What permanent damage? CO binds permanently to hemoglobin, but as red cells are replaced, the carboxyhemoglobin is gotten rid of, so what permanent damage are you talking about?
The problem is that even a slight lack of oxygen results in a brain injury…. General cognitive ability, headaches, propensity to be distracted/disorganized/inability to multi-task etc. Then there are related physical symptoms which are things like inability to keep one’s balance, vertigo… for men, the inability to maintain an erection. By no means a complete list but it is in these areas that problems arise.