The same gas that poisons us is also made inside our bodies as a signal
Carbon monoxide has no colour, no smell and at first no sting, which is why it is the most common kind of fatal poisoning in many countries. Yet our own cells produce small amounts of it when breaking down haem, and it works as a chemical messenger. Toxicity, as so often, depends on the dose.
The danger comes from how tightly the gas grips haemoglobin. It forms carboxyhaemoglobin, which cannot carry oxygen, and it also interferes with other iron-containing proteins such as myoglobin and the mitochondrial enzymes cells use to burn fuel. The organs that need the most oxygen, the brain and heart, suffer first. It is produced whenever carbon-based fuel burns incompletely, in stoves, heaters, engines and generators.
Early symptoms are easily mistaken for flu or food poisoning: headache, most often dull and frontal, along with nausea, dizziness and fatigue. Heavier exposure brings a racing heart, confusion, seizures and unconsciousness. The famous cherry-red complexion is mostly seen after death, when victims can look unusually lifelike, so doctors do not rely on it. A worrying feature is delayed damage: problems with memory, mood and movement can appear 2 to 40 days later in up to half of those poisoned. Doctors typically diagnose poisoning when carboxyhaemoglobin exceeds 3% in non-smokers or 10% in smokers; the highest level anyone is reported to have survived is 73%.
Scale is hard to pin down because many mild cases go unnoticed. The United States records more than 20,000 emergency visits a year, and poisonings peak in winter, when heaters run hard and power cuts send people to fuel-burning appliances, some meant only for outdoor use. In Australia, 95% of carbon monoxide deaths are linked to gas space heaters. Tolerance varies from person to person, as physiologist Esther Killick showed, shaped by factors such as anaemia, heart disease and altitude.
Knowledge of the gas's harm is ancient, but the link to blood came in 1793, when James Watt and Thomas Beddoes were investigating it as a possible medicine, and Claude Bernard confirmed it between 1846 and 1857. That therapeutic interest has returned: researchers are developing carbon monoxide-releasing molecules. The gas has no known harmful effect on photosynthesising plants, and some bacteria actually consume it.
Source: Carbon monoxide poisoning
