If a carbon monoxide alarm is sounding right now, stop reading and act:
- Get every person and pet outside into fresh air immediately.
- Call 911 from outside, not from inside the building.
- Do not go back in — for a phone, a pet, a bag, or to look for the cause — until responders say the building is safe.
Everything below assumes you have already done that, or that you are reading this before an alarm ever sounds, which is the right time.
Why the instruction is “leave,” not “investigate”
CO is invisible and odorless, so your senses cannot tell you whether the concentration is falling or rising. Symptoms are unreliable in the other direction too: CDC describes them as flu-like — headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. (Source: CDC, Carbon Monoxide Poisoning Basics.)
Confusion is the detail that makes self-assessment dangerous. The exposure impairs the judgment you would need to decide whether to leave. People have died while deciding whether the alarm was real.
An alarm sounding means the assumption “there is no CO here” has already been disproved. The only reliable move is distance.
The response, step by step
1. Move to fresh air. Outside is best. Everyone: children, older adults, anyone who sleeps deeply, anyone with mobility limits, and pets. Assign responsibility for people who cannot self-evacuate before the event, not during it.
2. Account for everyone. From outside. If someone is missing, tell responders — do not re-enter to search.
3. Call 911 from outside. Report a sounding CO alarm and whether anyone has symptoms. Guidance across CPSC and fire services is consistent: move to fresh air and call emergency services. (Source: CPSC, Carbon Monoxide Information Center.)
4. If you can do it safely on the way out, turn off the obvious source. If a generator, grill, or fuel-burning heater is running and switching it off is on your path to the door, do it. If it is not, leave anyway. Never move deeper into a building to reach an appliance.
5. Do not silence the alarm and stay. Silencing removes the warning without removing the gas.
6. Wait outside. Let responders measure the air with instruments and tell you what is safe.
Symptoms: who to worry about first
Suspect CO whenever symptoms appear in a pattern rather than in a person:
- Multiple people or pets feeling ill at the same time is a major warning sign.
- Symptoms that improve when you leave the building and return when you go back point strongly at the building.
- Pets often show effects first, because of their size and where they spend time.
Some people are affected sooner or more severely, including infants and young children, older adults, pregnant people, and anyone with heart or lung conditions. (Source: CDC clinical guidance on CO poisoning.) Anyone with symptoms needs medical evaluation — tell the clinician you suspect carbon monoxide, because that changes what they look for.
Never use “we feel fine” to decide the air is safe. Absence of symptoms is not a measurement.
Who is allowed to say the building is safe
Only the fire department, emergency responders, or the utility. Not you, not a neighbor, not a landlord over the phone, and not a consumer CO alarm that has stopped sounding.
An alarm that goes quiet after ventilation tells you the concentration at that sensor fell below its threshold. It does not tell you the source is off, the venting is fixed, or the level will not climb again once windows close. Re-entry is a decision made with instruments by people trained to use them.
The same applies after the event. Do not resume using the suspected appliance until a qualified professional has inspected it. The fuel-burning heater limits warning explains why “it seemed fine before” is not evidence.
Before you go back to normal
Once responders have cleared the building:
- Find and fix the source. A CO alarm is not a nuisance to be reset. Fuel appliance service, venting, chimney, and flue work belong to qualified professionals.
- Do not re-run the equipment that caused it. Especially a generator. The generator CO warning covers placement, and no placement fixes a machine that has already proved it can reach your living space.
- Check the alarm itself. If it activated correctly, it did its job. If it is past its service life, replace it — see the testing and replacement schedule.
- Re-check placement. If the alarm was not audible from the bedrooms, the response you just executed was luck. The placement guide fixes that.
- Write down what happened. Date, time, which alarm, who had symptoms, what equipment was running, what responders found, and what the professional did. That record matters for medical follow-up, for the landlord, and for insurance.
Practise the response before you need it
The confusion symptom is exactly why this must be rehearsed rather than reasoned out in the moment.
Decide in advance: who wakes whom, who takes the children, who takes pets, who calls 911, where everyone meets outside, and who cannot self-evacuate. Households that plan around a fire escape route already have most of this — use the same plan. The family communication plan covers accounting for people when phones are unreliable.
Rehearse it in the same breath as the fire drill. It costs ten minutes a year.
Alarm-response quick card
| Situation | Action |
|---|---|
| Alarm sounds, nobody has symptoms | Everyone out, call 911 from outside, wait for responders |
| Alarm sounds, someone has symptoms | Everyone out, call 911 immediately, report symptoms, seek medical evaluation |
| Symptoms but no alarm | Leave, get fresh air, call 911 or a clinician — a missing or expired alarm is not evidence of safe air |
| Alarm stops after you open windows | Still leave; a quiet alarm is not a cleared building |
| Alarm chirps intermittently | Likely low battery or end-of-life — check the manual; do not assume, and never remove the battery and stop there |
Print it, keep it with the household emergency records, and make sure the person most likely to be alone at home has read it.
What this page will never say
It will not tell you a safe concentration to tolerate, because a consumer alarm is not an instrument for making that judgment. It will not give you a threshold at which to stay. It will not tell you a portable detector clears a room. And it does not replace 911, a clinician, or the responders standing in your driveway with a meter.
Return to the carbon monoxide and combustion boundary for prevention, and the extreme cold and emergency heating hub for the cold-weather decisions that create most of these events.
Sources reviewed
ReadyLience reviewed current carbon-monoxide information and alarm guidance from CPSC, CO poisoning basics and clinical guidance from CDC, and heating-safety guidance from the U.S. Fire Administration. Symptom lists are reproduced as published warning signs, never as a diagnostic tool. This page states no safe exposure level and no self-clearance procedure, because only emergency responders or the utility can determine that a building is safe to re-enter. Clinicians control medical evaluation; qualified fuel, HVAC, chimney, and electrical professionals control source repair. The page remains in expert review.