Module 20 - Special Situations Afloat
Carbon Monoxide and Gas Safety
What you will learn
Recognise a credible carbon-monoxide warning, stop exposure and escalate for medical help without relying on smell or a single symptom.
Carbon monoxide is an invisible exposure problem. The Yachtmaster decision is to believe the alarm or symptom pattern, move people into clean air and remove the source only when that can be done without another casualty.
Carbon monoxide is present in the exhaust of engines and fossil-fuel appliances. It is colourless, tasteless and odourless, so a cabin that smells normal is not evidence that it is safe. Wind, aerodynamic effects, nearby structures and obstructed outlets can return exhaust into a cockpit or accommodation even when the vessel is underway.
Headache, dizziness, nausea, vomiting, unusual tiredness, confusion, stomach pain and shortness of breath can resemble seasickness or fatigue. One symptom is not a diagnosis, but an alarm sounding or several people developing a similar pattern is operational evidence that demands action. Do not send another person into a suspect space merely to confirm the cause.
Move exposed people into open air, stop the suspected source if that is safe from outside the contaminated space, ventilate and seek medical attention. Preserve vessel control while this happens. A symptomatic helm may need to be relieved; an engine may be needed for immediate control, so stopping it must be coordinated with a safe alternative rather than treated as an isolated switch action.
Fit a marine-suitable alarm in the locations and manner specified by its manufacturer, test it with its test control and do not ignore an activation. An alarm is a layer of defence, not permission to obstruct exhaust outlets, neglect maintenance or close every ventilation path.
| Evidence | Immediate meaning | Command response |
|---|---|---|
| CO alarm | Treat as a real exposure warning | Open air, control source, medical advice |
| Shared headache or confusion | Not safely explained as routine fatigue | Remove exposure and relieve affected roles |
| No smell | Does not detect or exclude CO | Use alarms, ventilation and source control |
| Symptoms ease outside | Exposure remains credible | Do not re-enter or restart without resolution |
Respond to a shared symptom pattern
Two people under a cockpit canopy develop headache and nausea while the engine has been idling. A third person assumes it is seasickness because no exhaust smell is obvious.
1. Treat the pattern as exposure
Move everyone into clean open air and prevent re-entry; absence of smell does not clear the space.
2. Control vessel and source
Relieve any affected watchkeeper and stop the engine or appliance when a safe control alternative is established.
3. Escalate medically
Contact the Coastguard or emergency medical service, report the suspected exposure and follow professional advice.
4. Do not reset the evidence
Record alarm state, symptoms, times, running equipment and actions; do not resume operation merely because symptoms ease in fresh air.
Sense check: If several people improve after leaving one space, that strengthens the exposure warning; it does not prove the space is safe to re-enter.
Common mistake or limitation
- Waiting for a smell, visible smoke or a second alarm before acting.
- Treating several unwell crew as seasick without comparing where they were and which equipment was running.
- Sending an unprotected crew member below to investigate a space that may still contain CO.
- Restarting the source after ventilation without finding how exhaust entered and without appropriate inspection.
Recap
- CO cannot be ruled out by sight, taste or smell.
- A sounding alarm or shared symptom pattern is enough to stop exposure and escalate.
- Fresh air, safe source control, vessel control and medical advice happen as one coordinated response.
- Testing alarms and keeping exhaust paths clear are continuing controls, not one-off checks.
Optional quick check
Section 1 of 12
Two crew become confused and nauseous beneath a canopy while the engine is running, but nobody smells exhaust. What is the defensible response?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Special Situations Afloat
This public lesson is part of Revision Module 20. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.