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The Dangers of Carbon Monoxide

Carbon Monoxide (CO) is a poisonous gas produced by the incomplete combustion of carbon based fuels such as gas, petrol, paraffin, coal, oil, wood etc. Most fuel burning appliance produce some carbon monoxide during normal operation which is vented safely outside by a flue or exhaust pipe. Dangerously high levels of carbon monoxide can be generated quickly from appliances that are poorly maintained, faulty, have restricted ventilation, develop blocked or cracked flues/exhausts. Very high levels can also be generated when appliances that are designed to be used in the open air are used inside. Adverse weather conditions can also cause a carbon monoxide build up from correctly operating appliances.

Sources of Carbon Monoxide Aboard Boats

Carbon monoxide can be generated by a number of appliances such as cookers, heaters, gas-fridges, portable generators, inboard engines, outboard engines, barbeques etc

Effects on the Body

When breathed in the lungs, carbon monoxide is absorbed into the bloodstream in preference to oxygen. This severely restricts the ability of the blood to transfer oxygen to the vital organs such as the heart and brain. Instead of the normal process of oxygen binding to the bloodís haemoglobin, carbon monoxide binds in its place to form carboxyhaemoglobin (COHb). As a result, the bodyís organs and tissue are starved of oxygen. 

The level of carbon monoxide absorbed by the body will depend on the concentration present and the period of exposure. Exposure to a low concentration for a long period can result in the same carboxyhaemoglobin level as exposure to a higher concentration for a shorter period of time. A number of other factors will affect the level of absorption by the body such as the level of physical activity at the time, a personís size, weight, age and general condition of health. Once absorbed, carbon monoxide can take many hours to desorb from the body upon exposure to clean air. Following high level exposure or long-term exposure permanent damage to the body (eg brain damage) can occur.


The symptoms of carbon monoxide poisoning closely resemble those of a cold or flu and so can be difficult to diagnose. Symptoms can include tiredness, drowsiness, headaches, dizziness, nausea, vomiting, chest pains, breathlessness, stomach pains, erratic behaviour and visual problems. High level exposure will result in unconsciousness and death. An idea of symptoms for different exposure levels is given in the following table,

CO Concentration

Exposure Time


30 ppm

8 hours

Maximum exposure allowed in UK workplace for 8 hour period


2 to 3 hours

Mild headache, fatigue, nausea and dizziness

400 ppm

1 to 2 hours

Serious headache, other symptoms intensify, life threatening after 3 hours

800 ppm

45 minutes

Dizziness, nausea, and convulsions. Unconscious within 2 hours, death within 2 to 3 hours

1,600 ppm

20 minutes

Headache, dizziness and nausea. Death within 1 hour

3,200 ppm

5 to 10 minutes

Headache, dizziness and nausea. Death within 1 hour

6,400 ppm

1 to 2 minutes

Headache, dizziness and nausea. Death within 5 to 30 minutes

12,800 ppm

1 to 3 minutes


ppm = parts per million

What to do in the Event of a Carbon Monoxide Alarm

The general advice given when a carbon monoxide alarm signals an alarm condition is to

  • switch off the appliance and do not reuse until checked
  • ventilate the area
  • seek medical advice

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Nereus Alarms Ltd
9 Britannia Road, Poole, Dorset. BH14 8AZ  UK
Tel: +44 (0)1202 731886