Construction hazardous substances: Carbon monoxide

Carbon monoxide (CO) is a colourless and tasteless poisonous gas which can be produced in dangerous amounts when using  petrol powered equipment and gas appliances. Carbon monoxide can kill. This page tells you how to control these risks and why.

What you must do

The Control of Substances Hazardous to Health (COSHH) Regulations says you must protect against the risks from carbon monoxide. Follow the Assess, Control and Review model. Pay particular attention to:


Identify and assess: Identify those tasks/situations where significant levels of carbon monoxide may occur. This may include:

  • using petrol powered equipment, such as generators and cut-off saws, indoors or in enclosed spaces (including trenches / excavations). Even after a few minutes this can create significant levels of carbon monoxide which can kill.
  • using equipment involving liquefied petroleum gas (LPG) in enclosed spaces – eg work equipment or heaters / cookers in welfare facilities
  • refurbishing existing buildings which disrupts gas flues or ventilation systems;
  • inadequately installing new gas appliances


Prevent: Think about eliminating or reducing carbon monoxide risks. Use safer alternatives, such as electrical tools, where you can.  Never use petrol powered equipment indoors or in enclosed spaces unless the ventilation needs have been fully assessed. Seek expert advice to help you with this assessment. Specially selected mechanical extraction that vents to the outside will almost always be needed. Consider also:

  • locating petrol generators etc outside  in well ventilated spaces so that fumes disperse and cannot gather or drift into building openings.
  • the impact of refurbishment work on existing gas-fired systems during the planning stage
  • future maintenance access to flue systems concealed within voids etc

Control: Even if you minimise some of the risk this way, you may still do  work that might create carbon monoxide. Control the risk by:

  • Ventilation – make sure any facilities using LPG have adequate ventilation at both high and low level. Check it is not blocked, eg fixed grilles covered by newspaper or rags in cold weather to "stop draughts"..
  • Detectors – use personal/mounted carbon monoxide / oxygen detectors where appropriate.
  • Installation – ensure a competent person carries out all LPG / gas installation or refurbishment work. Gas work undertaken in homes and certain commercial premises must be done by a Gas Safe registered engineer.
  • Other risks – you may have to consider other risks as well such as fire and explosion associated with petrol /LPG or confined space work.

Train: Workers need to know how to use the controls properly. They also need to be aware of the signs and symptoms of carbon monoxide poisoning.


Supervise: Ensure that controls are effective and used by the workers.

Maintain: Properly maintain all equipment. LPG equipment can be particularly vulnerable. Blocked or partially blocked burners can lead to higher carbon monoxide levels and flame failure. LPG can also leak from damage to hoses etc.

What you should know

Carbon monoxide (CO) is a colourless, odourless, tasteless, poisonous gas produced by incomplete burning of carbon-based fuels like gas, petrol, wood and coal. Signs that indicate incomplete combustion is occurring and CO may be produced include;

  • yellow or orange rather than blue flames (except fuel effect fires or flueless appliances which display this colour flame)
  • soot or yellow/brown staining around or on appliances
  • pilot lights that frequently blow out
  • increased condensation inside windows

CO stops your blood from bringing oxygen to cells, tissues, and organs. Early symptoms of CO poisoning can be confused with food poisoning, viral infections, flu or simple tiredness. They include drowsiness, headaches, breathlessness and nausea. CO levels can quickly kill without warning.

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Updated 2023-04-11