Carbon Monoxide Safety
Carbon Monoxide: Symptoms, Alarms and What to Do
Boiler and Heating Guides > Carbon Monoxide
Carbon monoxide is a poisonous gas you cannot smell; if exposure is suspected or an alarm sounds, get outside, seek medical help through NHS 111 or 999 for serious symptoms, then call 0800 111 999. Act even if nobody feels ill. This guide explains symptoms, alarms and subsequent appliance checks for London households. If you smell gas, leave and call the National Gas Emergency Service on 0800 111 999 from safety.
Call with your postcode, the boiler make and any fault code showing.
- Gas Safe registered: 648977
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Suspected Carbon Monoxide Leak or Alarm Sounding: What to Do Now
A suspected carbon monoxide leak or sounding alarm means leave for fresh air immediately and obtain medical help.
Get everyone outside
Leave the affected building. Stopping appliances or opening windows must never delay escape or require you to go back inside.
Seek medical help
Call NHS 111 about suspected exposure, even if symptoms improve outside. Call 999 for the serious symptoms listed below. Medical help must not wait for a gas call or repair booking.
Report the hazard
From a safe place, call the National Gas Emergency Service on 0800 111 999 and explain the suspected exposure or sounding alarm.
Stay outside
Follow emergency-service advice. Do not re-enter until you have been told it is safe.
Call 999 after suspected exposure if anyone has difficulty breathing, sudden confusion, unconsciousness, weakness, chest pain or muscle pain. Do not drive yourself to A&E.
Treat every sounding CO alarm as real. Do not reset the boiler, investigate the source or silence the alarm to continue using appliances.
Carbon Monoxide Symptoms: Recognise the Warning Signs
Carbon monoxide symptoms can include headache, dizziness, nausea, tiredness and breathlessness, and can resemble common illnesses.
Possible warning signs include:
- Headache or dizziness.
- Feeling sick or vomiting.
- Unusual tiredness.
- Breathlessness.
- Weakness, confusion, chest pain or muscle pain, which require the urgent response above.
Symptoms vary between people and may come and go. Feeling better outdoors does not establish that you are safe or that exposure did not happen. You do not need several symptoms before taking action.
Recognition is not a diagnosis. Follow the emergency steps above if exposure is suspected; healthcare professionals assess whether poisoning has occurred. Do not return indoors to see whether symptoms come back.
What Causes Carbon Monoxide in a Home or Gas Boiler?
Carbon monoxide can form when fuel burns incompletely, while faults affecting combustion, flues or ventilation can create a hazard indoors.
Faulty combustion
Boilers and gas cooking appliances are possible sources. Poor installation, faults or inadequate maintenance can affect how fuel burns. Stop using a suspect appliance and follow the emergency response.
Flue or ventilation problems
Combustion gases need a safe route outside, and appliances need their intended air supply. Problems can allow gases into living spaces. Leave assessment to a qualified engineer.
Outdoor equipment used indoors
Barbecues and camping stoves can produce dangerous carbon monoxide indoors. Never use them inside.
Soot or staining may be warning signs, but their absence does not establish safety. Do not assume the boiler is responsible, or that a room-sealed boiler cannot present a hazard. Never open the boiler casing or work on gas.
Carbon Monoxide and a Smell of Gas: Different Hazards
Carbon monoxide has no smell, so a gas smell is a separate warning requiring emergency action.
- Suspected CO or a sounding CO alarm: Get outside, obtain medical help and then report the hazard using the sequence above. A lack of smell gives no reassurance.
- Smell of escaping gas: Leave, avoid ignition sources and call 0800 111 999 from safety. Do not operate electrical switches or use flames. Read what to do if you smell gas for the full response.
These hazards can coexist. Neither should wait for a repair booking.
Carbon Monoxide Poisoning Treatment: What Medical Care Involves
Carbon monoxide poisoning treatment requires medical assessment and may include oxygen given in hospital.
Hospital staff may test your blood for evidence of exposure and decide what treatment you need. Tell them about the suspected source, your symptoms and when you left the building. Follow their advice even if you feel better.
Getting outside removes you from the suspected source, but it does not replace medical assessment. A gas engineer cannot diagnose or treat poisoning. Medical care comes first; appliance investigation follows once emergency needs have been addressed.
Carbon Monoxide Testing: A Person, a Room and a Boiler Need Different Checks
Carbon monoxide testing can assess a person’s exposure, detect CO in room air or investigate an appliance, and each answers a different question.
A person
Healthcare professionals use symptoms, exposure history and appropriate tests. Blood testing may measure carboxyhaemoglobin, formed when CO binds to haemoglobin. This does not establish whether a building is safe.
A room
An audible household CO alarm warns of carbon monoxide in the air. It cannot diagnose poisoning or identify which appliance is responsible. Its test button does not certify a boiler.
A boiler
A suitably qualified Gas Safe registered engineer assesses the appliance, flue and ventilation, using flue gas analysis where appropriate. Boiler combustion readings cannot diagnose a person.
No single check replaces the others. A quiet alarm must not override symptoms or a previous alarm event.
Carbon Monoxide Levels and PPM: Why a Reading Cannot Declare a Home Safe
A carbon monoxide reading in ppm measures concentration, but one reading cannot establish that a home or person is safe.
Room-air reading
Describes CO concentration where and when measured; it cannot rule out earlier exposure.
Blood-test result
Helps clinicians assess exposure; it does not clear an appliance for use.
Boiler flue measurement
Helps an engineer assess combustion; it is not a household exposure limit.
PPM means parts per million. Exposure duration and individual susceptibility matter alongside concentration. A search for “50 ppm carbon monoxide” should not become a reason to stay inside or a limit you use to judge safety.
A later low reading or silent alarm does not cancel symptoms or an earlier alarm. Follow emergency advice.
Reduce the Risk: Annual Servicing, Clear Ventilation and CO Alarms
Regular appliance maintenance, unobstructed ventilation and a correctly maintained audible CO alarm help reduce risk after the immediate emergency has been resolved.
CO Alarm Care After the Emergency Has Been Resolved
Routine alarm testing, positioning and replacement must follow the instructions for your actual alarm.
- Use the correct manufacturer’s manual.
- Test it as the manufacturer directs.
- Check its replacement date.
- Keep its intended position unobstructed.
Do not consult the manual instead of leaving when an alarm sounds.
For rented homes in England within the regulations’ scope, a CO alarm is required in any room used as living accommodation containing a fixed combustion appliance, excluding gas cookers. That legal exception does not mean a gas cooker cannot produce carbon monoxide.
- Arrange servicing: Maintenance follows the appliance manufacturer’s schedule, with combustion and flue checks as applicable. An annual boiler service helps identify problems requiring attention. Keep the service record.
- Keep ventilation unobstructed: Do not cover permanent vents. Leave flue assessment and repairs to a suitably qualified engineer; never dismantle or clear a flue yourself.
- Maintain an audible CO alarm: An alarm provides a warning and complements appliance maintenance. It does not replace servicing or professional investigation after suspected exposure.
Boiler repair, servicing and gas work
When to Call a Gas Safe Engineer for Boiler Repairs in London
Arrange a Gas Safe engineer’s assessment after emergency services have made the situation safe and medical help has been obtained. Suspected CO ends routine homeowner checks: never remove the casing, reset or restart a suspect appliance. You can then arrange boiler assessment and repair in London.
All Service 4 U has a 24/7 phone line. Our Gas Safe registered engineers undertake domestic natural gas boiler work; our business registration is 648977. Making the emergency safe may still leave the appliance needing a separately arranged assessment before use.
Have appliance details and any warning notice ready only if already available safely.
Discuss boiler repairs after the emergency: 020 3740 6785
020 3740 6785
Frequently Asked Questions
How Quickly Can Carbon Monoxide Poisoning Develop?
There is no single reliable timeline because effects depend on exposure and individual circumstances. A short, substantial exposure and repeated or prolonged exposure can both cause harm. Act on suspected exposure instead of timing symptoms. Seek medical follow-up for later health problems and explain the exposure, even if you initially felt better.
Can a Normal Finger Pulse Oximeter Rule Out Carbon Monoxide Poisoning?
No, a normal reading on a conventional finger pulse oximeter cannot rule out carbon monoxide poisoning. Ordinary oxygen-saturation devices differ from specialist CO-oximetry and clinical testing for carboxyhaemoglobin. Symptoms or possible exposure still require the emergency response. Do not delay medical help to repeat readings or obtain another device.
Does Smoking Affect Carbon Monoxide Test Results?
Smoking can contribute to carbon monoxide exposure and affect medical interpretation of test results. Tell the clinician about smoking as well as suspected appliance exposure. Smoking does not establish the cause of symptoms or make a sounding alarm safe to ignore. Leave interpretation to the healthcare team.
Can Carbon Monoxide Affect Dogs?
Yes, dogs can be poisoned by carbon monoxide and suspected exposure needs urgent veterinary advice. Take pets outside with the household if this will not delay escape. Never re-enter to retrieve an animal. Once safely outside, contact an emergency vet and explain the suspected exposure. Human medical needs remain urgent too.
What Does an Unsafe Warning Label on My Boiler Mean?
An unsafe warning label means follow the engineer’s instructions and do not use the appliance until the unsafe condition has been resolved. At Risk and Immediately Dangerous findings carry a do-not-use warning. Keep the warning notice for subsequent assessment. The label alone does not establish that replacement is the only remedy.
How Do I Check the Engineer Is Qualified to Work on My Boiler?
Check the engineer’s Gas Safe ID card and confirm their current qualifications cover the required boiler work. Check identity, validity and relevant work categories, and verify the details through the Gas Safe Register. Registration does not mean every engineer covers every appliance. Credential checks must never delay emergency calls.
Still have a question or need urgent assistance? Call 020 3740 6785, tell us what has happened and we will tell you what happens next.