Esther Killick and the CO Self-Experiments
The English physiologist (1902–1960) who through the 1930s repeatedly sealed herself in a chamber with carbon monoxide, becoming hypoxic and sometimes losing consciousness, publishing in the Journal of Physiology in 1936. Humans do acclimatise — by raising red cell mass, the same compensation as altitude or smoking. But the adaptation is symptomatic rather than protective: it raises baseline burden and deletes the warning headache while binding chemistry is unchanged.
**Esther Killick** (1902–1960) was an English physiologist who conducted the only systematic study of human acclimatisation to carbon monoxide — using herself as the sole subject. ## The experiments Through the 1930s, Killick sealed herself weekly in a chamber containing carbon monoxide, becoming progressively hypoxic and on occasion losing consciousness. She recorded her own symptoms, exposure levels and physiological responses across repeated sessions over months. Her results were published in the *Journal of Physiology* in 1936 as "The acclimatization of the human subject to atmospheres containing low concentrations of carbon monoxide." ## The finding Humans **do** acclimatise. Over months of repeated exposure her symptoms at a given concentration diminished substantially. The mechanism is the familiar compensation for chronic hypoxia: increased red blood cell production, raising total oxygen-carrying capacity to offset the fraction lost to carboxyhaemoglobin. It is the same adaptation produced by altitude and by heavy smoking. ## Why the result is the opposite of reassuring The adaptation is **symptomatic rather than protective**, and it makes an acclimatised person more dangerous to themselves in three ways. **The binding chemistry is unchanged.** CO still binds haemoglobin ~200 times more avidly than oxygen. Against an acute high-concentration exposure — a cracked heat exchanger, a running engine in a closed space — extra red cells buy very little. **Baseline burden rises.** The acclimatised individual carries more carboxyhaemoglobin at rest, so any acute exposure starts from a worse position. **The warning system is deleted.** The headache and nausea are the alarm. Losing them means losing the only signal a person has, since nobody can sense the gas directly. See Why No Animal Can Sense Carbon Monoxide. So the person most acclimatised to a chronically leaking appliance is the one least likely to notice the night it becomes lethal — which inverts the intuition that a hardened body is a safer body. See Carbon Monoxide Poisoning: Why You Cannot Build Useful Tolerance. ## As a piece of scientific history Killick belongs to the tradition of physiologists who used themselves as subjects — a practice that produced real results under an ethical framework no modern review board would approve. Repeated deliberate self-induced unconsciousness in a CO chamber over months is a considerable personal risk, and it remains the only human acclimatisation dataset of its kind, because nobody can ethically repeat it.