Carbon dioxide (CO₂) is what people exhale; indoor levels of 400 to 2,000 ppm are normal and harmless, and the number is used as a ventilation indicator. Carbon monoxide (CO) is a toxic gas from incomplete combustion; a few hundred ppm is dangerous within hours. A CO₂ monitor cannot detect CO and a CO alarm cannot measure CO₂. Homes with any combustion appliance need a CO alarm regardless of what else they have.
Side by side
| Carbon dioxide, CO₂ | Carbon monoxide, CO | |
|---|---|---|
| Molecule | One carbon, two oxygen | One carbon, one oxygen |
| Main indoor source | People breathing; some from gas cooking | Incomplete combustion: faulty boilers, blocked flues, generators, cars in garages, charcoal indoors |
| Normal indoor level | 400 to 1,500 ppm | 0 to a few ppm |
| Level of concern | Above 1,000 to 1,500 ppm: stale air, drowsiness | Above roughly 30 to 70 ppm sustained: headache, then serious harm |
| Toxicity | Low; occupational limit 5,000 ppm over 8 hours | High; binds to hemoglobin and starves tissues of oxygen |
| Sensor | NDIR (infrared absorption) | Electrochemical cell |
| Purpose of the device | Ventilation and air-quality indicator | Life-safety alarm |
Why the confusion is dangerous
Product listings sometimes describe a CO₂ monitor as a “carbon detector” or place CO and CO₂ in the same headline. A buyer who installs a CO₂ monitor believing it protects against a faulty boiler has no protection at all. The reverse mistake, expecting a CO alarm to tell you the office is stuffy, is merely unhelpful.
What each device should be
CO alarm
A certified alarm (UL 2034 in the US, EN 50291 in Europe) with an electrochemical sensor, installed near sleeping areas and on every level with a combustion appliance, replaced at the end of its stated sensor life (usually 7 to 10 years). It sounds; it does not need to display a precise number.
CO₂ monitor
An NDIR monitor placed at breathing height in the occupied rooms you care about, read as a trend. It is a comfort and health-optimization tool, not a safety alarm. See the best CO₂ monitors guide.
Combination devices
Some air quality monitors include both an NDIR CO₂ sensor and an electrochemical CO sensor, and some smoke-CO alarms add a CO₂ readout. They can be convenient, but a CO function inside a consumer monitor is not a substitute for a certified alarm unless the device itself carries the alarm certification.
Symptoms to know
- High CO₂: stuffiness, drowsiness, mild headache that clears quickly with fresh air.
- CO exposure: headache, dizziness, nausea, confusion, flu-like symptoms that affect everyone in the building and improve when away from it. Pets sick at the same time is a classic sign. Leave immediately and call emergency services.
For the levels at which CO₂ becomes a ventilation problem, see what a good indoor CO₂ level is. For gas leak and CO detectors, see the gas detection category.
Frequently Asked Questions
Does a CO₂ monitor detect carbon monoxide?
No. NDIR CO₂ sensors respond only to carbon dioxide. Carbon monoxide needs an electrochemical CO alarm, which is a separate device.
What CO level is dangerous?
Alarms are required to sound before 70 ppm sustained for a few hours; 150 to 400 ppm causes serious symptoms within hours, and over 800 ppm can be fatal in under two hours. Any CO alarm activation means leave and call for help.
Can a gas stove raise CO₂?
Yes, combustion produces CO₂ and a gas hob without a working extractor hood can push a kitchen well past 1,500 ppm. A poorly burning hob or appliance can also produce CO, which is why kitchens with gas appliances should have a CO alarm.