What does the TVOC or VOC index value represent?

The Sensirion VOC Index is a relative, unitless signal from 1 to 500. A value of 100 represents the sensor's recent typical indoor gas composition; values above or below 100 show deterioration or improvement relative to that adaptive baseline. It is not a direct TVOC concentration.

Does a high VOC index always mean toxic air?

No. The index is not a universal health threshold and cannot identify or quantify individual chemicals. Use it to investigate changes and ventilation needs, not to declare air safe or unsafe.

What should I do when the VOC index rises?

Identify recent sources such as cleaning, cooking, painting or new materials. Remove or isolate the source when practical, ventilate if outdoor conditions allow, and watch whether the index returns toward its recent baseline.

VOC Index Meaning: Scale, Baseline and Actions

VOC Index vs. TVOC concentration

The displayed value is not a TVOC concentration in ppb, ppm or mg/m³. TVOC concentration is an estimated amount of a mixture of volatile organic compounds, usually reported in concentration units and dependent on the calibration method. The Sensirion VOC Index instead compares the broad response of a metal-oxide sensor with its learned recent background and expresses the change on a logarithmic scale.

The sensor responds to many common indoor VOCs, but it does not separate benzene, formaldehyde, ethanol or any other individual compound. Sensirion documents conversion to an ethanol-equivalent TVOC value only for controlled laboratory conditions, so a simple index-to-ppb conversion should not be applied to VisiblAir readings.

How the 1–500 scale and baseline work

Model E-Lite uses the Sensirion SGP40. Model O and the current Model X2 use the Sensirion SEN55. All three report Sensirion's adaptive VOC Index with the manufacturer's default tuning:

100 is the recent typical condition: the algorithm maps the average indoor gas composition from roughly the past 24 hours to 100.
Above 100 means more intense than the learned background: a rise toward 500 represents a stronger VOC event relative to that room's recent history.
Below 100 means less intense than the learned background: a fall toward 1 represents improvement relative to recent history.

Because the baseline adapts, 100 is not a universal definition of clean air. Two rooms at 100 can have different VOC mixtures and absolute concentrations, and a persistent source can gradually become part of the learned background.

Stabilization and adaptation

Sensirion specifies a one-hour switch-on time for both the SGP40 and SEN55. After startup, short-term readings can be less settled, so look for a sustained pattern rather than acting on one sample. The default algorithm continuously learns its offset and gain over a 12-hour time scale; older events become progressively less important and are nearly forgotten after about twice that period. High-VOC events temporarily pause part of that learning so a spike is not immediately treated as normal.

This adaptation is useful for detecting changes in different indoor environments, but it also means that the index is best read as a time series. Compare a room with its own recent pattern instead of comparing one isolated number between buildings.

Reading, interpretation and action examples

Reading or pattern	Possible context	Interpretation	Practical action
Below 100	Fresh air introduced after a room was occupied	VOC intensity is below the sensor's recent baseline; it does not prove that every compound is at a safe concentration.	Continue observing the trend and other measurements.
Near 100	Usual activities in a stable room	This is the learned recent condition, not a health target.	Use it as the room's reference for future changes.
Brief rise above 100	Cooking, cleaning spray or a scented product	A new VOC event is more intense than the recent background.	Identify the activity; use an exhaust fan or ventilate if outdoor conditions allow, then watch for recovery.
Sustained or repeated rise	Painting, solvents, new furnishings, flooring or cabinetry	A continuing source may be emitting VOCs. The index cannot identify which compounds are present.	Remove or isolate the source when practical, follow product safety instructions, ventilate, and track the trend.

Why this is not a health threshold

Different VOCs have different health effects and exposure limits. A non-selective index cannot determine the identity or concentration of each chemical, so no VOC Index number alone establishes that air is safe, unsafe or compliant with an occupational limit. If a known chemical release, strong symptoms or an emergency is suspected, leave the area as appropriate and follow the product label or guidance from qualified health, safety or emergency professionals.

Manufacturer documentation

Important note

Use the index to spot and investigate changes. Do not use it to diagnose a pollutant, convert directly to a concentration, or compare health risk from one room to another.

Best-Fit Products

These VisiblAir models report the Sensirion VOC Index described in this guide.

Model E-Lite Model O Model X2

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Also Check CO2 Accuracy

If CO2 readings matter in your space, review the calibration check process and target outdoor range.

Read the Calibration Guide