Home / Air and breathing

Carbon monoxide detector guide: UL 2034, placement and testing

By the Life in Good Form editors. Updated 2026-10-15.

We research products from makers' specs and health sources; we do not run hands-on tests. Links to Amazon may earn us a commission at no cost to you. How this works.

Where to put CO alarms

From CPSC's CO Alarms page and Carbon Monoxide Fact Sheet, and EPA's indoor air guidance.

What to look for when buying

• UL 2034. The CPSC says to choose an alarm that meets the current UL 2034 safety standard; EPA also lists the IAS 6-96 standard.
• Power. Battery-operated, or plug-in or hardwired with battery backup. CPSC says plug-in and hardwired alarms should have battery backup.
• Interconnection. Interconnected alarms are best; when one sounds, they all sound.
• Mounting. CO alarms may plug into a receptacle or mount high on the wall.
• Combination units. CPSC notes combination smoke and CO alarms are available.
• End-of-life signal. Newer alarms signal when they need to be replaced.

The CPSC also notes many states and local areas require CO alarms in homes, so check your local building code.

Testing and replacing

CPSC recommends testing smoke and CO alarms monthly and replacing batteries every year. Follow the maker's instructions: the CPSC notes the test button checks the circuitry, not the accuracy of the sensor. Alarms have a recommended replacement age listed in the product literature, and newer models have an end-of-life signal. A CO alarm is a backup, not a replacement, for proper installation and upkeep of fuel-burning appliances, the EPA says.

Pick a date you will remember, like the start of each month for testing, so the routine sticks.

Symptoms of CO poisoning

CO is a colorless, odorless gas, so people may not know they are exposed. The CPSC says early symptoms of low to moderate poisoning resemble the flu without a fever:

• headache
• fatigue
• shortness of breath
• nausea
• dizziness

Higher levels can cause mental confusion, vomiting, loss of muscle coordination, loss of consciousness and death. With fast, high-level exposures, such as a generator running indoors, people can become confused and lose muscle control without milder warning symptoms first.

How much CO is dangerous

CO is measured in parts per million (ppm). The CPSC says most people have no symptoms from prolonged exposure at about 1 to 70 ppm, though some heart patients may have more chest pain. As levels stay above 70 ppm, symptoms such as headache, fatigue and nausea become more noticeable, and at sustained levels above 150 to 200 ppm, disorientation, unconsciousness and death are possible. Effects depend on the level, how long the exposure lasts and each person's health. CO alarms are designed to sound before life-threatening levels are reached, and the CPSC says current models are less prone to nuisance alarms than earlier ones.

The CPSC also says CO alarms are available for boats and recreational vehicles and should be used there too.

If the alarm sounds

1. Do not try to find the source.
2. Move outside to fresh air immediately.
3. Call emergency services, the fire department, or 911.
4. Do a head count, and do not go back in until responders say it is safe.

If a malfunctioning appliance is the source, do not use it until trained personnel service it. If you feel CO symptoms, also see a doctor promptly and say you suspect CO poisoning.

Preventing CO in the first place

• Have the heating system, chimneys and flues professionally inspected and serviced every year.
• Never run a portable generator or gas engine tool in or near a garage, house or other enclosed space, even with doors and windows open.
• Never leave a car running in an attached garage.
• Never burn charcoal indoors, or use gas ranges, ovens or dryers to heat your home.
• Do not use unvented fuel-burning appliances in a room where people sleep.

For other home air topics, see our air quality monitor guide, and for smoke protection, our smoke detector guide.

What to look for when you buy

Questions people ask

Where should a carbon monoxide detector be placed?

On every level of the home and outside sleeping areas, per the CPSC; not in kitchens or above fuel appliances.

How often should I test a CO alarm?

Monthly, and replace batteries every year, the CPSC says.

What standard should a CO detector meet?

The current UL 2034 standard, per the CPSC.

What should I do if my CO alarm goes off?

Go outside to fresh air immediately and call 911 or the fire department; do not search for the source.

Do CO detectors expire?

Yes. They have a recommended replacement age, and newer models give an end-of-life signal.

Sources

• Carbon Monoxide Fact Sheet (cpsc.gov), U.S. Consumer Product Safety Commission
• CO Alarms (cpsc.gov), U.S. Consumer Product Safety Commission
• What about carbon monoxide detectors? (epa.gov), U.S. Environmental Protection Agency

Sources checked 2026-10-15.