Carbon monoxide: the warning signs and what to do

You can't see it or smell it. An alarm can.

The answer in 2 lines. Carbon monoxide (CO) is a gas you can't see or smell. It comes from anything that burns fuel: furnaces, water heaters, gas stoves, fireplaces, space heaters, generators, grills and cars. If your CO alarm goes off, or several people at home feel headache, dizziness or nausea, get everyone outside into fresh air right away and call 911 from outside. Prevention: CO alarms on every level, fuel-burning appliances checked every year, and never a generator or grill indoors.

The first 3 steps

Get everyone, pets included, outside into fresh air right away.
Call 911 from outside. For questions, Poison Help is 1-800-222-1222.
Don't go back in until firefighters or a professional say it's safe.

Stop: danger

Generator, grill, camp stove: never inside a home, garage, basement or crawl space, even with doors and windows open.
Car running in an attached garage, even with the garage door open: CO can enter the house.
Gas oven or stove used to heat your home: never.

Warning signs

Headache, dizziness, weakness, nausea or vomiting, chest pain, confusion: CDC says symptoms are often described as "flu-like".
Several people (or pets) at home feel sick at the same time.
You feel better outside and worse when you come back in.
People who are asleep or have been drinking can die before they feel any symptoms.

Where carbon monoxide comes from

CO forms when fuel burns. A faulty, badly vented or misused appliance can fill a home with it.

Gas or oil furnaces, boilers and water heaters.
Fireplaces and wood stoves with blocked chimneys.
Gas stoves and ovens.
Portable generators, grills, camp stoves, and cars or gas tools running in a garage.

Prevention, every day

Install CO alarms on every level of your home and outside sleeping areas. Test them monthly and replace batteries as the maker says.
Have fuel-burning appliances (furnace, water heater, chimney) checked by a qualified technician every year, ideally before the heating season.
Generators: run them only outside, far from the house. CPSC advises at least 20 feet away, with the exhaust pointed away from windows and doors.
Keep vents and chimneys clear of snow, leaves and nests.

What not to do

Unplugging or removing the batteries from a CO alarm that sounds: it's warning you.
Opening a window and staying inside when the alarm sounds: get out.
Blocking vents or air intakes to keep heat in.

Renters and state rules

Many states and cities require CO alarms in homes with fuel-burning appliances or attached garages, and many make the landlord responsible for installing them. The rules vary by state: check with your local fire department. In New York City, for example, HPD explains owners' and tenants' duties for smoke and CO alarms (NYC HPD Fire Safety).

If you rent and the furnace or water heater seems faulty, tell your landlord in writing right away.

Good to know

CDC and CPSC publish CO data regularly; the latest figures published on their sites are for earlier years, not 2026.
CO poisoning rises in winter and after storms, when people run generators and heaters.
A smoke alarm doesn't detect CO unless it's a combination smoke/CO alarm: check the label.

In Philadelphia, Kim and her two kids have headaches every evening since it got cold. One night the new CO alarm sounds. She gets everyone outside and calls 911. Firefighters find high CO levels from the furnace. A technician repairs it before the family goes back in.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on October 18, 2026, for the United States. Confidence: high for the symptoms, the emergency steps and the generator rule; medium for local CO alarm requirements, which vary by state and city. This guide describes common cases, not a diagnosis.