Skip to content
Siren & Safe

Carbon Monoxide Detectors

Carbon Monoxide Poisoning Symptoms

It presents like flu without a fever, and it improves when you leave the building. That pattern is the most useful diagnostic signal available to a household.

By Ryder M.Last reviewed

Dimly lit hallway in a home at night

If you suspect carbon monoxide poisoning, get everyone outside into fresh air and call 911 from outside. Do not go back in. The symptoms are headache, dizziness, weakness, nausea, vomiting, chest pain and confusion — a picture that looks like flu but without a fever, and which often improves when you leave the house and returns when you come back. That pattern, and the fact that several people or pets in the same building are affected at once, are the signals worth knowing.

There are no product links anywhere on this page, deliberately. It is a page someone may read while feeling unwell, and putting a buy button on it would be indefensible. The detector roundup is one click away from the hub if that is what you need.

What to do, in order

If an alarm is sounding, or if you suspect carbon monoxide for any reason, this is the sequence. Do not reorder it.

  1. Get everyone outside into fresh air

    Everyone, including pets, immediately. Do not stop to open windows, switch off appliances, get dressed or find out what is happening. Fresh air is the treatment and every minute inside continues the exposure.

  2. Call 911 from outside

    Tell them you suspect carbon monoxide and say how many people were inside and whether anyone is unwell. The fire service carries meters and will tell you whether it is safe to go back in.

  3. Account for everyone

    If someone is still inside, tell the responders — do not go back in for them. A rescuer overcome by the same gas is the classic way one casualty becomes two.

  4. Stay out until you are told otherwise

    Not until the alarm stops. An alarm that has stopped sounding can mean the concentration fell below the reset threshold, not that the source is fixed.

  5. Get medical attention for anyone with symptoms

    Tell the clinician you suspect carbon monoxide, because it changes what they test for. A blood test for carboxyhemoglobin is the diagnostic, and it is time-sensitive — the level falls once you are breathing fresh air, so being seen sooner gives a better answer.

  6. Do not use the appliance again until it has been checked

    By a qualified technician, with the flue inspected. An alarm that sounded once will sound again, and the second time may be at night.

The symptoms

Carbon monoxide binds to hemoglobin far more readily than oxygen does, displacing oxygen from the blood. The symptoms are what oxygen starvation looks like, and they escalate with concentration and duration.

Early and mild: headache — often described as dull and band-like — dizziness, weakness, tiredness, nausea, shortness of breath on mild exertion.

Moderate: worsening headache, vomiting, drowsiness, confusion, impaired coordination, blurred vision, chest pain.

Severe: loss of consciousness, seizures, cardiac effects, and death. At high concentrations people can be incapacitated before they understand what is happening, which is why the alarm rather than the symptom is the intended warning.

CDC's material on carbon monoxide describes the same progression and emphasizes that people who are asleep or intoxicated can die before ever experiencing symptoms — which is the single most important fact on this page, and the reason the alarm outside the bedrooms matters more than any other position.

Carbon monoxide or flu?

This is not a diagnostic tool and it does not replace calling 911. It is a pattern worth recognizing, because CO poisoning is routinely mistaken for a virus.

SignalPoints toward CO
FeverUsually absent with CO. Present with flu.
Symptoms improve when you leave the houseStrongly suggestive of CO. Flu travels with you.
Symptoms return when you come homeStrongly suggestive of CO.
Several people in the building affected at onceSuggestive, especially without the usual staggered onset of an infection
Pets unwell too, or lethargicSuggestive. Smaller animals are often affected first
Worse during the heating season, or after a new applianceSuggestive
Sore throat, runny nose, coughPoints away from CO, toward a virus
Headache that is worst on wakingSuggestive — overnight exposure in a bedroom

If the pattern fits, leave the building and call 911 rather than watching for another day. Getting it wrong in that direction costs you an hour outside.

Who is affected soonest

The mechanism is oxygen displacement, so anyone with less reserve is affected at lower concentrations and sooner.

Infants and young children, because of body mass and faster respiration. Anyone pregnant, because fetal hemoglobin binds carbon monoxide more readily and the fetus has less reserve than the person carrying it. Older adults. Anyone with heart disease, angina, COPD, asthma or another chronic cardiorespiratory condition, for whom reduced oxygen delivery is precisely the thing their condition already limits. Pets, particularly small ones.

This is also why low-level monitors exist as a product category. A UL 2034 listed alarm is forbidden from sounding below 30 ppm, a threshold chosen to prevent nuisance alarms in a healthy adult population — and sustained exposure below that can still matter for the groups above.

The chronic, low-level version

Not every exposure is a dramatic event. A cracked heat exchanger, a partially blocked flue or a badly adjusted burner can produce a modest leak that never trips a listed alarm and goes on for a whole heating season.

What that looks like: recurring headaches, tiredness, poor sleep, nausea or difficulty concentrating, in more than one person in the household, worse in winter, better on holiday. It gets attributed to stress, to a stuffy house, to a virus going around.

If that description fits your household, two things are worth doing. Get every fuel-burning appliance and flue professionally serviced — which is the thing that fixes the cause rather than detecting it. And consider a monitor that reads below the alarm threshold, because a steady 12 ppm in a hallway turns a suspicion into a service call. Low-level CO monitors covers what exists and how patchy the market is.

Also mention it to a doctor. Carboxyhemoglobin testing is quick, and clinicians do not test for what nobody suggests.

After the event

Find the source before using anything again. A qualified technician, with the flue and the appliance both inspected. Until then, do not relight it.

Check the alarm's date while you are at it. An electrochemical sensor has a finite life, commonly 7 to 10 years, and an alarm that worked once at the end of its life is not evidence it will work again. Lifespan and replacement.

Look at coverage, not just the one alarm. If the alarm that sounded was the only one in the house, the gap is the rest of the house. Where to install CO detectors.

And if it turned out to be a false alarm — the alarm signaling end of life rather than detecting gas — that is a good outcome and still worth understanding, because the signals are genuinely easy to confuse. CO detector beeping codes.

Questions people actually ask

What are the symptoms of carbon monoxide poisoning?
Headache, dizziness, weakness, tiredness, nausea, vomiting, shortness of breath, chest pain and confusion, progressing to loss of consciousness and seizures at high exposure. It resembles flu but usually without a fever, and it often improves when you leave the building.
How do I know if it is carbon monoxide or the flu?
The distinguishing pattern is that CO symptoms improve when you leave the house and return when you come back, there is usually no fever, and more than one person — or a pet — in the same building is affected at once. If that pattern fits, leave and call 911 rather than waiting another day.
What should I do if my carbon monoxide alarm goes off?
Get everyone outside into fresh air immediately, then call 911 from outside. Do not open windows, switch off appliances or investigate first, and do not go back in — including for someone still inside. Tell the responders who is inside and let them do it.
Can you recover from carbon monoxide poisoning?
Many people recover fully with prompt treatment, which is why getting out and getting seen quickly matters. Severe or prolonged exposure can cause lasting neurological and cardiac effects, so anyone with symptoms should be assessed medically and should say that CO is suspected, because it changes what gets tested.
Why is carbon monoxide dangerous when you are asleep?
Because the early symptoms are the warning, and an unconscious person cannot notice them. People who are asleep or intoxicated can die from carbon monoxide without ever experiencing symptoms, which is why the alarm within earshot of the bedrooms is the most important one in the house.
Can pets be affected by carbon monoxide first?
Often, yes — smaller animals with faster respiration and lower body mass can show effects before people do. A pet that is unexpectedly lethargic or unwell alongside humans with headaches is a signal worth acting on rather than explaining away.

Sources