Carboxyhaemoglobin - blood

Carboxyhaemoglobin 0.2-2.0% of total haemoglobin in normal blood; up to 8.5% in heavy smokers.

In carbon monoxide poisoning, carboxyhaemoglobin levels provide an approximate guide to status.
Between 10-20%: headache and dyspnoea on exertion;
above 20%: confusion and irritability;
above 50%: unconsciousness, with death likely if exposure is prolonged;
above 70%: death is rapid.

Toxicity relates more to inhibition of mitochondrial cytochrome respiration than to interference with blood oxygen transport.