What to Do If You Suspect a Carbon Monoxide Leak

Your carbon monoxide alarm is going off — or multiple people in the house feel dizzy and nauseous at the same time. The instinct to investigate can be strong, but it's the wrong move: CO levels can rise quickly and you cannot judge safety by how you feel. Whether you're dealing with a carbon monoxide leak in an apartment, a house, or a rental, the safest response is the same — get everyone out first, then call for help. This page provides a step-by-step checklist for what to do if you suspect a carbon monoxide (CO) leak, covers apartment-specific steps, and explains the medical red flags that require urgent care.

Quick Safety Summary

• If a CO alarm sounds or you suspect exposure, leave to fresh air first — don't investigate indoors.
• Call emergency services for guidance and air testing before re-entering.
• Severe symptoms (confusion, fainting, chest pain, seizures) require immediate emergency care.
• In apartments, follow emergency steps first — then notify management for shared-system inspection.
• After the incident, fix the source and review prevention steps before resuming normal use.

Step-by-Step Checklist

1. Leave the building and move to fresh air right away.
2. Account for everyone (children, older adults, pets).
3. Call emergency services or your local fire department for advice and air testing.
4. Do not re-enter until responders say it's safe.
5. If it's safe and quick, open doors on your way out (do not delay evacuation).
6. If anyone has severe symptoms (confusion, fainting, seizures, chest pain, trouble breathing), seek emergency care.
7. If exposure happened during sleep or multiple people felt sick, mention this to responders/clinicians.
8. Once cleared, arrange a qualified inspection of fuel-burning appliances (furnace/boiler, water heater, fireplace, stove) and venting/chimney.
9. Do not use suspect appliances until they are inspected and repaired by qualified technicians.
10. Replace/upgrade CO alarms if needed; install alarms on every level and near sleeping areas.
11. Review prevention steps to avoid recurrence (maintenance + safe generator use).

Ventilation: What Helps vs. What Doesn't

A common first instinct when a carbon monoxide alarm goes off is to open windows and stay inside. This is a mistake that can have serious consequences. Here's why: opening windows may temporarily lower CO levels in the room, but it does not stop the source from producing more CO. If a furnace, water heater, or other appliance is continuously generating carbon monoxide, the levels can rise again as soon as ventilation changes — a door closes, wind shifts, or the appliance cycles on again. Meanwhile, you remain in the building exposed to a gas you cannot see or measure without professional equipment.

What actually helps when you suspect a CO leak: leave the building entirely and call for professional help from outside. Responders have calibrated meters that can detect CO levels throughout the building and pinpoint the source. Ventilation can be part of the remediation process — but only after evacuation and only under professional guidance. The key principle for a co leak what to do situation is always the same: people first, air testing second, source repair third.

Another misconception: running fans or turning on the HVAC system will "clear" the CO. In some cases, running the HVAC can actually spread CO from one part of the building to another, or the HVAC system itself may be the source. Do not attempt to troubleshoot appliances while you're still inside — leave that to qualified technicians with the right equipment.

If You Live in an Apartment or Rental

A carbon monoxide leak in an apartment can be more complex than in a house because CO may originate from shared mechanical rooms, boilers, or a neighbor's unit. Your safety steps don't change — but your follow-up should include the building.

• Follow the emergency checklist first — don't wait for the landlord or manager.
• Notify building management/landlord after you are safe; request a qualified inspection of shared equipment and venting.
• If you believe the problem may affect other units, inform neighbors and management (responders may advise broader evacuation).
• Document the incident (alarm photo/time, symptoms, responder report) for follow-up and maintenance tracking.
• If your landlord is unresponsive, contact your local fire department or housing authority for guidance on your rights and next steps.

Medical Red Flags: When to Seek Urgent Care

CO exposure can escalate from mild headache to a medical emergency. Seek urgent care if anyone shows these signs during or after a suspected leak:

• Confusion, inability to stay awake, fainting, or seizures.
• Chest pain, shortness of breath, palpitations, or severe weakness.
• Severe headache with vomiting or worsening neurological symptoms.
• Symptoms in a child, older adult, pregnant person, or someone with heart disease.
• Symptoms that improve outdoors but return indoors (possible ongoing exposure).

When calling emergency services, mention that CO exposure is suspected. This helps responders prioritize air testing and helps clinicians order appropriate blood tests rather than treating symptoms as a routine illness.

After the Incident: Verify the Source and Prevent Recurrence

Once responders have cleared the building, the work isn't over. Identifying and fixing the CO source is essential to prevent another incident.

• Have fuel-burning appliances inspected and serviced; ensure venting/chimney paths are clear.
• If you used a generator, confirm it was outdoors and far from doors/windows; never use it in a garage.
• Check CO alarm placement and age; replace alarms that are expired or unreliable.
• If symptoms persist or new symptoms develop days later, seek medical evaluation (delayed effects can occur).

Before You Go Back Inside

Before returning home after a CO incident, make sure these conditions have been met:

1. A qualified professional has tested the entire home with a calibrated CO meter and confirmed levels are safe.
2. The source of CO has been identified and either repaired, replaced, or shut down.
3. If the source was an appliance, it has been serviced by a licensed technician and confirmed safe to operate.
4. CO alarms are present and working on every level of the home and near sleeping areas.
5. Ventilation paths (vents, flues, chimneys) have been checked and confirmed clear and properly connected.
6. If the incident involved a generator, it has been repositioned outdoors, at least 20 feet from the building, with exhaust directed away from any openings.

Do not accept verbal reassurance without testing. CO levels can be measured quickly with professional equipment, and a responsible technician will confirm readings before clearing you to return. If you have any doubt, ask to see the meter readings yourself. Your safety depends on verifying that the source is fixed — not just that the air seems better for the moment.

Read next: How to detect a CO leak. Common sources of carbon monoxide.

Sources & References

• CDC — Carbon Monoxide Poisoning
• US CPSC — Carbon Monoxide Information Center
• American Red Cross — Carbon Monoxide Prevention
• NHS — Carbon Monoxide Poisoning

Last updated: February 15, 2026