Carbon Monoxide vs Carbon Dioxide

You see a "CO2 monitor" listed online and wonder, would it protect your family from carbon monoxide? Or a news report mentions "carbon dioxide poisoning" and you're not sure if that's the same thing your CO alarm detects. The names carbon monoxide (CO) and carbon dioxide (CO2) look almost identical, but the gases are not interchangeable, and neither are the detectors. CO is a toxic poison gas produced by incomplete combustion in furnaces, generators, stoves, and cars. CO2 is a normal part of the air we exhale, typically a ventilation concern at home and an acute hazard mainly at very high concentrations in industrial or confined-space scenarios. This guide explains the key differences, symptoms, sources, and which detector matches the hazard you face.

This is general safety information, not medical advice. If you suspect active CO exposure or severe symptoms, move to fresh air and contact emergency services.

Key Takeaways

CO (carbon monoxide): toxic at relatively low concentrations; 28.01 g/mol, roughly 3 percent lighter than air; disperses uniformly indoors.
CO2 (carbon dioxide): 44.01 g/mol, about 52 percent heavier than air; pools in confined low spaces (cellars, fermentation tanks, dry-ice storage); residential issues are almost always ventilation, not acute toxicity.
Detectors: CO alarms use electrochemical sensors tuned to CO. CO2 monitors use NDIR sensors tuned to CO2. They are not interchangeable in either direction.
Placement: NFPA 72 Chapter 29 puts CO alarms on each level and outside each separate sleeping area, and permits wall mounting for stand-alone alarms.
If a CO alarm sounds, leave immediately and call emergency services; a CO2 monitor will not warn you about CO.

The "CO Rises" Myth Most Vendor Pages Repeat

Open the top consumer-content pages for "CO vs CO2" and you will find the claim that carbon monoxide is "lighter than air and rises to the ceiling" repeated by Brinks, Vivint, ADT, and most security-alarm vendor blogs. The molar masses say otherwise: CO 28.01 g/mol, dry air about 28.97 g/mol, a difference under 4 percent. CO is fractionally lighter, but the gap is too small to drive stratification in a room with normal air movement. Gaseous diffusion, convection from heat sources, HVAC airflow, and human movement dominate the distribution; CO mixes into a roughly uniform concentration through the rooms it can reach. The Institute for Environmental Research and Education and NFPA both note this directly.

The standard behind that is worth naming correctly, because most pages citing it are citing something that no longer exists. NFPA 720 was the dedicated CO detection standard, and its 2015 edition was the last one published. The withdrawal itself came later, on 14 August 2018, when the NFPA Standards Council approved moving the material into NFPA 72 in the Annual 2018 revision cycle. Confusing the final edition with the withdrawal vote is the usual way this gets misreported, and it costs three years. The requirements have sat in NFPA 72 since its 2019 edition, with household alarms in Chapter 29 and commercial system detection in Chapter 17.

Chapter 29 puts residential CO alarms outside each separate sleeping area and on every level of the dwelling including basements, and it sets the outer distance at 21 feet from any door to a sleeping room, measured along the path of travel rather than in a straight line through the wall. Five of the 51 US jurisdictions write their own distance into the state instrument, and the asymmetry is the interesting part: every one of the five is tighter than 21 feet and not one is looser, at ten feet in Florida, Minnesota and Tennessee and fifteen in Colorado and Illinois. Go down to city level and the symmetry breaks, because Wilmington, Delaware allows forty. A legislature drafting for a whole state has never yet decided that NFPA was too strict; a single city has (see where to place a CO detector for the state-by-state figures). On height it permits wall mounting for stand-alone alarms, where Chapter 17 requires ceiling installation for system detectors. The practical range comes from the manufacturer rather than the code, and the manufacturers disagree with each other, which is the subject of our placement height guide. What none of them support is the "CO rises so install at the ceiling" rule: choosing low wall, high wall or ceiling is an operational decision, and following the installation manual beats following the myth.

CO2 behaves differently. At 44.01 g/mol it is about 52 percent heavier than air and does pool in low confined spaces. This matters for cellars, fermentation rooms, dry-ice party fog releases, and (in extreme natural form) the 1986 Lake Nyos disaster in Cameroon, where a sudden CO2 release from an overturned volcanic lake killed 1,746 people in surrounding villages. The relevant takeaway for residential decisions is narrow: CO2 pools where ventilation does not reach; CO does not pool meaningfully even though most consumer guides claim it does.

CO vs CO2: Side-by-Side Comparison

Feature	Carbon Monoxide (CO)	Carbon Dioxide (CO2)
What it is	Poison gas from incomplete combustion	Gas naturally present in air; produced by breathing and combustion; used widely in industry
Can you smell it?	No (odorless, colorless)	No (odorless, colorless)
How it harms you	Prevents the body from using oxygen normally (poisoning)	At high levels, displaces oxygen and affects breathing/acid-base balance
Typical "home" scenario	Equipment failure or exhaust trapped indoors (furnace, generator, car in garage)	Most often "stuffy air" from low ventilation, not an actual "leak"
What to install	UL-listed CO alarms in recommended locations	Optional CO2 monitor for ventilation awareness (not a safety alarm for CO)
Urgency	High: CO alarm or symptoms → leave immediately and get help	Usually low at home; high urgency mainly for industrial/confined-space releases

Where Each Gas Comes From

Carbon monoxide (CO): incomplete combustion

CO forms when fuels do not burn completely. Common CO sources include malfunctioning furnaces/boilers, blocked chimneys or flues, gas stoves used improperly, generators, fireplaces, and vehicle exhaust in enclosed or attached garages. Because CO is invisible and odorless, you often cannot "notice" it until symptoms appear, which is why working CO alarms and basic maintenance matter.

Carbon dioxide (CO2): breathing, combustion, and industrial use

CO2 is produced by people and pets (exhaled breath) and by combustion. Indoors, CO2 is often used as a practical signal of ventilation: higher readings usually mean more occupants and/or less fresh-air exchange. A true high-CO2 emergency is more common in industrial settings or confined spaces (compressed CO2 releases, dry ice storage, fermentation rooms, tanks, cellars), not typical residential living areas.

What Each Common Appliance or Activity Produces

The two gases come from overlapping but not identical sources. Mapping the appliance to the gas it produces clarifies what each detector watches for.

Source	CO output	CO2 output	What it means at home
Properly tuned gas furnace	Trace (within vent)	Significant (vented outdoors)	Annual inspection catches early CO from cracked heat exchanger.
Cracked heat exchanger or blocked flue	High (into airflow)	High (into airflow)	The classic CO incident scenario. Alarm is the only warning.
Gas oven or stove (proper use)	Low	Moderate (kitchen-localized)	Range hood prevents CO2 buildup. CO alarm needed nearby for failure modes.
Gas oven used for room heating	Significant	High	Documented CO-incident pattern. The oven was never designed for sustained operation.
Portable generator	Very high (CPSC puts the emission rate at hundreds of times a car's)	High	CO is the lethal hazard. CPSC attributes ~100 US deaths per year to portable generators.
Wood-burning fireplace (good draft)	Trace (vented)	Significant (vented)	Flue blockage redirects both indoors. Maintain chimney annually.
Charcoal grill indoors (never do this)	Extremely high, especially when smoldering	High	Smoldering charcoal is the highest CO yield phase. Indoor or covered-porch use kills people every year.
Occupants breathing	None	Moderate (~40,000 ppm exhaled)	Why CO2 climbs in poorly ventilated rooms with people present.
Compressed CO2 (kegerator, soda machine)	None	High if released	Confined-space CO2 hazard. Not a CO scenario.

Compare the columns and the alarm landscape clarifies itself: anything that burns fuel needs a CO alarm nearby; anything with high occupancy or compressed CO2 in a small space benefits from a CO2 monitor for ventilation awareness. Most homes need the first; offices, classrooms, and tight-construction bedrooms benefit from the second.

Symptoms: Carbon Dioxide Poisoning vs Carbon Monoxide

CO poisoning symptoms (treat as urgent)

CO poisoning symptoms can look like a virus or food poisoning: headache, dizziness, weakness, nausea/vomiting, chest discomfort, and confusion. Severe exposure can cause loss of consciousness. A key clue is pattern: multiple people feel sick in the same space, symptoms improve when leaving, or a CO alarm activates.

High CO2 exposure symptoms

High concentrations of CO2 can cause headache, shortness of breath, faster breathing, dizziness, confusion, and in extreme cases collapse. These effects are usually associated with unusual high-CO2 environments (confined spaces or industrial releases), rather than everyday home ventilation issues. If you suspect a compressed-gas release or anyone has severe symptoms, leave the area and seek emergency help.

Signs of Carbon Dioxide in Your House

In most homes, "signs of carbon dioxide in your house" means signs of poor ventilation: rooms feel stuffy, occupants feel sleepy, odors linger. A CO2 monitor can help you decide when to open a window or check the HRV, but it does not replace a CO alarm.

The "1000 ppm" threshold people quote as the ASHRAE limit was real once and has outlived the standard that carried it by more than three decades. ASHRAE's own 2020 position document is blunt about it: "The 1989 edition of ASHRAE's ventilation standard, Standard 62 (subsequently Standard 62.1), had a CO2 limit of 1000 ppmv, but this was removed from subsequent editions due to its common misinterpretation." Persily and de Jonge trace the mechanics: the 1989 limit was 1800 mg/m3, roughly 1000 ppmv, and it sat inside the standard's IAQ Procedure rather than being a target for anyone's building; the whole contaminant table went in the 2004 version. The number itself is nineteenth-century, from Pettenkofer, who offered 1000 ppmv as a marker of inadequate ventilation and 700 ppmv for bedrooms, and who argued CO2 was a proxy for bad air rather than its cause. It remains a serviceable rough indicator. It is not a limit, and no current ASHRAE standard prescribes it as one.

Modern airtight construction (IECC 2018+, Passivhaus, Net-Zero) routinely runs CO2 above 1000-1500 ppm during occupied hours because the building envelope is tighter than the historical assumptions behind the old guidance. The fix is continuous mechanical ventilation. ASHRAE 62.2 sets the residential rate and is sold rather than published, but the IRC reproduces it: section M1505.4.3, Equation 15-1, gives (0.01 × floor area) + [7.5 × (bedrooms + 1)] cubic feet per minute, and Table M1505.4.3(1) lets you skip the arithmetic. A two-bedroom home under 1,500 square feet lands at 45 CFM on the table. The drivers are floor area and bedroom count; the standard does not ask how many people live there.

For compressed CO2 sources (kegerator cylinders, soda fountains, fire suppression systems), the hazard is qualitatively different: large volumes can displace breathable air in confined spaces. Treat any compressed-gas leak suspicion seriously: ventilate, evacuate if symptoms occur, call a professional.

Detectors: Do You Need a CO Alarm, a CO2 Monitor, or Both?

Do CO detectors detect CO2?

No. A standard CO alarm is designed to detect CO and alarm at specific thresholds over time. It does not measure CO2.

Do CO2 monitors detect CO?

No. CO2 monitors measure CO2 (often for indoor air quality/ventilation). They are not designed to warn you about CO poisoning.

Best-practice setup for most homes

Install CO alarms on every level and near sleeping areas (follow NFPA 72 and manufacturer instructions). Height is a manufacturer question and the ranges differ, so read the one in the box.
Use a CO2 monitor only if you want a ventilation indicator, especially in bedrooms, nurseries, home offices, and tight-envelope construction.
Maintain combustion appliances and never run engines or generators in enclosed spaces.

Why One Ventilation Upgrade Addresses Both

Most consumer guides treat CO and CO2 as separate hazards solved with separate devices. That framing is correct at the alarm layer (the sensors are not interchangeable) and incorrect at the building layer: mechanical ventilation that meets ASHRAE 62.2 lowers both indoor CO2 (by diluting occupant-generated gas) and the probability of CO buildup from minor combustion-source leakage (by maintaining airflow that does not let trace exhaust accumulate). Practical implications:

Heat recovery ventilator (HRV) or energy recovery ventilator (ERV): provides continuous balanced fresh air at low energy cost. Size it from IRC Table M1505.4.3(1) or Equation 15-1, (0.01 × floor area) + [7.5 × (bedrooms + 1)] CFM.
Range hood ducted outdoors: removes cooking-generated CO2, NO2, and ultrafine particulates that recirculating hoods cannot.
Bath fan with humidistat or motion control: meets a portion of ASHRAE 62.2 requirements while serving its primary moisture-removal role.
Manual window operation: usable backstop, but inconsistent. Tight-envelope homes built for energy efficiency need the mechanical layer.

The framing matters because most homeowners ask "what alarm should I buy?" when the better question is "is my building moving enough air?" The alarm catches failures; the ventilation prevents most of them.

What to Do If You Suspect CO

If your CO alarm sounds or you suspect carbon monoxide exposure, get everyone (including pets) to fresh air immediately and call emergency services from outside. Do not go back inside until responders confirm the building is safe, and have fuel-burning appliances inspected before using them again.

Full emergency checklist: What to do if you suspect a carbon monoxide leak.

Frequently Asked Questions

Is CO the same as CO2?

No. CO (carbon monoxide, one carbon plus one oxygen, 28.01 g/mol) is a toxic gas of incomplete combustion. CO2 (carbon dioxide, one carbon plus two oxygens, 44.01 g/mol) is a normal product of breathing and complete combustion that becomes dangerous mainly at very high concentrations.

Is the 1000 ppm indoor CO2 limit an ASHRAE standard?

No, and ASHRAE says so itself. Its 2020 position document records that "the 1989 edition of ASHRAE’s ventilation standard, Standard 62 (subsequently Standard 62.1), had a CO2 limit of 1000 ppmv, but this was removed from subsequent editions due to its common misinterpretation." The number is older than the standard: the same document credits it to Max Josef von Pettenkofer, who "proposed 1000 ppmv of CO2 as a marker of inadequate ventilation indoors and 700 ppmv for bedrooms", and to a nineteenth-century argument that CO2 was a proxy for stale air rather than the thing doing the harm. Standard 62.1 does use CO2 today, as a set point for demand-controlled ventilation, which is not the same as a limit. ASHRAE 62.2, the residential standard, prescribes a ventilation rate rather than a concentration.

Should CO alarms be installed on the ceiling because CO rises?

No. NFPA 72 Chapter 29, which took over the CO alarm requirements when NFPA 720 was withdrawn, permits wall mounting for stand-alone alarms rather than requiring the ceiling, and manufacturers set their own height bands: the Kidde range runs from six inches below the ceiling down to six inches above the floor. CO is about 3 percent lighter than air at the same temperature, which is not enough to drive stratification; turbulence, convection, and gaseous diffusion dominate and the gas distributes uniformly in occupied rooms.

Which is more dangerous in a typical home: carbon monoxide or carbon dioxide?

CO is the bigger acute danger because life-threatening poisoning starts at concentrations the human body cannot detect (35-200 ppm sustained). High indoor CO2 in residences is almost always a comfort and ventilation signal rather than an acute hazard. CO2 becomes an acute hazard in confined spaces (cellars, fermentation tanks, dry-ice storage, the 1986 Lake Nyos lake-overturn disaster in Cameroon that killed 1,746 people).

Do CO detectors detect CO2 (or vice versa)?

No, in either direction. Standard CO alarms use electrochemical sensors tuned to CO. CO2 monitors use non-dispersive infrared (NDIR) sensors tuned to CO2 absorption bands. The sensors are not interchangeable, and a normal CO2 reading on a CO2 monitor tells you nothing about CO.

Can high CO2 make you sleepy?

Sustained indoor CO2 above ~1500-2000 ppm is associated with reduced alertness, headache, and lower cognitive performance in controlled studies. The cause is debated (CO2 itself versus correlated low ventilation removing other pollutants), but the response is the same in practice: improve ventilation. This is not equivalent to CO poisoning.

Does tight modern construction raise indoor CO2 risk?

Modern airtight construction (IECC 2018+ or Passivhaus) routinely runs CO2 above 1000 ppm during occupied hours without mechanical ventilation. The fix is continuous whole-house mechanical ventilation. ASHRAE 62.2 is the parent standard and is sold rather than published, but the rate is reproduced in the building code most states adopt: IRC 2018 section M1505.4.3, Equation 15-1, sets it at (0.01 × floor area in square feet) + [7.5 × (number of bedrooms + 1)]. Note what drives it, floor area and bedroom count, not headcount.

Sources & References

Published: January 15, 2024