VOC (volatile organic compound) is a category: hundreds of carbon-based gases that evaporate at room temperature. TVOC (total VOC) is a number claiming to represent their combined concentration. A “VOC index” is a relative score with no concentration claim at all. On consumer monitors all three labels usually sit on the same metal-oxide sensor, which responds to the whole family without identifying any member. The honest label is the index; the µg/m³ label implies a precision the sensor does not have.
Three labels, one sensor
| Label | What it claims | What the sensor delivers |
|---|---|---|
| VOC | Presence of volatile organics | A combined resistance change; fine as a qualitative indicator |
| TVOC (µg/m³ or ppb) | Total mass or volume concentration | An estimate valid only if the air resembles the calibration gas; often off by a factor of two or more |
| VOC index (0 to 500) | Change relative to the device’s own baseline | Exactly that; honest about its limits |
Why TVOC concentrations are shaky
Metal-oxide sensors respond differently to each compound: strongly to alcohols and hydrogen, weakly to some aromatics, and not at all to a few. The firmware assumes the air is a particular reference gas (ethanol is common) and scales accordingly. Real indoor air is a shifting mixture, so the µg/m³ figure is the concentration of an imaginary gas that would have produced the same signal. Two monitors using different reference gases read different “totals” in the same room. Laboratory TVOC is defined by summing specific compounds measured by gas chromatography, and it does not agree with sensor TVOC either.
The sensor types
- Metal-oxide (MOX): nearly every consumer monitor. Cheap, sensitive, non-specific, drifts, needs baseline adaptation.
- Photoionization detector (PID): professional handheld VOC meters. Responds to compounds by ionization energy, calibrated to isobutylene; still non-specific but more stable and quantitative. Not used in consumer monitors.
- Electrochemical formaldehyde cells: specific to formaldehyde; found in some consumer monitors. See what formaldehyde is and how monitors detect it.
What a consumer monitor can honestly say
- VOC levels rose or fell, and when.
- Roughly how large the change was relative to the room’s normal state.
- That a chemical source is present when readings stay high after ventilation.
It cannot honestly say that a room is at 250 µg/m³ of any particular thing, or that a reading is safe or unsafe. See what TVOC is for reading the number in context.
How to use the reading
Learn the baseline, watch for departures, and act on them: ventilate, find the product or material responsible, retest. For anything with health stakes, such as suspected formaldehyde from new flooring, use a dedicated sensor or a laboratory test rather than a TVOC figure. Combination monitors with an honest index alongside NDIR CO₂ are in the office air quality guide; see also CO₂ vs VOC monitoring.
Recommended Tools
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Frequently Asked Questions
Is VOC the same as TVOC on a monitor?
In practice usually yes: both labels sit on a metal-oxide sensor reporting a combined signal. Strictly, VOC is the class of chemicals and TVOC is the total concentration of them.
What is a VOC index?
A relative scale (typically 0 to 500, with 100 as the device's learned average) that reports how current air compares with the recent baseline. It makes no claim to a concentration.
Can any consumer monitor identify a specific VOC?
Only formaldehyde, via a dedicated electrochemical sensor. Identifying other individual compounds needs laboratory methods such as gas chromatography.
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