CO Detector vs CO2 Monitor: What You Actually Need (2026 Guide)
Do you need both a CO detector and a CO2 monitor? Yes, if you have gas appliances, a wood stove, or an attached garage, because carbon monoxide is the immediate safety concern and a UL 2034 CO alarm is the only device that wakes you up at 3 a.m. when something goes wrong. A CO2 monitor is the secondary tool. It tells you whether your office or bedroom is getting enough outdoor air, but it will not save your life.
Last updated: 2026-10-06 · By the CleanAirHomeLab team
Affiliate disclosure: Some product links below earn us a small commission at no cost to you. Our recommendations are based on EPA, CDC, CPSC, NFPA, and ASHRAE primary sources, not on commission rates. We are not your doctor or your gas company. If your CO alarm sounds, leave the home and call 911 or your gas utility.
Safety First: CO and CO2 Are NOT the Same Gas
A CO2 monitor will NOT alert you to deadly carbon monoxide. If you have any combustion source in the home (gas furnace, gas water heater, gas stove, wood stove, fireplace, or attached garage), you need a UL 2034 listed CO alarm. Period. The CO2 monitor is for ventilation, not safety. The CDC says more than 400 Americans die each year from unintentional CO poisoning not linked to fires.
Quick Answer: Which Device Do You Need?
- CO alarm (UL 2034): Required if you have any gas appliance, wood stove, fireplace, or attached garage.
- CO2 monitor (NDIR): Optional. Useful for ventilation tuning in offices, bedrooms, and classrooms.
- CO2 and ventilation: ASHRAE says a reading under 1,000 ppm is at best a sign of how much outdoor air each person gets. Its Standard 62.1 sets no CO2 limit. If the number keeps climbing, open a window or run HVAC fresh-air mode.
- Placement: CO alarm on every level and outside sleeping areas (CPSC). CO2 monitor near where you sit or sleep.
- Replace: Follow the maker's instructions, or every 5 years (CDC). Check the date stamp.
Install a UL 2034 CO alarm outside every sleeping area. The CPSC says to put one on every level of the home too.
CO vs CO2: The Physics, Side by Side
The two gases share two letters and little else. Carbon monoxide is one carbon atom bonded to one oxygen atom. The CPSC calls it a deadly, colorless, odorless, poisonous gas. It is made by the incomplete burning of fuels. Carbon dioxide is one carbon atom bonded to two oxygen atoms. You breathe it out with every breath. It builds up indoors when a room does not get enough outdoor air. Here is the comparison.
| Property | Carbon Monoxide (CO) | Carbon Dioxide (CO2) |
|---|---|---|
| Source indoors | The EPA lists unvented kerosene and gas space heaters, leaking chimneys and furnaces, back-drafting from furnaces, gas water heaters, wood stoves and fireplaces, gas stoves, generators and other gas-powered tools, car exhaust from attached garages, and tobacco smoke | People breathing out. ASHRAE calls it occupant-generated CO2. It builds up when a room gets too little outdoor air |
| Detection method | Electrochemical sensor (UL 2034 listed alarms) | NDIR (non-dispersive infrared) optical sensor |
| Reference levels | None of the sources we read sets a limit for homes. EPA outdoor air standard: 9 ppm over 8 hours and 35 ppm over 1 hour. OSHA workplace limit: 50 ppm over 8 hours. | ASHRAE Standard 62.1 sets no indoor CO2 limit. OSHA workplace limit: 5,000 ppm over 8 hours. |
| Dangerous concentration | CPSC: symptoms become more noticeable above 70 ppm. At sustained levels above 150 to 200 ppm, disorientation, unconsciousness and death are possible. NIOSH IDLH: 1,200 ppm. | NIOSH short-term limit: 30,000 ppm. NIOSH IDLH: 40,000 ppm. |
Figures come from the EPA National Ambient Air Quality Standards (outdoor air), OSHA 29 CFR 1910.1000 Table Z-1 (workplaces), the NIOSH Pocket Guide, the CPSC, and the ASHRAE position document on indoor carbon dioxide. None of these is a limit written for homes.
Why People Confuse CO and CO2
The names look almost identical and both are invisible and odorless. Marketing language muddies it further. Some smart home gadgets get sold as air quality monitors with a CO2 chart on the app, and shoppers assume the same gadget will warn them about a furnace problem. It will not. The CO2 sensor inside an Aranet4 or Airthings Wave Plus cannot read carbon monoxide. The two molecules absorb infrared light at different wavelengths, and the sensor is tuned to one wavelength only.
The other source of confusion is the symptom overlap. The CDC lists headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion as the most common symptoms of CO poisoning. NIOSH lists headache and dizziness among the symptoms of high CO2 exposure too. The difference is the danger. The CPSC says that at sustained CO levels above 150 to 200 ppm, disorientation, unconsciousness, and death are possible. Any time multiple people in the same home feel headaches and nausea at the same time, the first move should be to leave the house and call 911 or the gas utility.
One more thing to clear up: a CO alarm and a smoke alarm are not the same either. A smoke alarm warns you when there is smoke from a fire. A CO alarm warns you when there is a dangerous level of carbon monoxide. The Red Cross says to have both. Combination units exist and those count as both. A plain smoke detector by itself will not catch a CO leak from a cracked heat exchanger or a backdrafting water heater.
2 CO2 Monitors Compared
The table lists two CO2 monitors. The Aranet4 Home uses an NDIR sensor. The Airthings Wave Plus adds radon and TVOC tracking to CO2, which works for whole-home indoor air. Specs come from each maker's own page. We do not name a CO alarm model. The one we listed before is discontinued, so pick any alarm listed to UL 2034.
| Model | Type | Sensor | Display | Battery | Price | Best For | Link |
|---|---|---|---|---|---|---|---|
| Aranet4 Home | CO2 monitor | NDIR | E-Ink display + phone app | Up to 4 years (maker figure) | Check current price | Office, bedroom, classroom ventilation tuning | See on Amazon |
| Airthings Wave Plus | Multi-gas (radon + CO2 + TVOC + humidity + temp + air pressure) | NDIR (CO2) + alpha spectrometry (radon) | App + glow ring | 2 AA, 16 months on Bluetooth (maker figure) | $249.99 list price | Whole-home indoor air quality dashboard | See on Amazon |
The CPSC says to install a CO alarm that meets the current UL 2034 safety standard, so check the package for that mark. CO2 monitors are not life-safety devices.
6 Real Scenarios: Which Device(s) Do You Need?
Pick the row that matches your home. The recommendation column gives you the short answer.
| Your Situation | Recommendation |
|---|---|
| I have gas appliances or an attached garage | CO alarm mandatory. UL 2034 listed, outside every sleeping area. CO2 monitor optional. |
| I work from home in a small office (under 100 sq ft) | CO2 monitor useful for ventilation tuning. CO alarm only required if you have gas appliances or share a wall with an attached garage. |
| I just moved into an old house | Get both. CO alarm for any legacy combustion appliances and the chimney. CO2 monitor to baseline how well the home actually ventilates. |
| I have a wood stove or fireplace | CO alarm mandatory. Put alarms on every level and outside sleeping areas. The CDC says to have your chimney checked or cleaned every year. |
| I run a gas generator during power outages | CO alarm mandatory. CPSC: never use a portable generator inside a home or garage, even if doors and windows are open. Use it outside only, at least 20 feet from the home, with the exhaust facing away. |
| I am tracking allergens, dust, or VOCs primarily | Skip the standalone CO2 monitor. Get a full air quality monitor. See our picks at the best AQI monitor page. |
CO Alarm Placement in Plain English
NFPA 720 used to be the standard for installing CO alarms. NFPA withdrew it in 2018 and moved its requirements into NFPA 72. The short version from the CPSC: install CO alarms on every level of the home and outside sleeping areas.
- Outside every sleeping area: The CPSC says outside sleeping areas. The CDC says near every sleeping area. If you get only one detector, the EPA says to place it near the sleeping areas and make certain the alarm is loud enough to wake you up.
- Every level: Basement, main floor, and any second story. The EPA says each floor of the home needs a separate detector.
- Mounting height: Follow the installation instructions in the maker's booklet. The EPA says it is important to follow them exactly.
- Kitchens and fuel-burning appliances: The CPSC does not recommend installing CO alarms in kitchens or above fuel-burning appliances.
- Test it, and replace it on time: Test the alarm the way the maker's booklet says. For a battery unit, the EPA says to test weekly and replace the battery at least once a year. Replace the alarm following the maker's instructions or every 5 years (CDC).
The National Conference of State Legislatures says most states have adopted requirements for carbon monoxide detectors. The rules differ from state to state, so check yours. Even if your state does not require a CO alarm, install one if you have any fuel-burning appliance or an attached garage.
When a CO2 Monitor Is Worth Having
A CO2 monitor is a ventilation gauge. It tells you whether the air in the room is being refreshed fast enough to clear what people exhale. ASHRAE says indoor CO2 is not an overall measure of air quality, but it can be a useful tool if you know its limits. It can help in the situations below.
Work-from-home office
CO2 climbs in a small office with the door closed. ASHRAE says some studies found lower scores on thinking tasks at about 1,000 ppm, while other studies found no effect, so the evidence is inconsistent. A rising number still tells you the room needs more outdoor air. Open the door, crack a window, or run the HVAC fresh-air mode. A CO2 monitor on your desk tells you when to act.
Bedroom with the door closed
CO2 builds up overnight in a closed bedroom. ASHRAE says the evidence for direct effects of CO2 on sleep at common indoor levels is inconsistent. A monitor on the nightstand shows whether opening the door or a window brings the number down in your room.
Classroom or daycare
ASHRAE says the gap between indoor and outdoor CO2 can be used to judge outdoor air ventilation, if the measurement is done with care. A CO2 monitor can help show which rooms need more outdoor air.
Tight new-construction home
A tight home has less natural air exchange. A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) brings fresh air in without losing as much conditioned air. A CO2 monitor can show whether the rooms you use are getting enough of it.
Wine cellar or grow room
Fermentation gives off CO2, and it can build up in a small sealed room. NIOSH lists asphyxia among the effects of very high CO2 exposure and sets its Immediately Dangerous to Life or Health level at 40,000 ppm. A CO2 monitor with a high-level alarm gives you warning before you walk in.
Generators, Wood Stoves, and Fireplaces
The CPSC says more than 200 people in the United States die every year from accidental CO poisoning from consumer products that is not fire related. More than 100 of those deaths are linked to portable generators. The Red Cross says CO poisonings often increase during power outages, when people depend on generators and grills.
For generators, CPSC guidance is firm. Never use a portable generator inside a home or garage, even if doors and windows are open. Use it outside only, at least 20 feet away from the home, with the exhaust facing away. The CDC says more than 20 feet from any window, door, or vent. Some portable generators have a CO sensor that shuts the engine off. A voluntary standard, PGMA G300-2018, calls for the shutoff before CO measured just above the generator passes 400 ppm as a 10-minute average or 800 ppm at any moment. A second standard, UL 2201, sets those levels at 150 ppm and 400 ppm. In a 2023 notice, the CPSC estimated that about 30 percent of portable generators met the PGMA G300 sensor and shutoff rules as of 2022. The CPSC also said a shutoff system alone is not enough to protect people. A shutoff never makes it safe to run a generator indoors.
For wood stoves and fireplaces, the EPA lists leaking chimneys and back-drafting from wood stoves and fireplaces among the sources of CO indoors. The combustion gases that should go up the chimney instead come back into the room. A CO alarm will warn you. The CDC says to have your chimney checked or cleaned every year. The Red Cross says to have all heating systems, including chimneys and vents, inspected and serviced every year by a professional.
You should also test for radon. The EPA says all homes should be tested. We cover the math at our radon mitigation cost guide and the test-kit picks at our best radon detectors page. Radon is the long-term cancer risk. CO is the acute poisoning risk. Both can coexist in the same basement.
What to Do If Your CO Alarm Goes Off
A carbon monoxide alarm makes four fast beeps when there is a dangerous level of CO, the Red Cross says. The CPSC says never to ignore a sounding CO alarm. Every step below comes from the Red Cross or the CPSC.
- Get everyone out of the home right away. Pets too. (Red Cross)
- Call 911 once you are outside. (Red Cross)
- Count heads to check that everyone is out. (CPSC)
- Do not go back inside until emergency responders say it is safe. (Red Cross and CPSC)
- If a broken appliance caused the CO, do not use it again until trained people have serviced it. (CPSC)
A single chirp now and then is a different signal from the four-beep alarm. Check the maker's booklet for what it means on your unit. The Red Cross says sensors and batteries expire over time and become less effective, so write down the expiration date on your alarm. If it is past the replacement date the maker gives, replace the whole alarm rather than just the battery.
Common Questions
Does a smoke detector detect CO?+
No. A standard smoke detector reacts to smoke particles or heat, not to carbon monoxide. CO is invisible and odorless, and the sensor inside a smoke alarm cannot pick it up. You need a separate CO alarm. The CPSC says to install one that meets the current UL 2034 safety standard. Some combo units include both a smoke sensor and a CO sensor in one housing, and those are fine, but a plain smoke detector by itself will not alert you to a CO leak. The CDC says more than 400 Americans die each year from unintentional CO poisoning not linked to fires.
Does a CO2 monitor replace a CO alarm?+
No. CO and CO2 are different gases with different sensors. A CO2 monitor uses an NDIR optical sensor tuned to carbon dioxide. A CO alarm uses an electrochemical sensor tuned to carbon monoxide, which is a poisonous gas. The two sensors do not cross-read. If you have any combustion appliance in the home, you need a UL 2034 listed CO alarm. A CO2 monitor is a ventilation tool, not a safety device.
What CO ppm is dangerous?+
The CPSC says most people will not have symptoms from prolonged exposure to CO at about 1 to 70 ppm, though some heart patients might have more chest pain. As levels rise and stay above 70 ppm, symptoms become more noticeable and can include headache, fatigue and nausea. At sustained levels above 150 to 200 ppm, the CPSC says disorientation, unconsciousness and death are possible. For workers, the OSHA limit is 50 ppm as an 8-hour average, and NIOSH lists 1,200 ppm as Immediately Dangerous to Life or Health. The EPA outdoor air standard for CO is 9 ppm over 8 hours and 35 ppm over 1 hour. That is a standard for outdoor air, not a limit for homes. If your CO alarm sounds, go outside to fresh air and call 911.
What CO2 ppm is concerning?+
NOAA measured a monthly average of about 428 ppm of CO2 in outdoor air at Mauna Loa in August 2026. ASHRAE says its Standard 62.1 does not set a limit for indoor CO2. A 1,000 ppm figure was in the 1989 edition and was later removed. ASHRAE says a level below 1,000 ppm is at best a sign of how much outdoor air each person is getting. It also says the evidence that CO2 itself affects health, sleep or work at common indoor levels is inconsistent. For workplaces, the OSHA limit is 5,000 ppm as an 8-hour average. NIOSH lists 40,000 ppm as Immediately Dangerous to Life or Health. A CO2 monitor is a ventilation gauge.
How often should I replace a CO alarm?+
Follow the maker's instructions. The CDC says to replace a CO detector following the manufacturer's instructions or every 5 years. The sensor wears out over time. Check the date on the back of the alarm. If it has no date and you do not know how old it is, replace it.
Where should I install a CO alarm?+
The CPSC says to install CO alarms on every level of the home and outside sleeping areas. The CDC says near every sleeping area. The EPA says each floor of the home needs its own detector. The CPSC does not recommend installing CO alarms in kitchens or above fuel-burning appliances. For mounting height and spacing, follow the maker's instructions. NFPA 720 used to hold the installation rules for CO alarms. NFPA withdrew it in 2018 and moved its requirements into NFPA 72.
Where should I put a CO2 monitor?+
Near where you sit or sleep. The whole point of a CO2 monitor is to measure the air you actually breathe, not the air at the ceiling or behind a vent. Keep it away from open windows, exterior doors, and HVAC supply vents because those skew the reading. Do not exhale directly on the sensor. For office work, put it within arms reach of your chair. For sleep tracking, put it on the nightstand. Follow the maker's placement instructions. The Aranet4 and Airthings Wave Plus are battery powered so you can place them anywhere.
Do I need both a CO alarm and a CO2 monitor?+
You need a CO alarm if you have any combustion source: gas furnace, gas water heater, gas range, wood stove, fireplace, attached garage, or you use a portable generator near the home. The CO2 monitor is optional. It helps if you work from home in a small office, want to tune your bedroom ventilation, or are tracking indoor air quality alongside our top picks at the best air quality monitor page. CO comes first, CO2 second.
References and Primary Sources
- CDC, Carbon Monoxide Poisoning Basics, read 2026-10-05. Each year more than 400 Americans die from unintentional CO poisoning not linked to fires, more than 100,000 visit an emergency department and more than 14,000 are hospitalized. Install CO detectors near every sleeping area. Replace them following the maker's instructions or every 5 years. Use generators outside, more than 20 feet from any window, door or vent. Have your chimney checked or cleaned every year.
- US CPSC, Carbon Monoxide Information Center, read 2026-10-05. More than 200 deaths a year from consumer products, more than 100 of them linked to portable generators. Generators outside only, at least 20 feet from homes. CO alarms on every level and outside sleeping areas.
- US CPSC, Carbon Monoxide Questions and Answers, read 2026-10-05. Symptoms by CO level (70 ppm, 150 to 200 ppm), the UL 2034 advice, no alarms in kitchens or above fuel-burning appliances. What to do when the alarm sounds, re-read 2026-10-06: move outside to fresh air, call 911, do a head count, do not go back in until responders say so, and do not run a faulty appliance until trained people have serviced it.
- US CPSC, Safety Standard for Portable Generators, Federal Register, April 20, 2023, read 2026-10-05. PGMA G300-2018 and UL 2201 shutoff levels and the estimate that about 30 percent of generators met PGMA G300 as of 2022.
- US EPA, National Ambient Air Quality Standards table, read 2026-10-05. Outdoor air standard for CO: 9 ppm over 8 hours and 35 ppm over 1 hour. This is not a home limit.
- US EPA, Where should I place a carbon monoxide detector?, read 2026-10-05. Each floor needs a separate detector. Follow the maker's instructions. Test battery units weekly.
- US EPA, Carbon Monoxide's Impact on Indoor Air Quality, read 2026-10-05 and again 2026-10-06. Its list of indoor CO sources, used in the table above.
- OSHA, 29 CFR 1910.1000 Table Z-1, read 2026-10-05. Workplace limits: 50 ppm for carbon monoxide and 5,000 ppm for carbon dioxide.
- NIOSH, Pocket Guide to Chemical Hazards: Carbon Monoxide, read 2026-10-05. 1,200 ppm Immediately Dangerous to Life or Health.
- NIOSH, Pocket Guide to Chemical Hazards: Carbon Dioxide, read 2026-10-05. 30,000 ppm short-term limit and 40,000 ppm Immediately Dangerous to Life or Health.
- ASHRAE, Position Document on Indoor Carbon Dioxide (approved February 12, 2025), read 2026-10-05 and again 2026-10-06. Standard 62.1 does not set a limit for indoor CO2. The evidence for direct effects of CO2 at common indoor levels is inconsistent. It calls indoor CO2 occupant-generated.
- NOAA Global Monitoring Laboratory, Trends in Atmospheric Carbon Dioxide, read 2026-10-05. Mauna Loa monthly mean for August 2026: 427.55 ppm.
- NFPA, NFPA 720 Standard for the Installation of Carbon Monoxide (CO) Detection and Warning Equipment, read 2026-10-05. Withdrawn in 2018. Its requirements were incorporated into NFPA 72.
- National Conference of State Legislatures, Carbon Monoxide Detector Installation Statutes, read 2026-10-05. Most states have adopted requirements for CO detectors.
- UL, UL 2034 Standard for Single and Multiple Station Carbon Monoxide Alarms, read 2026-10-05. The standard the CPSC says a CO alarm should meet.
- American Red Cross, Carbon Monoxide Poisoning Safety, read 2026-10-05 and again 2026-10-06. Get everyone out, pets too; call 911 once outside; do not go back in until responders say it is safe; the four-beep alarm sound; yearly inspection of heating systems.
- Maker pages, read 2026-10-05: Aranet4 Home and Airthings Wave Plus. Product specs in the comparison table.
Sources and last review
Specifications on this page are taken from the documents below, on the dates shown. We do not physically test products; read our methodology. Page last reviewed .
This list covers the maker list price in the comparison table. Every other figure on this page is cited in the References section above.
- Airthings (Wave Plus product page, re-read) (manufacturer), retrieved 2026-10-06
Ready to Get the Right Device?
Install a UL 2034 CO alarm outside every sleeping area so that the household wakes before exposure passes the toxic threshold. Add a CO2 monitor only if you want to tune ventilation in an office or bedroom.
Affiliate Disclosure: Some product links on this page earn commissions at no extra cost to you. Our safety guidance is based on CDC, CPSC, EPA, OSHA, NIOSH, NFPA, ASHRAE, and UL primary sources, not commission rates. Full disclosure.