A Carbon Monoxide Alarm Sounds on a Boat: Fresh Air, Account, Call—and Stay Out

Treat the alarm as real

Announce the alarm, stop ordinary tasks and direct everyone toward uncontaminated open air. Do not send one person deeper into the cabin to check a generator, heater or alarm display. CO cannot be confirmed or excluded by smell, and early symptoms can resemble seasickness, fatigue or alcohol effects.

If anyone has headache, dizziness, weakness, nausea, vomiting, chest pain, confusion, unusual sleepiness, collapse or difficulty breathing, say “possible carbon monoxide exposure” to responders. Sleeping or impaired people may not report symptoms. The marine CO alarm guide covers selection and placement; this page covers the response.

Move people before equipment

Evacuate the cabin, enclosed cockpit, canvas enclosure, machinery space and stern exposure area to genuinely fresh air. A swim platform or aft deck can remain in exhaust, so do not assume “outside” is automatically safe. Move upwind and away from running engines or generators when possible without creating fall, propeller or traffic danger.

Put life jackets on anyone exposed on deck or being moved between boats. Keep the group together and assign one adult to count names. If a person is unconscious or cannot move, call for emergency help immediately and follow dispatcher or responder instructions; do not create a second victim by entering a dangerous atmosphere without training and equipment.

Make the emergency call with position first

On the water, use VHF channel 16 or properly registered DSC when there is grave and imminent danger, then communicate on voice as directed. Give the boat name or description, exact position, “possible carbon monoxide exposure,” the number of people, symptoms and whether anyone is unconscious or in the water. Use 911 or the nearest emergency service where it is the effective route.

Do not transport a seriously affected person yourself unless emergency professionals direct it. CDC advises immediate fresh air and emergency contact. Follow the dispatcher for breathing support or CPR; oxygen should be administered only by people trained and equipped to do so safely.

Shut sources down only without re-entering danger

If engines, generator, heater, stove or other fuel-burning equipment can be shut down from a safe position, do so according to the exact boat and appliance instructions. Do not reach into an enclosed machinery space, touch unknown electrical controls in a fuel-vapor situation or delay evacuation to protect equipment.

Neighboring exhaust can also be the source. Coast Guard material specifically warns not to assume a false alarm when the boat's own engine and generator are off. Move away from a running neighboring boat when safe and tell responders what equipment was operating nearby.

Use the fresh-air–account–call–stay-out card

This card is the citation-ready authority asset. Place a copy beside the float plan and emergency contacts, and rehearse the words without generating a test alarm.

Do not troubleshoot by resetting

An alarm that stops after a door opens has not proved the atmosphere safe or the source corrected. A silence button, power cycle or battery removal can erase the warning while CO remains. Follow the alarm manual only after everyone is out and responders have addressed the exposure.

Do not use a consumer handheld meter as permission to re-enter. Sensor location, response time, calibration and atmosphere matter. Fire-rescue or other trained personnel with appropriate instruments should determine when the space can be entered. Preserve the alarm and its history for service.

Symptoms do not set a safe threshold

People can be affected differently, and CDC lists headache, dizziness, weakness, nausea, vomiting, chest pain and confusion among common symptoms. Loss of consciousness can occur, and someone sleeping or intoxicated may die before recognizing symptoms. The absence of a headache in one person does not clear the boat.

Tell medical personnel about pregnancy, age, heart or lung conditions and any loss of consciousness, but do not use those details to triage the group yourself. Everyone with suspected exposure should follow professional medical direction. A pulse oximeter cannot rule out CO poisoning and should not delay the call.

Control the boat without trapping the crew

If the boat is underway, the operator must also prevent collision, grounding or a person-overboard event. Reduce exposure and obtain emergency guidance while maintaining only the control necessary for immediate safety. Do not leave the helm unattended or crowd everyone onto a small aft platform in exhaust.

If at a dock, move people ashore and upwind where safe, warn neighboring crews and prevent anyone from boarding. If anchored or moored, a running neighboring generator may require relocation under responder direction. The emergency planning guide helps assign communication and people-control roles before an incident.

Re-entry requires evidence, not elapsed time

Do not re-enter because windows have been open for several minutes, the alarm is quiet or the engine has stopped. Ventilation time depends on the source, boat layout, wind and concentration. Responders should address the atmosphere; a qualified marine technician should address exhaust, ventilation, appliance and alarm causes.

The boat, generator, engine, heater, stove and alarm manuals remain model-specific authority. Inspection may need to include exhaust routing and leaks, generator enclosure, ventilation openings, canvas configuration, backdraft conditions and neighboring exhaust. Do not restart any suspected source until the responsible professional releases it.

Preserve the alarm and exposure record

After emergency care, record the time, position, people present, symptoms, equipment operating, neighboring sources, weather, canvas or window configuration, alarm model and indicator behavior. Save responder findings and photographs without dismantling equipment. The record supports medical care and technical diagnosis.

Replace an alarm only under its manual or qualified service guidance, and do not discard it if the manufacturer or investigator may need it. Update the boat record with inspection, repair, alarm age, test procedure and return-to-service evidence.

Prevent the next exposure without blaming the alarm

Repair exhaust and fuel-burning equipment, restore required ventilation, keep exhaust outlets clear, and maintain marine alarms according to the boat and alarm makers. Educate every passenger about symptoms and stern exposure before departure. CDC advises staying at least 20 feet from the nearest boat running an engine or generator when docked, beached or anchored.

Do not defeat the alarm because a nuisance event is suspected. A sensor fault still requires the manual's fault response and service; it does not prove the space is safe. The safe ending is documented medical follow-up where indicated, measured atmospheric clearance, corrected source and a functioning approved alarm—not a reset that makes the cabin quiet.