What Is a Carbon Monoxide Alarm? | Life-Saving Gas Detector

A carbon monoxide alarm is a safety device that detects dangerous CO gas and sounds an audible warning before levels become deadly.

Carbon monoxide is the leading cause of accidental poisoning deaths in the United States, and because the gas is completely invisible, odorless, and tasteless, a detector is the only way to know it is accumulating before it reaches dangerous levels. A carbon monoxide alarm continuously samples the air and emits a loud warning — typically 85 decibels or louder — when it detects a hazardous concentration. Here is what you need to know about how these life-saving devices work, where to install them, and how to keep them reliable.

How Do Carbon Monoxide Detectors Work?

CO alarms measure both the concentration of carbon monoxide in the air and how long it stays at that level. The CPSC’s CO alarm guidelines specify time-weighted trigger points that prevent false alarms from short harmless spikes while still catching dangerous buildups. At 70 parts per million (ppm) sustained for one to four hours, the alarm sounds. At 150 ppm it triggers within 10 to 50 minutes, and at 400 ppm it sounds within 4 to 15 minutes. Below 30 ppm, no alarm is triggered.

Three sensor technologies are used in modern detectors. Electrochemical sensors — the most common type — use electrodes immersed in a chemical solution that reacts specifically to CO and generates a current that triggers the alarm. Biomimetic sensors employ a gel disk that darkens as it absorbs carbon monoxide; when the discoloration reaches a set level, the alarm sounds. Metal oxide semiconductor sensors use a heated tin dioxide film — when CO contacts the film, it alters the electrical current flowing across it, which triggers the alarm. All three types are effective, with electrochemical sensors being the most widely adopted in current models.

Where Should You Install CO Alarms?

Place a CO alarm on every level of your home, outside each sleeping area, and inside any bedroom that contains a fuel-burning appliance such as a gas stove, furnace, or water heater. Also install one near doors leading to an attached garage, since vehicle exhaust is a major source of CO. For wall mounting, position the alarm about five feet from the floor and at least six inches from any conjoining wall. For ceiling mounting, keep it at least six inches from any wall. Because CO mixes evenly with air, wall and ceiling placements work equally well. Avoid installing within 10 feet of fuel-burning appliances, in bathrooms, or near vents, fans, and windows where airflow could prevent the sensor from reading accurately.

If you are shopping for a unit, our tested roundup of the best carbon monoxide alarms covers top-rated models for different home setups and budgets.

Maintenance, Testing, and Replacement Schedule

Test your CO alarm monthly by pressing the test button. A working unit will sound four beeps, pause, then four more beeps for roughly five to six seconds. If your alarm uses replaceable batteries, swap them every six months. The entire unit must be replaced every five to seven years — the sensor degrades over time and may not detect CO accurately after that point. Look for the manufacture date on the back or side label; some models also chirp every 30 to 60 seconds when they reach end of life. A chirp at that interval almost always means a low battery or a unit that needs replacement.

Power options include battery-only units, plug-in AC models that must include a battery backup for power outages, and hardwired units connected directly to your home’s electrical system. AC-powered alarms with battery backup remain functional during outages, which matters because CO risks can increase when heating equipment malfunctions during a power loss.

What to Do When the Alarm Sounds

If your CO alarm emits a continuous high-pitched pattern — distinct from the intermittent chirp of a low battery — treat it as a genuine emergency. Evacuate everyone immediately, including pets. Call 911 from outside the home. Do not re-enter until emergency responders say it is safe. A CO alarm is a backup to proper appliance maintenance; regular inspection of fuel-burning equipment is the best prevention against CO buildup.

FAQs

How often should you replace a CO alarm?

Replace the entire unit every five to seven years. The exact lifespan depends on the model and manufacturer; check the label on the back or side for the manufacture date and replacement date. After that point, the sensors degrade and may not detect CO accurately.

Why does my CO alarm keep chirping?

A chirp every 30 to 60 seconds almost always means the battery is low or the unit has reached its end of life. Replace the battery first. If the chirping continues with a fresh battery, the alarm itself needs replacement. Some models also chirp to indicate a sensor malfunction; consult the manual for your specific unit.

Can a CO alarm detect natural gas or smoke?

No. A standard carbon monoxide alarm only detects CO gas — it will not sense natural gas, propane, or smoke. For those threats, you need a separate natural gas detector or a smoke alarm. Combination units that detect both CO and smoke are available and provide multi-threat coverage from a single device.