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Carbon Monoxide

What is it?

Carbon Monoxide or CO is a natural gas created by the incomplete combustion of fossil fuels. It has no smell, no taste and no colour, making it extremely hard to detect. The effects of carbon monoxide on humans and animals can be deadly - around 40 people die each year in the UK from CO poisoning.

Fossil fuels are used extensively in our lives and are used every day in our homes. Gas, oil, coal and even wood burnt in boilers and engines can emit CO, as can fossil fuels used in water heaters, oil burners, cookers, gas fires, open fires and solid fuel appliances. When these appliances are installed correctly, maintained and serviced regularly, the chance of CO building up are greatly reduced. Most victims of CO poisoning are exposed to gasses that are not vented adequately by heating equipment.

If you use an appliance that runs off any fossil fuel you should make sure:

  • there is adequate ventilation;
  • the appliance has been installed correctly; 
  • it is serviced yearly; 
  • it is well maintained; 
  • you consider investing in a CO detector - it could save your life.

How are people poisoned and what happens?

Our bodies will quickly and easily absorb carbon monoxide. In fact our bodies will actively seek to absorb CO over oxygen where it is present, even though the gas is fatal to us. If CO builds up in the air you breathe, you will begin to feel its effects. Once inhaled, carbon monoxide combines with oxygen carrying hemoglobin to form carboxyhemoglobin. Carboxyhemoglobin inhibits the transference of oxygen around the body, starving the organs of oxygen. This oxygen starvation particularly affects the heart, brain and central nervous system.

Symptoms are often flu-like and their severity depends on the amount of carbon monoxide inhaled. The below guide shows symptoms experienced by an healthy adult at the relevant exposure level. The CO is measured in Parts Per Million (ppm) in relation to air.

ppm CO

Time Exposed


 50 ppm   N/A Safety level as specified by the Health and Safety Executive
 200 ppm   2-3 hours Slight headache, some may experience slight dizziness, nausea, chest tightness, fatigue, lack of concentration, memory loss.
 400 ppm   1-2 hours Headache, worsening to a sever headache with prolonged exposure, dizziness, nausea, fatigue, tightness across the chest, memory loss, lack of concentration.
 800 ppm   45 minutes Headache, dizziness, nausea, convulsions within 45 minutes. Unconscious within 2 hours. Death can occur within 2-3 hours.
 1600 ppm   20 minutes Headache, dizziness, nausea, convulsions, unconsciousness. Death can occur within 1 hour.
 3200 ppm   5-10 minutes Headache, dizziness, nausea, convulsions, unconsciousness. Death can occur within 1 hour.
 6400 ppm   1-2 minutes Headache, dizziness, nausea, convulsions, unconsciousness. Death in approximately 30 minutes.
 12,800 ppm   1-3 minutes Death


People who are poisoned over a length of time will often display symptoms ranging from headache, dizziness, nausea, fatigue, lack of concentration, memory loss, disorientation, irritability and tightness across the chest. As the level of CO in the body increases the symptoms will develop to include confusion, weakness, lack of co-ordination, lack of balance, problems with the heart, cerebral edema, convulsions, coma and death.

Effects on the heart include rapid and irregular heartbeat, decreasing blood pressure, cardiac arrhythmias, heart blocks, ventricular ectopic beats, heat attack and death. When subject to CO poisoning the brain can develop cerebral edema (swelling). Cerebral edema causes brain cells to crush into each other through swelling, thus killing the compacted cells. Carbon monoxide related cerebral edema can cause irreversible damage to the brain which in turn can effect the nervous system.

CO poisoning can be miss-diagnosed as flu or gastroenteritis through its symptoms. Due to its effects on the brain and nervous system it can also be miss diagnosed as a neurological or psychiatric disorder.

Who is at risk?

Although anyone who inhales an unsafe level of CO gas will suffer poisoning to some degree (depending on the amount of CO in the air) children, the elderly and pregnant women succumb quicker to the effects of CO poisoning. It is also suggested that acute CO exposure can give an expectant mother a non-lethal dose of CO, but can harm an unborn child and lead to miscarriage. Pets display symptoms at a faster rate and will show the same kind of effects as humans (dizziness, nausea, fatigue, disorientation, irritability, weakness, lack of co-ordination, convulsions, coma, death).

People already suffering from a heart condition can succumb more rapidly to carbon monoxide, as can those with respiratory health problems. Smokers are at a higher risk as they will already have elevated levels of Carboxyhemoglobin in their blood.

CO poisoning - the facts surrounding gas appliances

A lot of people assume "it will never happen to me". The chances are that it won't, but it does pay to take precautions to make that risk even less likely. If you know the facts surrounding CO poisoning, you can act to reduce your risk.

The facts below are taken from "A review of carbon monoxide incident information for 2000/2001". Published by the Health and Safety Executive, the review looks at incidents involving piped natural gas and LPG.

  • The months of November, December and January (when the central heating system it at its highest use) have the highest rates for CO poisoning. The heating season (starting from the first cold days of autumn when the central heating system is first put into use, to the time when it is turned off in the spring) is the period where most poisonings happen.
  • Most gas related CO poisoning happens in homes where the owner is the occupier (66% according the HSE figures). It is up to you to make sure appliances are installed correctly by a competent installer, are well maintained, serviced regularly (all people installing and servicing gas appliances have to be Gas Safe Registered by law) and you should consider installing a CO alarm.
  • By law landlords have to get all appliances serviced yearly, keep them well maintained (including the flue) and have safety checks (gas safety certificate) completed at least every 12 months. The landlord has to keep a record of safety checks made and make it available to the tenant if requested.
  • It is reported that the most common faults causing appliances to produce CO gas is a lack of servicing and flue/terminal faults. Next most common is poisoning caused by a lack of ventilation.
  • Properties built pre 1945 accounted for 50% of CO poisonings in the financial year between 2000 and 2001. 20% of poisonings occurred in properties built from 1946 - 1965, 20% in properties from 1966-1980, 4% in properties from 1981 - 1991 and 3% from properties post 1992 (3% were unclassified).
  • People are most likely to be poisoned in their bedroom or living room.
  • Central heating accounts for 80% of incidents and 87% of fatal and non-fatal casualties for the year 2000/2001. Appliances involved in incidents ranged from back boilers, floor standing boilers, wall mounted boilers, wall mounted combi boilers, warm air units, cookers, space heaters and water heaters. Wall mounted boilers accounted for 34% of incidents, floor standing boilers accounted for 25% and wall mounted combi boilers accounted for 24%.
  • At 59% of incident sites, the appliance had been installed as new.
  • The majority (76%) of appliances involved in a CO poisoning incident are open flued. When these incidents occurred, in 57% of cases the flue was not installed to appropriate standards.

Avoiding carbon monoxide poisoning

Everyone who owns an appliance that uses fossil fuels (gas, oil, coal, wood) should:

  • check the flame colour of the appliance (if it is burning orange it is not operating properly and could be emitting CO gas); 
  • have your appliance(s) serviced every year by someone qualified to work on them. All people working with gas have to be Gas Safe Registered by law; 
  • maintain all appliances adequately - check in the case of boilers that the flue is not blocked, remove any debris, obstructions or plants from the flue area; 
  • make sure there is adequate ventilation in your home, not only to vent out any noxious gasses but also to allow oxygen and fresh air into the home; 
  • invest in a CO detector which can recognise low CO levels and warn you of any change; 
  • if you are a tenant you should have a safety certificate. Your landlord has to by law have appliances checked yearly. Check if your landlord has fitted a CO detector; 
  • if you suffer from headache, dizziness, nausea, fatigue, lack of concentration, memory loss, disorientation, irritability and tightness across the chest you may be suffering from CO poisoning. Ask your doctor for a CohB test; 
  • in mild cases of CO poisoning, victims recover when administered oxygen or simply brought into fresh air. If you start feeling the effects of CO poisoning, get outside of the premises immediately; 
  • don't use open flames, ovens etc to heat your home - use appliances only for the purpose they are intended; 
  • remember it is not just your boilers that are at risk, anything that goes through the process of combustion can produce carbon monoxide. Cookers, water heaters, oil burners, gas fires, open fires, engines and solid fuel appliances to name but a few can all produce CO.

Finding a Gas Safe Registered installer

It is illegal to work on gas unless you are registered with Gas Safe. Many CIPHE members are registered with Gas Safe, you can search for an IPHE Registered Plumber by clicking here (if they are registered with Gas Safe it will state they can work on gas). You can also use the Gas Safe Register website to search for Registered installers, please go to