Hazardous gases on vessels

Toxic gases can quickly build up on board vessels and kill a crew member or passenger in minutes. Identify equipment, appliances and systems that create hazardous gases to reduce the risks.
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3 steps to reduce risks

Conduct a risk assessment as part of your safety management system (SMS).

Identify equipment, appliances and systems that create hazardous gases. Properly store and maintain them.

Ensure control measures are in place to eliminate or reduce the risk to your crew and passengers. 

Dangers of gas exposure

Exhaust from vessel engines, generators, appliances and sewerage systems produces gas emissions that may be:

  • toxic
  • highly explosive. 

If left unmonitored, unmaintained, and unmanaged, gases can accumulate, leading to:

  • poisoning
  • explosions and fires.

Risks and prevention

Carbon monoxide poisoning 

Carbon monoxide is an odourless, tasteless and colourless gas.  It is produced when a carbon-based fuel, such as petrol, diesel, propane, charcoal or oil, burns. 

Inhaling high concentrations of carbon monoxide can cause death in minutes.  


  • Engines, generators and fuel-burning appliances: These include:
    • BBQs, charcoal briquettes/beads and grills 
    • outdoor heaters 
    • gas lanterns 
    • tools with gasoline engines (e.g. pressure washers, saws) 
    • portable generators 
    • boat engine exhausts.
  • Confined areas: Never operate carbon monoxide-emitting appliances indoors or in small, poorly ventilated spaces.


Some common symptoms of carbon monoxide poisoning include:

  • nausea
  • vomiting
  • headaches
  • dizziness or weakness
  • eye irritation 
  • fainting.

Symptoms of high exposure to carbon monoxide include:

  • loss of consciousness
  • seizures
  • permanent brain injury
  • death.

Carbon monoxide poisoning can be easily mistaken for seasickness or intoxication.  If unsure, call for medical assistance. Failure to identify and treat carbon monoxide poisoning can have fatal consequences. 


  • Avoid being seated or standing close to engines or exhaust systems, particularly when the vessel is idling.
  • Ensure passengers do not linger on the swimming deck and ladder at the rear of the vessel due to its proximity to exhaust fumes.
  • Only use fossil-fuel burning appliances, engines, tools and equipment outdoors. Consider other factors like wind direction, keep fumes away from open windows and cabin doors.
  • Dive boats with cylinder charging compressors need to ensure that the compressor air intake is well away from propulsion engine exhausts to reduce the risk of ignition.
  • Monitor and service equipment regularly to ensure there are no leaks. Check equipment as part of the pre-start routines.
  • Consider installing gas detection monitors.


Hydrogen sulphide poisoning

Hydrogen sulphide is a colourless, flammable, corrosive, poisonous gas that smells like rotten eggs and is produced naturally from decaying organic matter.

Prolonged exposure desensitises a person to the smell - higher levels will dull a person’s sense of smell instantly. Inhalation of acute concentrations of hydrogen sulphide can be fatal.  


  • Sewerage systems: Hydrogen sulphide can be produced in bilge systems, oily water tanks and fuel tanks but most it's most commonly produced in sewerage systems.  Concentrations will continue to build in sewage tanks until they are emptied. 
  • Fishing vessels: Hydrogen sulphide can also be produced on fishing vessels by decaying fish, fish waste or offal.
  • Low-lying, enclosed areas: Hydrogen sulphide is heavier than air and can travel along the ground. It collects in low-lying and enclosed, poorly ventilated areas.
  • Ignition sources and sparks: Hydrogen sulphide is highly flammable. It can easily combust and explode and must be kept away from ignition sources or sparks.


Common symptoms of hydrogen sulphide poisoning:

  • headache
  • dizziness
  • staggering
  • sudden collapse
  • acute eye, nose and throat irritation
  • shortness of breath
  • tightness in chest
  • wheezing
  • loss of olfactory senses (loss of smell and taste).

Acute symptoms of hydrogen sulphide poisoning at high concentrations:

  • convulsions
  • pulmonary edema (fluid in the lungs)
  • coma
  • death (in very high concentrations almost instantly).


  • Complete a thorough risk assessment and ensure crew are aware of the mitigation measures in place for their safety.
  • Pump out sewage tanks after each voyage/charter.
  • Check that sewage tanks are empty prior to commencing each voyage. Empty them if they contain waste.
  • Include the monitoring, maintenance, and servicing of sewerage systems in your vessel risk assessment in your SMS. 
  • Make the crew aware of hydrogen sulphide and other hazardous gases on board in safety induction. Include a procedure to manage this occurrence in the vessel’s emergency plan/procedures.
  • Immediately alert the master when a rotten egg smell is first detected and enact emergency procedures, including calling 000.
  • Due to the gas being heavier than air, hydrogen sulphide will accumulate on lower sections in vessels. Stay clear of these zones and move to an open area in an emergency.
  • Stay vigilant about your surroundings and the possibility of inhaling hazardous substances when dealing with incidents on board.  Contact HAZMAT for guidance as needed.
  • Do not put yourself and your crew in danger when helping others. If someone has collapsed inside from any hazardous gases, go outside.


Carbon dioxide poisoning 

Carbon dioxide (CO2) is a naturally occurring gas. Carbon dioxide is heavier than air and accumulates on the ground.  Similar to carbon monoxide, carbon dioxide is colourless, tasteless and odourless. 

We exhale carbon dioxide, and in low quantities it does not pose a risk to people.  However, a few minutes of exposure to high concentrations of carbon dioxide can be fatal, particularly if a person collapses to the floor or is working in low lying areas in a vessel.


  • CO2 cylinders: Carbon dioxide contained in CO2 cylinders is commonly used in the hospitality industry to carbonate drinks such as on tap beers, ciders and post-mix soft drinks.
  • Confined areas: Many passenger vessels use and store CO2 cylinders, usually in small cupboards. If there is a leak, gas emissions can build up and pose a serious risk to people when they open the cupboard or enter the confined area where these cylinders are being stored. Asphyxiation and possible death can occur within minutes. 


Acute carbon dioxide poisoning stops the respiratory and circulatory systems in the body which can cause:

  • tachycardia (fast heart rate)
  • cardiac arrythmias
  • impaired consciousness.

More serious symptoms of high exposure to carbon dioxide are:

  • convulsions
  • coma 
  • asphyxiation
  • death.


  • Include the use, storage and maintenance of CO2 in your risk assessment, emergency plans and crew induction within the vessel's SMS.
  • Store CO2 cylinders in open, well-ventilated areas.
  • Do not store CO2 cylinders in small rooms or cupboards without CO2 monitors.
  • Check CO2 cylinders for leaks regularly.
  • Make crew aware of the risks of CO2 exposure in your vessel induction and daily prestart briefings, and provide information on what to do in case there is a gas leak.
  • Consider installing gas monitor devices near the source.


Next steps: Address the risk of hazardous gases on your vessel by conducting a risk assessment as part of your safety management system.