Carbon Monoxide: Symptoms, Dangers & What to Do if Exposed

Gas safety sits at the top of properties for homeowners and landlords.

Central to this is the management of carbon monoxide (CO), a toxic, tasteless, and scent-free gas often referred to as a “silent killer.”

In the UK, there are approximately 20 deaths a year caused by CO leaks. These are almost always from unsafe gas appliances, poor installation, or a lack of professional maintenance.

Understanding how to manage this poisonous gas will protect your family or tenants.

Recognising the invisible: Symptoms & medical realities

Carbon monoxide poisoning is frequently misdiagnosed in clinical settings.

When inhaled, it forms carboxyhemoglobin. This effectively starves the heart, brain, and vital organs of oxygen.

Because the gas cannot be seen or smelled, victims often mistake early symptoms for common ailments, such as fatigue, the flu, or even a persistent hangover.

The five common symptoms of CO poisoning include:

1. Nausea
2. Headaches
3. Dizziness
4. Shortness of breath
5. Loss of consciousness

Vulnerability is not uniform. Children, the elderly, and those with respiratory or cardiovascular conditions are at significantly higher risk from the same concentration of CO. Pets often display symptoms before humans. This makes them an inadvertent early warning indicator – hence the use of canaries in coal mines (to detect CO).

If symptoms dissipate when you are away from the property but return upon re-entry, CO exposure is the primary suspect.

If this happens, you must visit your GP immediately to rule out poisoning.

If you suspect a gas leak or the presence of carbon monoxide, take immediate action:

• National Gas Emergency Service: 0800 111 999
• HSE Gas Safety Advice Line: 0800 300 363

Physical signs in people are often the first warning. But the home environment also provides clues that an appliance is failing.

Physical warning signs in your appliances

Your appliances provide visual clues of incomplete combustion long before total mechanical breakdowns occur. Look out for the following:

• Flame Colour: Gas flames should be a crisp blue colour. Persistent yellow or orange flames are a sign of incorrect gas-to-air ratios and incomplete combustion
• Staining: Look for soot or yellow/brown scorching around the edges of appliances or on the walls behind them
• Condensation: An unexplained increase in condensation on windows, especially if in the room where the appliance is located, could be a sign of a failing flue.

Immediate actions to take

1. Switch off all gas appliances and isolate the supply (if possible)
2. Open doors and windows to maximize ventilation and introduce fresh air
3. Evacuate the property immediately. Stay as calm as you can. A raised heart rate accelerates the absorption of CO into the bloodstream
4. Seek medical help.

Do not re-enter the property until it has been inspected and cleared by a Gas Safe specialist.

Standard alarms vs. low-level monitors

Most households now feature a CO alarm. But traditional detection methods may still leave occupants vulnerable to chronic health issues.

Standard consumer alarms, governed by BS EN 50291, are calibrated to alert only when CO reaches 50–150ppm. This isthe threshold for acute, life-threatening exposure.

But they are not designed to detect insidious low-level exposure.

Low-level monitors, by contrast, track concentrations from as low as 1–9ppm, which is a level at which chronic exposure has been linked to persistent neurological symptoms. This could be present long before a standard alarm would sound.

Many in the HVAC industry increasingly advocate for these low-level monitors that track air quality more granularly. They identify concentrations that fall well below the threshold of standard emergency alarms.

Understanding these differences is vital. The impact of exposure can be devastating even when a standard alarm stays silent.

Landlord legal obligations: compliance, fines, and liability

Landlord Gas Safety Record (LGSR/CP12)

Landlords must organise a gas safety check every 12 months. This must cover every appliance and flue.

And they must provide a copy of the LGSR (also known as CP12) record to tenants within 28 days of the check and retain records for themselves for at least two years.

CO Alarm Legal Requirement (England)

Since October 2022, landlords in England are legally required to install a CO alarm in every room that contains a gas appliance. This includes any room where the only appliance is a boiler.

Alarms must comply with BS EN 50291 or face notice from the local authority, with a civil penalty of up to £5,000.

Financial and legal consequences

• Criminal liability (Gas Safety Regulations 1998): Failing to arrange an annual gas safety check is a criminal offence under the Gas Safety (Installation and Use) Regulations 1998. It carries an unlimited fine and up to six months’ imprisonment. And it can prevent a landlord from pursuing possession proceedings.
• Civil penalties: Local authorities can issue fines of up to £30,000 per offence for safety and licensing failures.
• Rent Repayment Orders (RROs): Non-compliance can trigger an RRO, forcing landlords to repay up to 12 months’ rent to tenants or the local council.
• Shared risk: Legal liability is often a “shared risk” between landlords and letting agents. Prosecution can target either or both parties if oversight is found.
• Potential audit trigger: Administrative oversights in selective licensing can trigger a full council audit, including a deep-dive into EPC and Gas Safety compliance.

Ventilation and depressurisation

Modern energy-efficient construction has changed building physics. Houses now often have great insulation, but they are highly susceptible to dangerous pressure differentials.

Depressurisation and “backdrafting”

In airtight homes, high-airflow mechanical extraction can pull air out of the building faster than it can be replaced. For example, powerful kitchen hoods.

This creates depressurisation, which is a vacuum effect. When the internal pressure drops, it can cause a failed flue draught.

This is ‘backdrafting’, when toxic combustion gases are pulled back down the chimney (and into living spaces).

Unvented gas ranges

Special caution is required for unvented gas ranges. These appliances rely entirely on room volume and secondary ventilation for safety.

Without a dedicated extraction strategy, they can quickly elevate CO and pollutant levels to hazardous concentrations, espeically in modern buildings that have low leakage.

Carbon monoxide myth-busting

Relying on “common knowledge” can be a fatal error. As specialists, we must clarify these six dangerous misconceptions:

Myth 1: “CO has a slight metallic smell”

CO is entirely odourless. If you smell gas, you are smelling the “stenching agent” added to natural gas; CO itself remains undetectable to the nose.

Myth 2: “Only older appliances leak”

New appliances are just as vulnerable if the installation is poor or if flues become blocked by debris or bird nests.

Myth 3: “A CO alarm is all the protection you need”

An alarm is a reactive backstop. Only professional maintenance and flue-gas analysis can prevent a leak from occurring.

Myth 4: “Low levels aren’t dangerous”

Chronic exposure to low levels can lead to permanent neurological damage. Any detection requires immediate intervention.

Myth 5: “Symptoms are always obvious and sudden”

Symptoms are subtle and frequently misdiagnosed as the flu or general fatigue.

Myth 6: “Opening windows fixes the problem”

Ventilation is a temporary mitigator, not a solution. You must leave the property to stop the physiological absorption.

Strategic maintenance: Gas Safety Check vs. boiler service

Homeowners and landlords often conflate these two services, which can be a costly mistake.

• Gas safety check: A mandatory inspection to confirm that appliances meet Regulation 26(9) of the Gas Safety (Installation and Use) Regulations 1998. It ensures pressure, combustion, and ventilation are safe.
• Boiler service: A preventative maintenance procedure. It involves cleaning components, checking seals, and optimizing efficiency. Skipping this often leads to total boiler failure and emergency call-out charges that can exceed £300.

Both procedures are essential. However, only one (the Gas Safety Check) is a legal requirement for rental compliance.

The Gas Safe Register: Verifying qualifications

It is a legal and safety prerequisite to use only engineers listed on the official Gas Safe Register.

The verification process

Always inspect the engineer’s ID card before work begins. A valid card must feature a security hologram and a specific licence number.

You must also check the back of the card, where qualifications are appliance-specific (and must be updated every 5 years).

If an engineer is qualified for boilers but not gas fires, they can’t legally touch the latter.

Industry safety codes

When an engineer identifies a fault, they use the following safety classifications:

• ID (Immediately Dangerous): The appliance is a risk to life. By law, the engineer must disconnect the gas supply immediately.
• AR (At Risk): Faults exist that could become dangerous. The appliance is typically isolated until repaired.
• NCS (Not to Current Standards): The appliance is safe but does not meet current gas safety standards; upgrades are recommended.

Carbon monoxide safety today: The data behind the danger

Despite rigorous regulation, gas hazards remain a prevalent threat in UK homes.

Fire and rescue services attended 1,623 CO incidents in 2019, resulting in 212 casualties and 17 fatalities. 

And official inspections identified 3,168 gas fittings classed as “At Risk” or “Immediately Dangerous” between 2020–21, underscoring the prevalence of substandard workmanship.

Get your home appliances covered with Hometree

Our home appliance cover includes unlimited repairs, an annual boiler service, zero parts & labour charges, and 24/7 claims support to keep your home warm. Subject to exclusions and T&Cs apply.

With no hidden fees and coverage across England, Scotland, and Wales, we’ve got you covered.

You can get covered today or contact us for a free quote.