Carbon Monoxide Safety

By Tom Burden, last updated 6/8/2026

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Carbon monoxide safety belongs on every boat owner’s maintenance checklist, especially on boats with cabins, generators, inboard engines, propane appliances, heaters or enclosed cockpit spaces. Carbon monoxide (CO) is invisible, odorless and tasteless, and symptoms can be mistaken for seasickness, dehydration or fatigue. That makes prevention, ventilation, mechanical inspection and properly installed carbon monoxide detectors the best defenses.

The danger of carbon monoxide on boats
Where CO collects on boats
Symptoms of CO poisoning
U.S. Coast Guard tips to avoid CO exposure
Inspect your boat for CO hazards
What to look for in CO detectors
What to do if CO poisoning is suspected
Carbon monoxide safety FAQ

The Danger of Carbon Monoxide on Boats

Carbon monoxide is produced whenever carbon-based fuels burn incompletely. On boats, the most common sources are gasoline or diesel engines, gas generators, cooking ranges, cabin heaters and other fuel-burning equipment. CO can build up inside cabins, partially enclosed cockpits, beneath canvas enclosures, near swim platforms, under hardtops and around stern areas where exhaust can linger.

The risk is not limited to the boat producing the exhaust. A boat idling nearby, a generator running on a rafted vessel, or exhaust drifting across a dock can create dangerous exposure aboard another boat. Downwind operation, following seas, canvas enclosures and poor ventilation can all increase risk by trapping or redirecting exhaust into occupied spaces.

CO is dangerous because it binds to hemoglobin in the bloodstream much more readily than oxygen, reducing the body’s ability to carry oxygen. Early symptoms often look like ordinary boating discomfort: headache, dizziness, weakness, nausea, vomiting, chest pain or confusion. High exposure can cause unconsciousness, drowning or death.

Where CO Collects on Boats

CO can collect anywhere exhaust is produced or trapped. The highest-risk areas are not always obvious, especially when air movement changes around a moving or docked boat.

Cabins and sleeping spaces: Enclosed accommodation spaces can trap CO from engines, generators, heaters or nearby boats.
Canvas-enclosed cockpits: Eisenglass, dodgers, biminis and full enclosures can reduce ventilation and allow exhaust to accumulate.
Stern and swim platform areas: Exhaust can collect around the transom, under swim platforms or behind the boat while the engine or generator is running.
Raft-ups and docks: Exhaust from neighboring boats can enter your cabin or cockpit, especially when wind pushes fumes toward your boat.
Following seas or downwind running: Exhaust can be drawn forward into the cockpit or cabin instead of dispersing aft.
Generator compartments: A leaking exhaust hose, failed clamp, cracked fitting or poor cooling-water flow can allow CO to enter the boat.

Symptoms of CO Poisoning

The rate at which CO is absorbed into the bloodstream depends on the concentration in the air and the length of exposure. Symptoms can progress quickly, and people who are sleeping, intoxicated or already fatigued may not recognize early warning signs. The table below lists symptoms of CO poisoning which can vary from a slight headache to death.

100 ppm	.01%	Slight headache in two to three hours
200 ppm	.02%	Slight headache within two to three hours; loss of judgement
400 ppm	.04%	Frontal headache within one to two hours
800 ppm	.08%	Dizziness, nausea and convulsions within 45 minutes. Insensible in two hours.
1,600 ppm	.16%	Headache, dizziness and nausea within 20 minutes. Death in less than two hours.
3,200 ppm	.32%	Headache, dizziness and nausea in five to ten minutes. Death within 30 minutes.
6,400 ppm	.64%	Headache and dizziness in one to two minutes. Death in less than 20 minutes.
12,800 ppm	1.28%	Death in less than three minutes.

If more than one person aboard develops headache, dizziness, nausea, weakness or confusion at the same time, assume CO exposure is possible until proven otherwise. Move everyone to fresh air immediately, shut down engines and generators if it is safe to do so, and seek emergency medical help.

U.S. Coast Guard’s Tips to Avoid CO Overexposure

Operate combustion devices such as stoves, heaters and generators in well-ventilated areas.
Supplement natural ventilation with fans or forced air. Keep forward-facing hatches open to allow fresh air circulation in accommodation spaces, even in inclement weather.
Close the hatches and set your course away from the path of exhaust fumes when motoring downwind in a following sea.
Install CO detectors in each cabin. It’s the most effective defense against a potentially fatal problem.
Avoid any activity on the rear deck, swim platform, and around exhaust pipes while the engine or generator is running.
CO exposure danger also exists on deck, especially when a boat idles at the dock or seawall where exhaust can accumulate. Even when not running your engine, beware of CO accumulation from other boats that idle at the dock to windward.

Inspect Your Boat for CO Hazards

Most CO problems on boats trace back to fuel-burning equipment, failed exhaust components, poor ventilation or operating habits that trap exhaust. A detector is important, but it is not a substitute for inspection and maintenance.

Each Trip

Take the following actions whenever you use your boat. Do not operate the vessel if any of these problems exist.

Make sure you know where CO exhaust outlets are located on your vessel.
Educate all passengers about the symptoms of CO poisoning and where CO may accumulate.
When docked or rafted with another boat, be aware of exhaust emissions from the other boat.
Confirm that cooling water flows from the wet exhaust outlet when the engine or generator is started.
Listen for any change in exhaust sound that could indicate a failure of an exhaust component.
Test the operation of each carbon monoxide detector by pressing the test button.
Confirm that cockpit canvas, hatches and ventilation openings are arranged so exhaust can escape instead of being drawn into occupied spaces.

Once a Month

Make sure all exhaust clamps are in place and secure.
Look for exhaust leaking from exhaust system components. Signs include rust and/or black streaking, water leaks, or corroded or cracked fittings.
Inspect rubber exhaust hoses for burned, cracked or deteriorated sections. All rubber hoses should be pliable and free of kinks.
Check generator sound shields, exhaust hose runs and nearby bulkheads for soot marks, water leaks or unusual heat discoloration.

Annually by a Qualified Marine Technician

Replace exhaust hoses if any evidence is found of cracking, charring or deterioration.
Ensure that your engines and generators are properly tuned and well maintained.
Inspect each water pump impeller and check the condition of the water pump housing. Replace if necessary.
Inspect each of the metallic exhaust components for cracks, rust, leaks or looseness. Pay particular attention to the cylinder head, exhaust manifold and water injection elbow.
Clean, inspect and confirm proper operation of the generator cooling water anti-siphon valve if equipped.
Confirm that installed CO detectors are within their service life and replace any detector that has reached its manufacturer’s replacement date.

What to Look for in CO Detectors

Marine-rated certification: Use carbon monoxide detectors intended for marine use and tested to UL 2034 requirements for recreational boats. Household CO detectors are not built for the same vibration, humidity, temperature swings and installation conditions found aboard boats.

Time-weighted averaging: A marine carbon monoxide detector should compute the time-weighted average of the CO concentration in the air. The time-weighted average measurement process constantly monitors CO levels and helps reduce nuisance alarms while still alerting occupants before exposure becomes dangerous.

Multi-channel monitoring: This function allows detectors to measure fumes in different cabins. For example, the carbon monoxide detectors CMD5-MD-R allows up to ten detectors to be linked together. When one detector’s alarm activates, all connected detectors will alarm to alert boat inhabitants in other locations to the presence of CO.

Sensitivity: CO alarms are designed to alarm based on both concentration and exposure time. A properly functioning detector provides warning before occupants experience serious overexposure symptoms.

Sensor mounting position: Install carbon monoxide detectors or remote sensors in each enclosed accommodation space and sleeping area, following the detector manufacturer’s instructions. Avoid locations near hatches, portholes, vents or areas where water could contact the device.

Replacement date: CO detector sensors have a finite service life. Mark the installation date, follow the manufacturer’s replacement schedule and replace the unit by the printed replacement date even if the power light still works.

What to Do if CO Poisoning Is Suspected

Do not wait for symptoms to become severe. If a CO alarm sounds, if passengers show symptoms, or if exhaust is suspected inside the boat, treat the situation as a serious safety event.

Move everyone to fresh air immediately.
Shut down engines, generators, heaters and other combustion devices if it is safe to do so.
Open hatches, ports and doors to ventilate the boat.
Account for all passengers, including anyone sleeping below.
Call 911, hail the Coast Guard or seek emergency medical help if symptoms are present.
Do not re-enter enclosed spaces until the source is identified, the boat is ventilated and the detector no longer alarms.
Have the engine, generator, exhaust and fuel-burning systems inspected before operating again.

Carbon Monoxide Safety FAQ

Do all boats need a carbon monoxide detector?

Any boat with an enclosed accommodation space should have marine carbon monoxide detectors installed. Boats with cabins, sleeping spaces, inboard engines, generators, heaters, propane appliances or enclosed cockpits have the highest risk. Even boats without generators can be exposed to CO from their own engine or from nearby boats at a dock or raft-up.

Where should carbon monoxide detectors be installed on a boat?

Install CO detectors in enclosed accommodation spaces and sleeping areas, following the detector manufacturer’s instructions. Avoid placing them where spray, rain, hatch leaks or direct airflow from vents could affect the sensor. A single detector in the main cabin may not be enough if the boat has separate sleeping compartments.

Can carbon monoxide poisoning feel like seasickness?

Yes. Headache, dizziness, nausea, weakness and vomiting can look like seasickness, heat stress or dehydration. If more than one person feels sick at the same time, or symptoms improve when people move into fresh air, treat CO exposure as a possibility and take immediate action.

Is the swim platform safe when the engine or generator is running?

No. Avoid sitting, swimming, boarding, towing or playing near the stern, swim platform or exhaust outlets while the engine or generator is running. CO can collect around the transom and under swim platforms, and exposure can happen outdoors as well as inside the cabin.

How often should boat carbon monoxide detectors be replaced?

Follow the manufacturer’s replacement date and service-life instructions. CO sensors do not last forever, and an old detector may power on without providing reliable protection. Test detectors before each trip and replace expired units immediately.

More Information

For more about carbon monoxide poisoning and what you can do to prevent it, read Carbon Monoxide Poisoning a Dangerous Combination, which originally appeared in United States Coast Guard Boating Safety Circular 86.

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