Carbon monoxide deaths
- Finding
Carbon monoxide binds haemoglobin far more than oxygen does; after death it leaves cherry-red lividity and a high carboxyhaemoglobin.
What it is
Carbon monoxide (CO) is a colourless, odourless gas formed by incomplete combustion: fires, braziers and charcoal in closed rooms, poorly vented heaters and stoves, engines in garages. It binds haemoglobin with an affinity about 250 times that of oxygen and forms carboxyhaemoglobin, which carries no oxygen. Death is from hypoxia. In forensic medicine the tasks are to recognise the findings, measure carboxyhaemoglobin, establish whether the person was breathing during a fire, and determine the manner of death (accident, suicide or homicide), which depends on the scene and not only on the laboratory.
How to tell it apart
- Cherry-red lividity also appears with cold (refrigerated bodies or hypothermia) and in cyanide poisoning: confirm with carboxyhaemoglobin.
- Soot only around the nose and mouth can settle without breathing; soot below the vocal cords means breathing during the fire.
- A burnt body with normal carboxyhaemoglobin and no soot in the airway: consider death before the fire and look for injuries.
- Smokers have a slightly higher baseline carboxyhaemoglobin; it does not explain fatal levels.
How to recognise it
- Cherry-red or bright pink lividity; blood and muscles bright red.
- In fires: soot in the trachea and bronchi, sometimes in the oesophagus or stomach.
- Brain and lung oedema, which are non-specific.
- Scene: a combustion source in an enclosed space; several victims or animals affected in the same place.
- High blood carboxyhaemoglobin; fatal cases usually show very high levels, although deaths at lower levels occur in people with heart or lung disease.
How it forms
- CO displaces oxygen from haemoglobin and shifts the dissociation curve to the left: haemoglobin releases less oxygen to the tissues.
- It also binds myoglobin and enzymes of cellular respiration.
- The brain and heart, the organs most sensitive to hypoxia, fail first.
- The cherry-red colour comes from the colour of carboxyhaemoglobin itself.
Next step: what to do and document
- At a scene with an enclosed combustion source, ventilate and protect the teams before entering; there may be more victims.
- Describe the colour of the lividity and the scene (source, ventilation, doors and windows).
- Send blood (femoral if possible, with anticoagulant, tube full and tightly closed) for carboxyhaemoglobin and, in fires, for cyanide.
- In fire victims, record whether there is soot in the lower airway and relate it to the carboxyhaemoglobin.
- Do not decide the manner of death from the laboratory alone: it depends on the scene and the investigation.
Common mistakes
- Relying on the cherry-red colour without measuring carboxyhaemoglobin.
- Using pulse oximetry to rule out poisoning in the living: it does not distinguish carboxyhaemoglobin.
- Sending blood in a half-empty or open tube: the CO escapes.
- Concluding that every fire victim died from the fire without looking for earlier injuries.
Test yourself
Think of the answer, then tap the question to see it.
Which autopsy finding shows that a fire victim was breathing during the fire?
Soot in the airway below the vocal cords, together with raised carboxyhaemoglobin.
Why can pulse oximetry mislead in carbon monoxide poisoning?
Because a standard pulse oximeter does not distinguish carboxyhaemoglobin from oxyhaemoglobin and may show a normal saturation.
Besides carbon monoxide, what else causes cherry-red lividity?
Cold (refrigerated bodies or hypothermia) and cyanide poisoning; this is why carboxyhaemoglobin is measured to confirm.
What does a burnt body with normal carboxyhaemoglobin and no soot in the trachea suggest?
That the person may have been dead before the fire; another cause of death, such as injuries, must be sought.
Connections
Educational content on forensic and legal medicine; not legal advice and no substitute for the technical guides of Colombia's National Institute of Legal Medicine and Forensic Sciences. Laws change: each page links the official text and says when it was last checked. For a real case, follow the guide in force and ask the competent authority.
Sources: Carbon monoxide poisoning (acute) (BMJ Clinical Evidence, 2008); Homicides disguised as fire deaths (Acta Medica Lituanica, 2023); Carbon monoxide suicide by charcoal-burning: a case report and review of the literature (The Pan African Medical Journal, 2021); Postmortem redistribution of drugs: a literature review (Forensic Science, Medicine and Pathology, 2024).
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Representative model, for educational purposes only.