Carbon Monoxide Poisoning: Why You Cannot Build Useful Tolerance

You cannot build useful resistance to carbon monoxide, and the closest thing to tolerance makes you **more** likely to die of it. The reason is that the mechanism is physical chemistry with no adaptive lever. ## Three simultaneous attacks **It out-competes oxygen for haemoglobin.** CO binds haemoglobin with roughly 200 times the affinity of oxygen, forming **carboxyhaemoglobin**. Blood that could be carrying oxygen carries CO instead. **It poisons the oxygen you are still carrying.** CO left-shifts the oxygen–haemoglobin dissociation curve, so the haemoglobin that *is* loaded with oxygen holds onto it more tightly and releases less of it to tissue. This second effect is frequently omitted and it matters — the damage is worse than the carboxyhaemoglobin percentage alone suggests. **It jams cellular respiration.** CO inhibits **cytochrome c oxidase**, the same mitochondrial enzyme that cyanide attacks, so even oxygen that reaches a cell cannot be used. No amount of fitness or prior exposure changes any of these binding affinities. See Carboxyhaemoglobin: Binding, Half-Life, and Reference Levels. ## The one documented acclimatisation, and why it is a trap **Esther Killick** performed the only systematic human acclimatisation study, on herself, in the 1930s — see Esther Killick and the CO Self-Experiments. Humans do acclimatise. The problem is *how*. The adaptation is the same one produced by altitude or smoking: more red blood cells, raising oxygen-carrying capacity. That does two unhelpful things. It raises your **baseline poison burden** — which is why a heavy smoker walks around at 10–15% carboxyhaemoglobin — and it does nothing against an acute dose, because the binding chemistry is unchanged. Worse, the acclimatisation is largely **symptomatic**. The headache and nausea *are* the warning system. Someone acclimatised to a slowly leaking furnace is the person least likely to notice the night it becomes lethal. This is a clean instance of the pattern in The Clean Single-Lever Pattern: Why Simple Biological Mechanisms Disappoint in Trials — the intuitive lever, toughen yourself up, pulls in the wrong direction. Compare the same reasoning applied to a different poison in Mithridatism: Building Poison Tolerance Through Repeated Exposure. ## Why you cannot sense it either CO is odourless, colourless, tasteless and non-irritating, and no animal has a receptor for it — humans are not unusually blind here. See Why No Animal Can Sense Carbon Monoxide. The cruelty is that CO impairs precisely the judgement needed to identify it. Drowsiness and confusion arrive early, and victims routinely rationalise symptoms as flu, hangover or exhaustion. There are documented cases of poisoned people constructing elaborate wrong explanations for their own symptoms rather than suspecting the gas. ## What actually works Recognition training is worth having as a backup — see Recognising Carbon Monoxide Poisoning: The Tells That Actually Work — but it can never be the primary defence, because the poison degrades the faculty doing the recognising. The primary defence is a working detector. Sealed ten-year-battery units on every level eliminate the commonest failure mode, which is not the absence of a detector but a detector with a dead battery someone meant to replace. See Electrochemical CO Detectors and the UL 2034 Time-Delay Curve.