What CO detectors actually do
Carbon monoxide is colorless, odorless, and mixes freely with air at household concentrations. The only way to know it is present is a detector. The CDC reports more than 400 Americans die from accidental CO poisoning each year, with another 50,000 emergency room visits. Most incidents happen at night, when sleeping occupants are unaware of rising levels until symptoms set in.
A CO alarm contains an electrochemical sensor that measures carbon monoxide in parts per million. When concentrations stay elevated long enough to pose a health risk, the alarm sounds. The CPSC recognizes only one safety standard for residential CO alarms: UL 2034. That standard sets specific concentration-time thresholds. The alarm must trigger at 400 ppm within 4 to 15 minutes, at 150 ppm within 10 to 50 minutes, and at 70 ppm within 60 to 240 minutes. These thresholds reflect how quickly CO affects the body at different exposure levels.
Common household sources include gas furnaces, gas water heaters, fireplaces, wood stoves, vehicles running in an attached garage, and portable generators. A cracked heat exchanger in a gas furnace is one of the most frequent causes of slow CO buildup, and it can produce elevated levels before any other symptom of appliance failure appears.
What makes a good one
UL 2034 certified
This is the only residential CO standard the CPSC recognizes. Every alarm on this page carries it. Do not buy a CO detector without this certification.
Electrochemical sensor
The most accurate and specific sensor type for CO detection. Cheaper metal-oxide sensors drift over time and generate more false alarms. All reputable residential models use electrochemical sensors.
Battery power or battery backup
Plug-in models must have a battery backup. A CO detector that goes dark during a power outage provides no protection, and power outages are exactly when portable generators are most likely running.
Digital ppm display
Not required, but worth it. A display shows current levels and peak memory, so you can tell whether a situation is improving or worsening before the alarm threshold is reached.
Our picks
Kidde KN-COPP-B-LPM
The most widely recommended battery-operated CO detector for household use. The digital display shows current ppm from 11 to 999 and stores peak-level memory so you can see the highest reading since the last reset. The electrochemical sensor is rated for 10 years and the unit carries a 10-year warranty. Wall or tabletop mounting. Three AA batteries are included.
Sensor type
Electrochemical
Power source
3 AA batteries
Display
Digital ppm, peak memory
Sensor life
10 years
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First Alert CO710
The sealed 10-year lithium battery means no battery changes for the life of the unit. When the sensor reaches end of life at 10 years, the whole unit gets replaced. This eliminates the risk of a dead battery going unnoticed. It also satisfies sealed-battery requirements in California, New York, and other states with stricter CO alarm codes. The digital display shows current ppm levels alongside room temperature.
Sensor type
Electrochemical
Power source
Sealed 10-year lithium
Display
Digital ppm, temperature
Sensor life
10 years
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Also worth knowing: combination alarms
Combination smoke and CO alarms
If you are replacing smoke alarms and CO detectors at the same time, a combination unit simplifies installation and reduces wall clutter. Kidde and First Alert both make combination models that meet UL 217 for smoke and UL 2034 for CO simultaneously. Combination units run $40 to $60 each, which is often less than buying separate smoke and CO alarms for the same locations. For homes with existing hardwired smoke alarm systems, a hardwired combination model like the First Alert SC9120B maintains interconnectivity and adds CO protection without running new wire.
Where to put them
The CPSC recommends at minimum one CO alarm in the hallway outside every separate sleeping area. A two-story home needs at least one per floor. If a fuel-burning appliance is located inside a bedroom or its attached bathroom, an additional alarm must be placed inside that room.
Mounting height does not significantly affect CO detection. Carbon monoxide mixes freely with room air, so ceiling or mid-wall placement both work. Follow the manufacturer's installation instructions. The CPSC does not specify a required mounting height, and NFPA 720 placement guidance is consistent with this.
Keep CO alarms away from cooking areas and humid bathrooms. Both produce false alarm conditions. The hallway just outside the sleeping area is the correct placement for most households.
What most households miss
CO alarms have an expiration date
The electrochemical sensor degrades over time. Most units have a 5 to 10 year sensor life. An expired sensor may still appear to function while no longer detecting CO reliably. Check the manufacture date printed on the back of any alarm currently in your home. Replace the entire unit at end of sensor life.
A power outage is when you need it most
Portable generators are the leading cause of CO poisoning deaths during disaster events. They are most often used precisely when the power is out. A plug-in CO alarm with no battery backup is offline at exactly that moment. Both picks on this page run on battery power through any outage.
One alarm does not cover multiple floors
CO from a basement furnace may not reach a second-floor bedroom before occupants absorb a dangerous dose. A single unit on the main floor is better than none, but a multi-story home needs at least one alarm per level.
Testing and replacement
Test every CO alarm once a month using the built-in test button. Some models test only the alarm circuitry. Others test the sensor itself. The user manual specifies which. If the unit supports sensor testing, run that function quarterly.
Replace the entire unit at end of sensor life. The manufacture date is printed on the back of the alarm. If the date is missing or the unit is older than 10 years, replace it. There is no reliable way to recalibrate or extend the life of an expired electrochemical CO sensor.
NWS recommendation
Start with two Kidde KN-COPP-B-LPM units. One in the hallway outside the primary sleeping area, one on each additional floor. The battery operation keeps them running through power outages and the digital display lets you verify levels before an alarm is warranted. If your state requires sealed-battery alarms, substitute the First Alert CO710 at each location.
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