(from here)One such study has been conducted by Dr. David G. Penney. He suggests that long periods of exposure to carbon monoxide, at low levels, can result in long-term effects. He calls this chronic carbon monoxide poisoning. This can cause effects in an individual for days, weeks, months, or even years. Results vary depending on the age and health of an individual.
Which supports the idea as a long term condition.
There's also more information at Toxnet