with many pellet stoves burning very cleanly, this may not be true of leaks from their exhaust area.
Be safe. Install one or multiple CO detectors.