Carbon monoxide has no smell, no colour and no taste. It is recognised by a pattern, not a sensation — and the pattern is that everybody in the house feels ill at once.

Is this a virus, or is it the house?

This is the question people actually face at eleven at night, and the answer is in the pattern rather than the symptoms. Tick everything that is true and the reasoning appears underneath — including when the honest answer is that a virus is more likely.

What it is, and why you cannot feel it coming

Carbon monoxide is what fuel produces when it burns without enough oxygen: gas, wood, coal, oil, petrol, paraffin, charcoal, pellets. A correctly burning, correctly vented appliance produces almost none. A blocked flue, a cracked heat exchanger, a starved flame or an engine running in an enclosed space produces a great deal.

• It binds haemoglobin far more tightly than oxygen does, so it displaces the oxygen your blood is carrying and keeps hold. That is why the effect builds over hours, and why fresh air — and, in hospital, high-flow oxygen — is the treatment.
• Sleep removes your only defence. Awake, you might notice the headache and open a window. Asleep, you simply get a deeper dose, which is why bedrooms and overnight exposures dominate the deaths.
• Low-level exposure over weeks looks like the modern condition: tiredness, poor sleep, morning headaches, difficulty concentrating, feeling run down, low mood. People change their diet and blame their job for months.
• High-level exposure is fast: confusion, unsteadiness, chest pain, breathlessness, vomiting, collapse, seizures. By the confusion stage people frequently cannot organise their own escape — which is the reason the alarm exists rather than the awareness.
• Cherry-red skin is a myth for the living. It is a post-mortem finding, not a bedside sign. Do not use skin colour to reassure yourself about anybody.

Where it comes from

Almost every case involves something ordinary that is faulty, blocked, or being used somewhere it was never meant to be.

The pattern, and what it is usually mistaken for

Since the symptoms belong to a dozen harmless illnesses, the discriminating information is where and when they happen, and to whom.

• More than one person ill at the same time, rather than one after another over days. Viruses spread through a household in sequence; a gas affects everybody in it at once.
• Everybody improves within hours of leaving the building and feels ill again after returning. This is the single most useful observation on this page.
• Pets are affected too, often first because they are smaller and sometimes closer to the floor. A bird that dies, a normally lively dog that is flat, a cat being sick.
• It tracks the heating. Worse when the boiler, fire or stove is running; started when the heating went on for the season; better in a warm week when nothing is lit.
• It is worse in one room or in one bed. Morning headaches in a particular bedroom are a specific and much-ignored clue.
• No fever, no sore throat, no obvious infection — and yet a headache, nausea and exhaustion that keep coming back.

The alarm: the only detector you actually have

1. Buy a carbon monoxide alarm, not a smoke alarm. Look for the standard on the box — EN 50291 in Europe, UL 2034 in North America — and buy from a shop rather than a marketplace listing full of unbranded lookalikes.
2. One per level, and one in every room with a fuel-burning appliance, plus the room you sleep in. If you buy only one, put it where you sleep, because that is where the fatal exposures happen.
3. Place it at breathing height — roughly head height when seated or lying — on a wall or shelf, one to three metres from the appliance. Carbon monoxide mixes with air rather than rising like smoke, so ceiling mounting is not required. Keep it out of cupboards, out of the path of steam, and away from windows and extractor fans.
4. Test it monthly with its own button, and never test it with a flame. The test button checks the electronics; a naked flame proves nothing and risks damage.
5. Note the end-of-life date. Sensors expire, typically after seven to ten years, and an expired alarm is decoration. Write the replacement year on the case with a marker the day you install it.
6. Learn what its beeps mean from its own leaflet: a single chirp every minute is usually a low battery, while a repeated urgent pattern is the alarm itself. Never solve a night-time alarm by taking the battery out and going back to bed. That decision has killed whole families.

If the alarm sounds, or you suspect it

1. Get everyone outside into fresh air, immediately. All the people, all the pets, in whatever they are wearing. Do not gather belongings, do not check rooms twice, do not send one person back for a phone. Confusion is a symptom, so decisions made inside are already impaired.
2. Turn off the appliance if it is on your way out, and open doors and windows as you pass them. Neither of these replaces leaving.
3. Call the emergency number or your national gas emergency line from outside, and use the words possible carbon monoxide. That sentence changes what the crew brings and how fast they come, because it tells them to expect several patients and to carry oxygen.
4. Say how many people were in the building, including anyone who has already gone to work or school feeling unwell, and anyone who is pregnant. Everybody exposed needs assessing, not only whoever feels worst.
5. Do not go back in until the fire service, the gas emergency service or a qualified engineer says the building is safe. Levels can rebuild in minutes and the source has not been found yet.
6. Get medical assessment even if you feel better in the air. Ask for the carboxyhaemoglobin blood test and say when you left the building. Feeling improved is what fresh air does; it is not evidence that the dose was small.

Who it harms first

• Unborn babies. Fetal haemoglobin holds carbon monoxide even more tightly than adult blood, so the fetus is harmed at exposures where the mother has only a headache. Pregnancy is a reason for immediate assessment and low thresholds, not for waiting to see.
• Babies and small children, who breathe faster relative to their size and often cannot tell you anything except by being unusually sleepy or sick.
• Older people, in whom confusion, a fall or worsening heart failure may be the entire presentation — and in whom it is routinely blamed on age.
• Anyone with heart or lung disease, where reduced oxygen delivery can trigger angina, a heart attack or an asthma-like crisis at a level someone else would tolerate.
• Pets, which are frequently the first casualty and the only warning a household gets. A dead bird or a suddenly listless dog in winter deserves a thought about the boiler.
• People who sleep where the appliance is — bedsits, converted garages, caravans, boat cabins, night workers sleeping in vehicles with a heater running.

The once-a-year afternoon

Raising it with someone else

• With a parent or grandparent: buy the alarm and fit it yourself rather than recommending it. The gap between agreeing and doing is where this risk lives, and the visit is the intervention.
• With a landlord: ask in writing for the current service certificate and a carbon monoxide alarm, keep the reply, and mention the appliance signs you have noticed. A written request changes what happens if it later goes wrong.
• With a friend who keeps having “winter flu”: ask the two discriminating questions — is anyone else in the house ill, and do they feel better away from home? It is a thirty-second conversation that occasionally saves someone.
• With students and people in shared houses, where nobody feels responsible for the boiler and everybody assumes somebody else checked: one alarm per floor, bought by whoever reads this first.
• On holiday or in rented accommodation abroad: a travel carbon monoxide alarm weighs nothing, and unfamiliar heaters, gas water heaters in bathrooms and unswept chimneys are exactly the conditions that cause tourist deaths every year.
• After an incident, help the household find the cause rather than only the treatment. A poisoning with no explanation found means the exposure is still there.

The drill: 16 decisions

Sixteen ordinary moments — a camping trip, a power cut, a chirping alarm at two in the morning, a whole family with headaches. Most have an instinctive answer that has killed people. Pick your move; every answer explains why.

The card

Print it and put it next to the boiler, or inside the caravan.

🖨️ Print the card