Carbon monoxide
Plain English. Practical detail. UK context.
Carbon monoxide, or CO, is a poisonous gas that cannot be seen or smelled. If exposure is suspected, go outside and seek medical advice promptly; do not re-enter until advised. Call 999 for serious symptoms described by the NHS, such as breathing difficulty, sudden confusion, loss of consciousness, weakness or chest pain.
Safety: gas work must only be done by a Gas Safe registered engineer who is qualified for that type of appliance. Check their ID card or the Gas Safe Register. If you smell gas, leave the property and call the gas emergency number: 0800 111 999 in Great Britain or 0800 002 001 in Northern Ireland.
Last reviewed: (editorial review against the public sources listed below).
Understand the sources without relying on smell
The NHS identifies appliances and fires burning gas, wood, oil or coal, as well as engines and portable generators, as potential sources in unsuitable conditions. Barbecues and camping stoves must not be used indoors or inside tents. A gas smell is a different warning: absence of smell does not prove absence of CO. Homeowners should not run an engine in an enclosed garage or use a cooker as improvised room heating. Trainees must recognise that combustion, ventilation and flue questions require competent assessment, not visual reassurance from a working appliance.
Symptoms are not a DIY diagnosis
The NHS lists headache, dizziness, sickness, weakness, confusion, breathing difficulty and other symptoms. They can vary and may worsen in an affected building. Symptoms can have other causes, but suspected exposure needs prompt medical advice rather than a home test or waiting to see whether it happens again. The NHS advises contacting 111 for suspected symptoms and 999 or emergency care for its specified serious signs. Do not drive yourself to emergency care. This article is not a diagnostic tool and gives no safe-exposure time or level.
Alarms support, not replace, prevention
Suitable CO alarms, correct installation and maintenance are important, but do not correct unsafe appliances or flues. Follow the actual alarm instructions and applicable building or tenancy requirements. England's Approved Document J gives relevant combustion and alarm guidance, with national differences elsewhere. Homeowners should keep appliances properly installed and maintained by competent people and preserve ventilation paths. Trainees should not silence or dismiss an alarm to continue work. An alarm's absence of sound is not a complete professional assessment of every possible combustion or installation fault.
What to agree with the tradesperson
A gas appliance concern requires appropriate Gas Safe competence; other fuels need their relevant specialist. Agree the investigation, defects, repairs and records, not simply replacement of an alarm. The NHS gives 0800 111 999 for suspected gas-appliance CO leakage; use the appropriate emergency service for the location and fuel. Medical help and leaving danger should not be delayed by paperwork or a contractor appointment. Prices and outcomes vary from job to job, so compare written, itemised quotations rather than relying on a typical figure. Keep relevant installation and maintenance records, while trainees learn to escalate concerns rather than claim a previous safety certificate proves current conditions are safe.
Frequently asked questions
Can I smell carbon monoxide?
No. CO is colourless and odourless; do not use smell to rule it out.
Does an alarm replace appliance servicing?
No. Appropriate installation, maintenance and ventilation remain necessary.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.