A CO₂ reading tells you how well a room is ventilated relative to the people in it. A VOC reading tells you whether something in the room is releasing volatile chemicals: cleaning products, paint, new furniture, cooking, cosmetics or people themselves. CO₂ is a direct measurement with a known meaning; VOC is a relative index from a sensor that reacts to hundreds of compounds at once. Read them together and they explain most changes in indoor air.
What each sensor measures
CO₂
NDIR sensors detect carbon dioxide specifically, by infrared absorption. The reading is in ppm and can be compared with guidelines. Its only significant indoor source is people (plus combustion), so it doubles as a ventilation meter. See how CO₂ monitors work.
VOC
Metal-oxide sensors respond to a broad family of volatile organic compounds: alcohols, aldehydes, ketones, terpenes, hydrocarbons and more. The sensor cannot tell them apart. Manufacturers convert the raw signal into a “TVOC” estimate or an index, often with an algorithm that adapts to the room’s baseline over time. Two monitors can report very different VOC numbers in the same room and both be behaving normally.
Typical sources
| Event | CO₂ | VOC |
|---|---|---|
| People entering a room | Steady rise | Mild rise (breath and skin emissions) |
| Window opened | Falls | Falls |
| Cleaning with sprays | No change | Sharp spike |
| Cooking with oil | Small rise (gas hob) | Spike |
| Hand sanitizer nearby | No change | Very sharp spike |
| New furniture, carpet, paint | No change | Elevated for days to weeks |
| Candles | Rise | Rise |
Reading them together
- CO₂ high, VOC low: too many people for the ventilation. Open a window.
- CO₂ low, VOC high: a chemical source with adequate ventilation. Find the source: recent cleaning, a new product, cooking.
- Both high: a closed, occupied room with a source. Ventilate first, then find the source.
- Both low: fine.
The trap: “eCO₂” from a VOC sensor
Some cheap monitors have only a VOC sensor and print a CO₂ number estimated from it. That number reacts to sanitizer and cooking, not to people, and is useless for ventilation. If a monitor’s CO₂ channel spikes when you use hand gel, it is not measuring CO₂. See NDIR vs metal-oxide sensors.
Which to buy
If you buy one sensor, buy CO₂; ventilation is the most common indoor air problem and the reading is actionable. If you want VOC as well, buy a combination monitor that names both sensors honestly, such as those in the office air quality monitors guide. Formaldehyde, a specific VOC of health concern, needs its own electrochemical sensor; a generic VOC index does not measure it.
Recommended Tools
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Frequently Asked Questions
Is VOC or CO₂ more important to monitor?
For ventilation decisions, CO₂. For detecting chemical sources such as cleaning products, new furniture or paint, VOC. Most people benefit from CO₂ first and VOC as a bonus.
Why does my VOC reading spike when the CO₂ reading does not?
Something is emitting chemicals without adding CO₂: hand sanitizer, cooking oil, perfume, a new rug or solvent. That is exactly what the VOC sensor is for.
What units is VOC reported in?
Usually an index (0 to 500) or an estimated µg/m³ or ppb of "total VOC". Metal-oxide sensors cannot identify compounds, so the number is a relative indicator, not a measurement of any specific chemical.
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