How to Respond When Your Carbon Monoxide Alarm Goes Off

Why this matters

Carbon monoxide (CO) is a gas you cannot see, smell or taste. It comes from anything that burns fuel: a furnace, a boiler, a water heater, a gas range, a fireplace, a car or a generator. When it builds up indoors it takes the place of oxygen in your blood, and the first signs (headache, dizziness, nausea, feeling tired or confused) are easy to mistake for the flu. People who are asleep may never feel them at all. A CO alarm that goes off is the only warning you get that you cannot dismiss as a cold, so the right response is the same every time: get out first, think second.

Before anything else: if the alarm is sounding its full alarm pattern (not a single short chirp every minute or so), go to Step 1 now and read the rest later, outside.

Step 1: Get everyone out to fresh air, pets included

Leave the house the moment the alarm sounds its alarm pattern. On most CO alarms that is a group of four quick beeps, a pause, and then four more, repeating; smoke alarms use three. The four-beep pattern comes from NFPA 72, the national alarm standard (it absorbed the old CO standard, NFPA 720), in whatever edition your local code has adopted, and alarms sold for homes are tested to UL 2034, which requires them to sound sooner the higher the CO level climbs. Wake everyone, gather the children and the pets, and go outside. If it is cold, grab coats and shoes on the way only if they are by the door; do not go room to room collecting things.

• Do not stop to open windows. It feels helpful, but every minute spent walking the house is a minute spent breathing the air you are trying to escape. Responders would rather ventilate it themselves after they have taken a reading.
• Do not hunt for the source. Turning appliances off, checking the furnace or looking into the garage keeps you inside. That is their job, with a meter.
• Count heads at one meeting spot. Pick somewhere you can all stand together: the sidewalk, the driveway, a neighbor's porch. Count people and pets out loud. If someone is missing, tell the dispatcher and the first responder who arrives; do not go back in yourself.

If a person will not wake up, or is too confused or weak to walk, call 911 immediately, from outside if you can get there, and tell them the CO alarm is sounding.

What breaks if you skip it: CO poisoning gets worse the longer you breathe it, and it clouds judgment. People who stay to investigate are the ones who end up needing an ambulance.

Step 2: Make the call from outside

Once everyone is out, call. Who you call depends on how people feel:

• If anyone has symptoms, call 911. Symptoms include headache, dizziness, nausea or vomiting, shortness of breath, confusion, unusual sleepiness, or simply everyone in the house feeling unwell at the same time. Say "our carbon monoxide alarm went off and someone feels sick." Infants, older adults, pregnant women and anyone with heart or lung problems are at higher risk, so mention them.
• If nobody has symptoms, follow your local guidance. Many fire departments want a 911 call for every CO alarm, symptoms or not, and will send a crew with a CO meter. Others ask you to use the fire department's non-emergency number, and in many areas the gas utility also responds to suspected CO from gas appliances, day or night. Find out which applies where you live now, on a calm day, and write the number in your house records. If you do not know when the alarm goes off, call 911. Nobody will be upset with you.

Tell the dispatcher what fuel-burning equipment you have (gas furnace, gas water heater, fireplace, attached garage) and whether anything unusual was going on, such as a car warming up in the garage or a generator running during an outage.

What breaks if you skip it: an alarm that stops on its own does not mean the CO is gone. Without someone measuring, you have no way to know whether it is safe to go back in.

Step 3: Stay out until someone with a meter clears the house

Do not go back inside until the fire department or the gas utility tells you it is safe. They will walk the house with a CO meter, usually ventilate it, and check the likely sources.

• The alarm going quiet is not an all-clear. Doors left open as you ran out can bring the reading down enough for the alarm to stop. The source is still there.
• Do not press silence and go back to bed. Most alarms let you silence them for a few minutes, and if CO is still present they sound again. That button exists to quiet the noise while you are already leaving, not to end the event.
• Some alarms remember a peak reading. If yours has a display or a peak-level button, tell the responders. It helps them judge how high the level got before the door opened.

Step 4: Leave the suspect equipment off until a licensed pro checks it

Responders may shut off an appliance or the gas supply to it, and sometimes they attach a tag saying it must not be used. Leave it exactly that way. The fix belongs to a licensed heating contractor or plumber, depending on the appliance, and sometimes the gas utility.

• Do not relight a pilot or restart the furnace "to see if it happens again." You would be recreating the condition that set off the alarm, with your family inside.
• Do not remove a tag a responder or the utility left. See related: What the Tags and Stickers a Tech Leaves Behind Mean.
• Ask what they found. Write down the time, the readings they mention, which appliance they suspect and the name of the department or utility. A contractor arriving the next morning works faster with that note.

If they find nothing, the house still gets checked by a pro before you go back to normal. A brief event can come from a vent that was blocked for an hour by snow or wind, and that condition can return the next storm.

Step 5: Get medical care if anyone felt sick

Anyone who had symptoms should be checked by a medical professional, even if they feel better after a while in fresh air. CO poisoning is diagnosed with a blood test or a breath test, and treatment is oxygen, sometimes in a special pressurized chamber for serious cases. Feeling better outside is itself a clue: if headaches that come and go are better when you are away from home and worse when you return, tell your doctor about your fuel-burning equipment.

One alarm, two different nights

The same alarm can call for a different first call depending on one thing: whether anyone is sick. Here are two nights in the same house, so you can see where the path forks and where it does not.

The house: two stories, a gas furnace and a gas water heater in the basement, an attached garage. A CO alarm in the upstairs hallway outside the bedrooms and one in the basement. Two adults, two children, one dog.

Night one, 2:10 a.m. The hallway alarm sounds four beeps, a pause, four beeps. One of the children says she has a headache and feels like throwing up. One parent carries the younger child, the other takes the dog and walks the older one out the front door. At the end of the driveway, at 2:13, they count: four people, one dog. The parent with the phone calls 911 and says, "Our CO alarm is going off and my daughter has a headache and nausea." The fire crew arrives, measures inside, finds an elevated level in the basement and upstairs, ventilates, and shuts off the furnace. An ambulance crew checks the family. The daughter is taken in for a blood test. The furnace stays off with a tag on it until a licensed heating contractor finds the problem the next day.

Night two, a different winter, 9:40 p.m. The basement alarm sounds its alarm pattern. Everyone feels fine. They leave the same way and count heads at the same spot. This time the house records file has the answer to "who do we call when nobody is sick": their fire department's guidance is to call 911 for any CO alarm, so they do, and say, "Our CO alarm is going off, nobody has symptoms." The crew measures, finds a reading near the water heater, and shuts off the gas to it. Nobody goes back in until the crew says so. A plumber licensed for gas work comes the next morning.

What stayed the same both nights: out first, headcount at one spot, the call made from outside, nobody back in until a meter said so, equipment left off until a pro checked it.

What changed: only who was told about symptoms and how urgently the medical side moved. If night two's family had lived somewhere whose guidance was the non-emergency line or the gas utility, the call would have gone there instead, but the rest of the night would have been identical.

Where people go wrong on a night like this: they pull the battery because the alarm is loud and they assume it is faulty, or they open every window and go back to bed once it stops. Both erase the only evidence that something is wrong. If you genuinely think the alarm is malfunctioning, that is still a question for someone with a meter, from outside.

How this differs from a chirp

A single short chirp every 30 to 60 seconds is not an alarm. It usually means a low battery, a fault, or that the alarm has reached the end of its life and needs replacing. That is not an evacuation, but it is not something to ignore either, because an alarm that is chirping may not be able to warn you. The patterns differ between models, and telling them apart belongs to its own card: see related: What Your Carbon Monoxide Alarm Is Telling You. If you cannot tell which one you are hearing, treat it as an alarm and get out.

After the house is cleared

• Have every fuel-burning appliance checked, not just the one that was blamed. A licensed heating contractor checks the furnace or boiler and its vent; a plumber licensed for gas work checks the water heater; a chimney professional checks a fireplace flue.
• Check outside vents after storms. High-efficiency furnaces and water heaters often vent through plastic pipes on a side wall, and snow drifts can cover them. Keeping them clear from the outside is safe to do; anything inside the equipment is not.
• Never run a generator, a grill or a car in the garage or near open windows. See related: Portable Generators and Your House: What Is Safe.
• Replace an alarm that sounded if the maker's instructions say to after a confirmed CO event, and test the rest. See related: How to Test Your Smoke and Carbon Monoxide Alarms.
• Write it down. Date, time, which alarm, who came, what they found, who repaired what. See related: Your House Records File and What Goes In It.

When to call whom

• 911: anyone sick, anyone who will not wake, anyone confused, or you do not know who else to call.
• Fire non-emergency line or the gas utility: only if your local guidance says so for an alarm with no symptoms.
• Licensed heating contractor, plumber or chimney pro: after responders clear the house, before any appliance is used again.

See related: When to Stop and Call a Licensed Pro Around the House.

References

• UL 2034, the standard residential CO alarms are listed to, which sets how quickly an alarm must sound at rising CO levels
• NFPA 72, National Fire Alarm and Signaling Code (CO detection requirements, formerly NFPA 720), in the edition your jurisdiction has adopted
• See related: Carbon Monoxide Testing + Response Reference
• See related: What Your Carbon Monoxide Alarm Is Telling You