St Emlyn’s Lesson Plans
Recognising carbon monoxide poisoning
Recognise a shared exposure, interpret a delayed COHb result and plan treatment and a safe destination.
Medical Students · 45-minute session
For learners
Before: Allow about 12 minutes in total.
When we meet: We will assess a household exposure and a delayed blood result, then discuss treatment, specialist advice and whether home is safe.
For facilitators
Send: the learner preparation link before teaching.
Bring: Board, UKHSA diagnostic algorithm and the case’s written COHb result with its oxygen and timing history.
Before the session
If you are attending this session, complete the core preparation below before you come. Allow about 12 minutes in total.
Core preparation
UKHSA carbon monoxide diagnostic algorithm · About 5 minutes
Open and read the one-page diagnostic algorithm linked on this page. Follow the exposure-history questions and the immediate actions for suspected poisoning.
https://www.gov.uk/government/publications/carbon-monoxide-co-algorithm-to-diagnose-poisoning
UKHSA toxicological overview · About 5 minutes
Read “Main points” and “Kinetics and metabolism”. Focus on symptoms, the effect of oxygen and time since exposure, and the limits of a carboxyhaemoglobin result. The detailed toxicology and references are optional.
https://www.gov.uk/government/publications/carbon-monoxide-properties-incident-management-and-toxicology/carbon-monoxide-toxicological-overview
Think about (allow 2 minutes): How would oxygen already given and time away from the source affect your interpretation of a low COHb result?
Picture the scene
Several members of a household attend after an engineer identifies a faulty gas fire. One has a headache; another felt better after leaving home.
By the end of the session
- Ask an exposure history that identifies people and places at risk.
- Explain why normal pulse oximetry or a low delayed COHb does not exclude poisoning.
- Describe immediate support, specialist discussion and safe discharge planning.
After the session
- Where in your usual headache or dizziness history could you add one question about home, heating and other affected people?
Facilitator guide
Send learners the preparation link several days before teaching. Allow about 12 minutes in total.
Before learners arrive
- Find the local NPIS/TOXBASE access route, relevant gas/emergency service process and oxygen equipment.
- Check how your blood-gas analyser reports COHb and who arranges follow-up after significant exposure.
Equipment: Board, UKHSA diagnostic algorithm and the case’s written COHb result with its oxygen and timing history.
Setup: Pairs interview each other as clinician and household member; then compare the information they would otherwise have missed.
If the preparation has not been completed
Replace the five-minute “Establish the essentials” slot. Display the UKHSA diagnostic algorithm and write “source, others affected, time away, oxygen” on the board. Give pairs two minutes to make an exposure question for each heading, two to discuss why a later low COHb could mislead, and one to agree the first escalation step.
Suggested session plan
- 0–5 min — Explore. Ask why five people arriving together should alter the history.
- 5–10 min — Establish the essentials. Build the five-heading exposure checklist.
- 10–32 min — Discuss & Apply. Give 10 minutes to environmental safety and 12 to the delayed-result case.
- 32–40 min — Consolidate. Ask pairs to explain the delayed COHb result and name the clinical and environmental checks needed before discharge.
- 40–45 min — Reflect & Transfer. Ask learners where an exposure question would fit into their usual headache or dizziness history.
Start the discussion
Read the opening scenario together. Give everyone a moment to think, then ask what concerns them and what they need to know next.
Cases and discussion
Use these cases, or invite an anonymised case from the group that addresses the same questions.
A household with a faulty heater
Five household members attend after a faulty appliance is found. They initially report feeling well; further questioning identifies intermittent morning headaches and a child who has been unusually sleepy.
Add this information as the discussion develops:
- One person says the headaches vanish at work.
- They intend to return home immediately and restart the heater because it is cold.
Ask the group:
- Who needs individual assessment?
- What exposure questions change your suspicion?
- What must be safe before anyone goes home?
Discussion notes
Points to draw out:
- Assess every exposed person individually; apparently mild or absent symptoms do not provide a complete exposure assessment.
- Ask about fuel-burning equipment, ventilation, duration, other occupants and pets, and symptom variation away from the building.
- Arrange appropriate clinical assessment and ensure the source is made safe by the relevant emergency or qualified service. Do not advise re-entry to a potentially contaminated environment.
- Ask about pregnancy and other vulnerability, and discuss concerning exposures with the senior clinician or poisons service.
Common misconceptions:
- Assuming only the person with the worst headache has been exposed.
- Reassuring solely because the appliance has now been switched off.
For a more experienced group: Ask how you would identify other occupants or neighbouring premises without placing staff or relatives at risk.
A reassuring number after oxygen
A 25-year-old has used a camping stove indoors. A friend found them drowsy with headache and vomiting. After prehospital oxygen and time away, COHb is 4%; ordinary pulse oximetry reads 99%.
Add this information as the discussion develops:
- The patient is improving but still has difficulty concentrating.
- Contrast with a person found unconscious after intentional exposure, with ongoing reduced consciousness and ECG abnormalities.
Ask the group:
- What can and cannot be concluded from 4%?
- What immediate support is needed while gathering information?
- What concerns require early specialist discussion?
Discussion notes
Points to draw out:
- COHb falls after removal from exposure and faster with oxygen; interpret the sample in that context. Its value does not reliably grade clinical severity.
- Conventional pulse oximetry cannot distinguish the relevant haemoglobin species. A normal reading must not override the exposure history.
- Give high-concentration oxygen and supportive ABC care for suspected significant poisoning; obtain suitable blood co-oximetry, ECG and further tests guided by presentation.
- Persistent neurological or cardiac features, pregnancy or severe exposure warrant urgent senior/NPIS discussion. Hyperbaric treatment is a specialist decision with uncertain benefit, not a routine destination triggered by one number.
Common misconceptions:
- Withholding oxygen to obtain an untreated sample.
- Teaching deliberate hyperventilation as a routine way to remove CO.
- Treating intentional exposure as only a toxicology problem.
For a more experienced group: Discuss how neurological follow-up and mental-health assessment fit alongside the immediate physical treatment.
Important clinical points
- Do not enter an unsafe environment to investigate the source.
- A normal conventional saturation does not rule out CO poisoning.
- Do not discharge someone back to a continuing source; provide advice about delayed neurological symptoms and how to seek review.
Closing the session
Use these questions to check what the group will take away. Go back over any clinical points that remain unclear.
- Explain a low COHb after oxygen in plain language.
- Name the exposure question you would ask before ordering more tests.
- Describe what makes discharge safe beyond symptom improvement.
Key teaching points:
- Ask about the source, other exposed people and symptoms away from the building.
- Interpret COHb alongside the exposure and oxygen timeline; ordinary saturation can mislead.
- Before discharge, assess recovery and confirm a safe destination.
Before leaving, ask each person to choose one action from After the session and say when they will try it. They complete that follow-up in practice, with supervision where needed.
Adapting the discussion
Uncertainty and local practice
Hyperbaric oxygen decisions depend on clinical features, specialist advice and access; avoid presenting a universal threshold.
If the discussion drifts
What would make it unsafe to reassure or discharge this particular person?
For a more experienced group
Use the intentional-exposure contrast to separate airway support, co-ingestion assessment and psychosocial needs.
A 30-minute version
30 minutes: opening 3, checklist 5, delayed-result case 12, home safety 5, consolidation 3, reflection 2.
Evidence and guidance
- UKHSA CO diagnostic algorithm — Exposure-led assessment.https://www.gov.uk/government/publications/carbon-monoxide-co-algorithm-to-diagnose-poisoning
- UKHSA CO toxicological overview — Timing, severity and delayed effects.https://www.gov.uk/government/publications/carbon-monoxide-properties-incident-management-and-toxicology/carbon-monoxide-toxicological-overview
- UKHSA CO incident management — Conventional oximetry limitation.https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1077807/UKHSA-incident-management-carbon-monoxide.pdf
Suggested curriculum links
GMC Outcomes for graduates / MLA 2026: Poisoning; environmental exposure and patient safety
Learners integrate exposure history, test limits and safe onward care.
Check these links against the learner’s stage and programme. Attendance at a teaching session does not establish competence.
First published 2020-09-25 · Simon McCormick.