Carboxyhaemoglobin: Binding, Half-Life, and Reference Levels
Non-smoker baseline 1–3%, heavy smoker 10–15%, generally lethal above 50% but potentially lethal at 10–30% with heart disease. Elimination half-life is 4–5 hours on room air, 40–80 minutes on 100% oxygen, about 23 minutes hyperbaric — which is why airing out a room doesn't clear your blood. Foetal haemoglobin binds CO even more avidly, and significant exposures carry a 10–30% risk of delayed neurological sequelae days to weeks later.
**Carboxyhaemoglobin (COHb)** is haemoglobin with carbon monoxide bound in place of oxygen, and its percentage of total haemoglobin is the standard measure of carbon monoxide exposure. ## Why the affinity matters so much CO binds haemoglobin roughly **200 times** more avidly than oxygen. The practical consequence is that very low airborne concentrations produce dangerous blood levels: air that is a fraction of a per cent CO can drive COHb to lethal territory over hours, because the equilibrium strongly favours CO. ## Reference levels - **Non-smoker baseline: 1–3%.** Everyone has some, because the body produces CO endogenously — see Gasotransmitters: The Lethal Gases Your Body Manufactures as Signals. - **Heavy smoker: 10–15%.** Chronically elevated, with a compensatory rise in red cell mass. - **Symptomatic: from around 10–20%**, with headache, nausea and impaired judgement. - **Generally lethal: above 50%.** - **Can be lethal at 10–30%** in people with existing heart disease, because the margin for reduced oxygen delivery is already gone. Symptom severity correlates only loosely with COHb percentage, partly because the left-shift of the dissociation curve and the cytochrome oxidase inhibition are not captured by the number. ## Elimination half-life This is the clinically decisive figure, and it drives treatment: - **Room air: 4–5 hours.** - **100% oxygen: 40–80 minutes.** - **Hyperbaric oxygen: about 23 minutes.** High-flow oxygen is the treatment because it accelerates displacement, and hyperbaric oxygen is used for severe poisoning — with the additional rationale of driving dissolved oxygen into plasma independent of haemoglobin. The half-life on room air explains a common and dangerous mistake: **airing out the house does not clear your blood.** It stops further uptake, and the CO already bound takes hours to leave. "Open the windows and go back to bed" is how people relapse. ## Vulnerable groups **Foetal haemoglobin binds CO even more avidly than adult haemoglobin**, and foetal levels lag and then exceed maternal levels. Foetal mortality exceeds 50% in severe maternal poisoning, and treatment thresholds for pregnant patients are correspondingly lower. Children, with higher metabolic rates and respiratory rates for their mass, take up CO faster than adults. So do pets. This produces the characteristic order of collapse — animals first, then children, then adults — described in Recognising Carbon Monoxide Poisoning: The Tells That Actually Work. ## Delayed neurological sequelae A significant exposure carries a **10–30% risk of delayed neurological sequelae** — cognitive impairment, personality change, movement disorders and memory deficits appearing days to weeks *after* apparent recovery. This is why significant exposures warrant medical assessment even when the patient feels fine afterwards.