Two Very Different Gases, Two Very Different Devices
One of the most common — and most dangerous — points of confusion in home safety is mixing up carbon dioxide (CO2) and carbon monoxide (CO). The names sound similar and both are invisible, odourless gases, but they behave completely differently, and the devices that detect them are not interchangeable.
What Is Carbon Monoxide (CO)?
Carbon monoxide is produced by incomplete combustion — faulty furnaces, blocked chimneys, gas stoves, car exhaust in an attached garage, or portable generators run indoors. CO binds to haemoglobin in your blood roughly 200 times more strongly than oxygen, which is why even small concentrations are lethal.
- 9 ppm — the EPA's outdoor 8-hour exposure limit
- 70 ppm — most CO alarms begin sounding within 1-4 hours
- 400+ ppm — life-threatening within hours
A CO detector is a legally required safety device in most jurisdictions if you have any fuel-burning appliance or an attached garage.
What Is Carbon Dioxide (CO2)?
Carbon dioxide is what you and everyone around you exhales. It is not acutely toxic at the levels found in buildings — instead, it is the best available proxy for ventilation quality. When CO2 accumulates, so do airborne pathogens, odours, and VOCs, and cognitive performance measurably declines.
- 420 ppm — outdoor baseline
- 800 ppm — the level below which most experts recommend staying
- 1,500+ ppm — stuffy, drowsy, unhealthy over long periods
See our complete indoor CO2 level guide for the full breakdown.
Can a CO2 Monitor Detect Carbon Monoxide?
No. A CO2 monitor uses an NDIR or photoacoustic sensor tuned to the infrared absorption wavelength of carbon dioxide. It is physically incapable of detecting carbon monoxide, and a CO alarm is equally blind to CO2. If a salesperson or product listing implies one device does both jobs with a single sensor, treat it as a red flag.
Which One Do You Need?
You need a CO detector (smoke/CO combo alarms count) if you have:
- Gas, oil, propane, or wood-burning appliances
- A fireplace or an attached garage
- Local laws requiring one — most do
You benefit from a CO2 monitor if you care about:
- Sleep quality in closed bedrooms
- Focus and productivity in a home office
- Ventilation and airborne infection risk in shared spaces
The honest answer for most households is both: a $25 CO alarm for life safety, and a CO2 monitor for day-to-day air quality. Browse our monitor directory or start with our best home monitors guide.
Key Takeaways
- CO (carbon monoxide) kills; a CO alarm is a mandatory safety device.
- CO2 (carbon dioxide) degrades air quality and cognition; a CO2 monitor is a wellness and ventilation tool.
- No consumer device measures both with one sensor — check the spec sheet, not the marketing name.