us Health & medical scares carbon monoxide alarm went off • co detector going off • outside after co alarm • headache after co exposure • nausea after co exposure • feeling sick after heater • possible carbon monoxide poisoning • co exposure symptoms • dizziness after being indoors • sleepy or confused after alarm • furnace carbon monoxide • gas stove carbon monoxide • generator fumes indoors • car exhaust in garage • apartment co alarm • family still inside • pets still inside • need medical evaluation for co • when to call poison control • when to call 911

What to do if…
a carbon monoxide alarm goes off and you are now outside with headache or nausea

Short answer

Assume possible carbon monoxide (CO) exposure: stay outside in fresh air and get urgent medical guidance. Do not go back inside until professionals say it’s safe.

Do not do these things

Do not re-enter to silence/reset the alarm or “check quickly.”
Do not assume it’s a false alarm because you don’t smell anything (CO can’t be smelled).
Do not run fans, the furnace, HVAC, or any fuel-burning appliance.
Do not use a generator, grill, camp stove, or car engine near an open door/window.
Do not drive yourself if you’re dizzy, confused, unusually sleepy, or very unwell.
Do not ignore symptoms that improve outside—CO exposure can look like a “flu” feeling.

What to do now

Stay outside and keep everyone out. Move to a safe spot away from the building (and away from idling vehicles). Keep pets with you if they’re already out.
Call for emergency help if anyone is significantly ill. Call 911 now if anyone has trouble breathing, chest pain, fainting/collapse, seizure, confusion, can’t stay awake, or symptoms are worsening.
If symptoms are mild/moderate (like headache or nausea), get expert guidance immediately.
- Call Poison Control (Poison Help): 1-800-222-1222 and say: “CO alarm went off; we left; now headache/nausea.”
- If you can’t get through quickly or you’re worried, call 911.
Have the home checked before anyone goes back in. From outside, call 911 to request emergency response (in many places this is the fire department), or call your gas utility emergency line if gas appliances are involved. Ask for a carbon monoxide investigation and follow instructions.
Account for everyone who was inside. If someone might still be inside, do not re-enter—call 911 and tell them a CO alarm is sounding and someone may still be inside.
Tell responders who is higher-risk. If anyone exposed is pregnant, an infant/child, older, or has heart/lung disease, say so. These groups may need faster evaluation even with “just a headache.”
If you’re sent for medical evaluation, bring the key details:
- time the alarm went off,
- how long people were inside,
- symptoms (headache, nausea, dizziness, confusion, sleepiness),
- possible sources (furnace, water heater, gas stove, fireplace, generator, car in/near garage). Hospitals can test for CO exposure and provide oxygen if needed.

What can wait

Troubleshooting the detector or deciding whether to replace it (don’t go back in to do this).
Investigating the exact source yourself.
Calling your landlord/HOA/insurance (after the immediate safety/medical steps are underway).
Retrieving belongings unless responders say it’s safe.

Important reassurance

Feeling headache or nausea after a CO alarm is a valid reason to treat this as real. You’ve already taken the most protective action by getting outside; getting prompt medical guidance and a professional safety check is the right next move.

Scope note

This guide covers first steps to reduce harm and get you assessed. Repairs, inspections, and follow-up prevention happen after the space is confirmed safe.

Important note

This is general information, not medical advice. If symptoms are severe, worsening, or you’re unsure, call 911. Do not re-enter the building until it has been evaluated and you’re told it’s safe.

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What to do if…a carbon monoxide alarm goes off and you are now outside with headache or nausea