If you get a headache, feel dizzy or feel sick while a ventless fireplace is running, treat it as possible carbon monoxide poisoning until a professional proves otherwise. The CDC lists headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion as the most common symptoms of CO poisoning. The U.S. Consumer Product Safety Commission’s guidance is to get outside to fresh air immediately and call the fire department from outside the home. Turn the unit off only if you can do it on your way out, and do not relight it until a qualified technician has checked it.
First: When It Is an Emergency
This section comes first because the order matters. Read the rest later.
Call 911 now if anyone is unconscious, not breathing, having seizures, confused, or cannot be woken. That is the line from Poison Control’s carbon monoxide guidance (National Capital Poison Center, read September 27, 2026): “If someone is unconscious, not breathing, or having seizures, call 911 immediately.”
For everyone else with symptoms, the CPSC’s Carbon Monoxide Questions and Answers page (read September 27, 2026) says: “get outside to fresh air immediately. Leave the home and call your fire department to report your symptoms from a neighbor’s home.” Poison Control also takes calls at 1-800-222-1222 and offers an online tool.
What that means in practice:
- Get everyone out, including pets.
- Turn the fireplace off only if the control is on your way out. Do not go back in for it.
- Call the fire department or 911 from outside.
- Do not go back inside to “air the room out” or check the alarm. Opening windows and staying in the room is not the same as leaving.
- Tell the responders and any clinician that a vent-free gas appliance was running.
Anyone who had symptoms should get medical advice, even if they feel better outside. Feeling better outside is not a sign it was nothing; it is one of the signs that it was CO.
Why a Ventless Fireplace Can Cause These Symptoms
A vent-free fireplace has no chimney. It takes its combustion air from your room and puts its exhaust back into your room. When everything is working as designed, the burn is meant to be clean. When something is off, such as a partly blocked burner, logs out of place, the wrong gas pressure or a unit run longer than its manual allows, more carbon monoxide ends up in the air you are breathing.
The CPSC describes carbon monoxide as “a deadly, colorless, odorless, poisonous gas” produced by the incomplete burning of fuels including natural gas and propane. You cannot see it or smell it. Any smell you notice is something else, and the absence of a smell tells you nothing. Our explainer on how a ventless fireplace produces carbon monoxide goes through the mechanism in detail.
The Ventless Fireplace Headache and Dizzy Pattern to Watch For
The hard part is that CO symptoms look like ordinary illness. The CDC’s carbon monoxide page (last reviewed January 12, 2026; read September 27, 2026) notes that “CO symptoms are often described as ‘flu-like.'” The pattern is what gives it away, and three published sources describe the same pattern.
| What you notice | What the source says | Source |
|---|---|---|
| Symptoms ease when you go outside or leave the house | Suspect CO when “headaches, nausea, and/or flu-like symptoms clear up when you go outdoors” | Poison Control |
| Symptoms get worse in one room and better elsewhere | Symptoms “may get worse when you spend time in an affected room or building and get better when you leave or go outside” | NHS (page reviewed December 16, 2025) |
| Several people feel ill at the same time | Suspect CO when “several people develop symptoms of headache, nausea, and fatigue or drowsiness at the same time” | Poison Control |
| Children or pets seem affected first | “Children and pets are often affected first” | Poison Control |
| Symptoms come and go | “The symptoms may come and go” | NHS |
If your headache arrives in the evening when the fire is on and is gone by the next morning at work, that is exactly the pattern these sources describe. It is worth taking seriously even if it has happened many times without getting worse.
Who is at greater risk
The CDC states that “infants, the elderly, and people with chronic heart disease, anemia, or breathing problems are more likely to get sick from CO.” It also warns that “people who are sleeping, drunk, or under the influence of other substances can die from CO poisoning before they have symptoms.” That second line is the reason a vent-free unit should never be left running while anyone in the room is asleep. Our guide to who should not use a ventless fireplace covers which households face the most risk.
Why the Oxygen Sensor Did Not Stop It
Owners often say the unit has a safety sensor, so it cannot be the fireplace. The oxygen depletion sensor (ODS) shuts the unit off when the oxygen level in the room falls too far. It does not measure carbon monoxide. A unit can produce more CO than it should because of a burner or log problem while oxygen in the room is still above the shutoff point, and the ODS will not react. Our explainer on what an oxygen depletion sensor does not catch covers this in full.
The CO alarm is the device built to warn you about carbon monoxide. It is a condition of using a vent-free unit, not an accessory. If you do not have one, do not relight the fireplace until you do. And if people have symptoms, do not treat a silent alarm as the all-clear: follow the emergency steps above either way, and have the alarm itself checked.
After Everyone Is Safe: What Happens to the Fireplace
Once the emergency is over and a clinician has seen anyone who had symptoms, the fireplace stays off until it has been inspected. This is not a job to diagnose yourself.
- Leave it off. Do not relight it “to see if it happens again.” Testing your own body against a gas appliance is not a test.
- Book a qualified technician who services vent-free units, or a licensed gas fitter. Tell them about the symptoms, when they happened, and how long the unit had been running.
- Ask for a CO reading with the unit running. The U.S. Department of Energy’s Building America guidance on unvented appliances describes testing the air near a running ventless fireplace with a hand-held CO monitor, and treats a result over 35 ppm as a failure that requires service and a retest or replacement.
- Ask what they found, in writing: burner condition, log placement, gas pressure, pilot and ODS condition, and any soot.
- Have your CO alarm checked or replaced if it did not sound, and ask the technician about where it is placed.
Anything involving the gas supply, the burner, the pilot or the controls is licensed work. Do not open the unit, clean the pilot or adjust the air shutter yourself.
What if the technician finds nothing?
Other things cause headaches, and a clean inspection is useful information. But if the pattern returns when the fire is on, the fireplace is still the first suspect, and it goes off again. Tell your doctor that a vent-free gas appliance is in the home. Many households in this position decide the unit is not worth the doubt, and our guide to ventless gas vs electric fireplaces sets out what changes if you switch.
What Not to Do
These are the responses that feel sensible and are not:
- Cracking a window and staying. A window may change the air in the room. It does not make it safe to stay in a room where people have CO symptoms.
- Waiting to see if it passes. Symptoms easing when the fire is off is a warning sign, not reassurance.
- Going back in to switch the unit off after you are outside. Leave that to the fire department.
- Taking pain relief and carrying on with the fire running. That masks the warning, not the cause.
- Bypassing or adjusting the ODS or controls. That removes a safety device from an appliance already causing problems.
Frequently Asked Questions
Can a ventless fireplace give you a headache?
It can. A ventless fireplace releases its exhaust into the room, and a fault can raise carbon monoxide. The CDC lists headache and dizziness among the most common CO symptoms. Treat a headache while it runs as possible CO until a technician has checked the unit.
What should I do if I feel dizzy near my ventless fireplace?
Follow the CPSC’s guidance: get outside to fresh air immediately and call the fire department from outside the home. Call 911 at once if anyone is unconscious, confused or having trouble breathing. Get medical advice afterward.
My CO alarm did not go off. Can it still be carbon monoxide?
Do not rule it out on that basis. Follow the emergency steps if people have symptoms, then have both the fireplace and the alarm checked. The oxygen sensor on the unit does not detect carbon monoxide.
When can I use the fireplace again?
Only after a qualified technician has inspected it, found the cause and confirmed it is working as designed, with a working CO alarm in place. If symptoms return when it runs, turn it off again and speak to your doctor.
This guide is based on CDC, CPSC, Poison Control (National Capital Poison Center), NHS and U.S. Department of Energy Building America guidance read on September 27, 2026. It is general safety information, not medical advice. If you or anyone in your home has symptoms, contact emergency services or a clinician.