What Is TVOC in Air Quality? An Index, Not a Concentration

TVOC stands for total volatile organic compounds, one figure summing every gas that evaporates from paint, furniture, cleaning products and cooking. A VOC is one compound; TVOC is hundreds of them added together, with very different toxicity flattened into the same number. The catch: most consumer monitors do not report a TVOC air quality figure as a concentration. They report an index scaled to your own room, so your 92 and someone else's 92 describe different air.

What is a VOC Index, and why is it not a concentration?

A VOC Index is a relative scale rather than a concentration. It runs 1 to 500, with 100 set to your own room's average over the past 24 hours, so the same reading in two homes can describe very different air. Most consumer monitors report this instead of a figure in µg/m³ or ppb.

Sensirion, whose sensors sit inside a large share of consumer air monitors, publishes how their VOC Index works:

Read that baseline row again, because it is the part that trips everyone up. The index is calibrated against your own room's last 24 hours, not against clean air. A reading of 100 in a freshly painted flat and a reading of 100 in a well-ventilated one mean the same thing relative to each room, and completely different things in absolute terms.

Three consequences follow.

1. Comparing your number to a friend's is meaningless. Each device is scaled to its own environment. A screenshot of someone's VOC Index tells you nothing about your air.
2. A flat reading does not mean clean air. If a room has been mildly contaminated for days, the baseline drifts up to match it, and the index settles back near 100. The number tracks change, not cleanliness.
3. The useful signal is movement. A spike when you open a bottle of solvent, and how long it takes to fall, tells you something real. The absolute value on a quiet afternoon tells you very little.

Sensirion's own reasoning is worth quoting, because it is unusually candid for a component datasheet. They say a true VOC concentration "cannot be properly provided under field conditions," and that the index instead "behaves like a human nose."

Read that as an admission. The ppb figure other devices print is closer to an estimate than a measurement.

What TVOC levels are safe, and what is a normal range?

There is no health-based safe TVOC level, and the most cited threshold is a statistical one rather than a toxicological one. The German Committee on Indoor Air Guide Values recommends avoiding concentrations above 950 µg/m³, which is 0.95 mg/m³, for precautionary reasons. It states directly that the figure is the 95th percentile of readings from the German Environmental Health Survey between 2014 and 2017.

That is worth restating in plain terms. The number is not "the level at which harm begins." It is "the level above which only 5 percent of surveyed German homes sat." A useful line to stay under, and not a safety threshold.

Where the TVOC level chart comes from

The five-band TVOC charts in circulation trace back to one exposure classification proposed by Lars Mølhave in 1990, reproduced by the European Collaborative Action on indoor air quality in its 1997 report on TVOC:

Two things about that table go unmentioned by most pages that publish it.

The measurements behind it came from gas chromatography calibrated to toluene, not from a consumer sensor. A lab instrument and a pocket meter are not producing the same quantity, so dropping one device's output into the other's scale is a stretch.

The same report that reproduced the table also states that "at present no precise guidance can be given on which levels of TVOC are of concern from a health and comfort point of view," and that "no consistent associations have been shown between measures of TVOC and discomfort or health effects."

For a sense of scale, the German 0.95 mg/m³ line sits inside Mølhave's middle band, the one he called multifactorial precisely because symptoms in that range cannot be pinned on TVOC alone.

TVOC levels in a government standard

Hong Kong publishes official TVOC classes in its guidance notes for offices and public places, measured as an 8-hour average:

That µg/m³ to ppbv pairing is worth pausing on, because it exposes the unit problem. Converting a mass concentration into parts per billion needs a molecular weight, and TVOC is hundreds of compounds with different ones. Every ppb figure for TVOC is quoted on an assumed basis, which is one reason two meters can disagree while both are internally consistent.

Reading your own number

Start by working out whether your monitor shows an index or a concentration, because the two are read in completely different ways. This table covers both.

A colour-coded chart with five health bands is that 1990 exposure classification with colours added. It sorts readings into rough tiers, which is useful. It is not a scale of harm, and the body that published it says so in the same document.

What causes high TVOC levels in a house?

New and recently disturbed materials cause most high TVOC readings in a home, and the effect fades as those materials finish releasing gas. The EPA lists the common household sources:

• Paints, varnishes, waxes and paint strippers
• Cleaning products, disinfectants and air fresheners
• Building materials and new furnishings
• Dry-cleaned clothing
• Office equipment including copiers and printers
• Hobby and craft supplies

Two additions worth knowing beyond that list.

Cooking is the most common daily spike in a normal home, and it is not a problem to solve. Frying and toasting release VOCs, the index climbs, and it falls again with the extractor on.

Activity level matters more than the room. The EPA notes that during paint stripping, levels can reach 1,000 times background outdoor concentrations. Nothing in ordinary daily use approaches that, which is a useful sense of scale when a reading looks alarming.

New furniture is the case people ask about most, because the smell is obvious and it lasts for weeks. That is off-gassing from new furniture, and the pattern of a slow decline over days is exactly what a VOC index is genuinely good at showing.

How to lower TVOC indoors

Ventilation is the fastest way to lower TVOC, because you are diluting gases that came from inside the room. Source control is the durable fix, because you are removing what emits them.

What works:

• Open a window. Same as with CO2, and for the same reason: the source is inside, so outdoor air dilutes it.
• Run the extractor while cooking and for a few minutes afterwards.
• Let new furniture and paint off-gas somewhere ventilated, or at least with the door open and a window cracked.
• Store solvents, paints and adhesives outside living space. A sealed tin still leaks.
• Choose low-emission materials when you are already buying. Look for a recognised emissions certification rather than the word "natural" on the box.

Houseplants do not meaningfully reduce VOCs at room scale. The chamber studies people cite used sealed containers and plant densities far beyond what fits in a home.

How the Desk Pro AI measures it

The Autonomous Desk Pro AI reports VOC as an index, using a Sensirion SEN55:

A reading of 92 in the app means the air is slightly cleaner than that room's own 24-hour average. Not clean in absolute terms, and not comparable to anyone else's 92.

Placement is the same principle as with indoor CO2 levels: a sensor at the desk describes the air you are breathing, a sensor in a hallway describes the building.

If you are buying any TVOC meter, the question worth asking is whether the number is an index or a concentration, and if it is a concentration, how it was derived. Most devices do not say.

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Frequently asked questions

What is a good VOC index?

A VOC Index at or just below 100 is good, because 100 is set to your own room's average over the past 24 hours. Anything under 100 means the air is cleaner than that room's recent normal. There is no universal good value, since the scale is local to your space.

Is 500 a high VOC reading?

500 is the top of the Sensirion VOC Index scale, so a reading sitting there means the index has hit its ceiling rather than reported a precise level. Something is emitting strongly. Ventilate, find the source, and watch how fast the number falls once the room clears.

What does TVOC mean in air quality?

TVOC means total volatile organic compounds, the combined total of gases evaporating from paint, furniture, cleaning products and cooking. It is one summed figure rather than a list of individual chemicals, which makes it useful for spotting change and unsuitable for judging health risk.

What is the difference between VOC and TVOC?

A VOC is one individual compound. TVOC is all of them added together as a single number. Most monitors label the reading "VOC" on screen but actually report TVOC, and many report it as an index rather than a concentration.

What TVOC level is safe in mg/m³?

No health-based safe level exists. The German Committee on Indoor Air Guide Values suggests staying below 0.95 mg/m³, or 950 µg/m³, as a precaution. Hong Kong's guidance notes class 0.2 mg/m³ as excellent and 0.6 mg/m³ as good over 8 hours. Neither figure marks where harm starts.

What level of TVOC is unhealthy?

No single figure marks the line, because the measure sums hundreds of compounds with very different toxicity. Mølhave's classification calls 3 to 25 mg/m³ a discomfort range, and readings above the German 0.95 mg/m³ line are worth investigating. Individual compounds like formaldehyde have their own limits a summed number cannot capture.

What causes high TVOC levels in a house?

New furniture, fresh paint, cleaning products, air fresheners and cooking are the usual causes. The EPA also lists varnishes, dry-cleaned clothing, printers and craft supplies. Most spikes trace to something recently introduced or recently disturbed, and they fade with ventilation once the source stops emitting.

What is HCHO, and how is it different from TVOC?

HCHO is formaldehyde, one specific compound with its own health limits. TVOC is the sum of hundreds of compounds, formaldehyde among them. A dedicated formaldehyde sensor is separate hardware from a VOC sensor, so if both readings on a combination meter always move together, check how the HCHO figure is derived.

Do TVOC levels rise in wildfire smoke?

Yes. The EPA lists volatile organic compounds among the components of wildfire smoke, so a VOC reading climbs during a smoke event. It will not give you a reliable ppb figure though, and the EPA identifies fine particles as the main health concern in smoke, so PM2.5 is the number to watch.

Do air purifiers remove TVOC?

Partly. Activated carbon adsorbs some VOCs, so unlike CO2 there is a real mechanism. But carbon saturates, capacity varies by compound, and a spent filter can re-release what it captured. Ventilation and removing the source work better and cost less.

Why does my TVOC reading stay high?

A continuing source is the usual answer: new furniture still off-gassing, stored solvents, or a recently painted surface. If the reading is an index rather than a concentration, a persistently elevated value means levels are still rising against the room's own recent baseline.

Conclusion

TVOC is a change detector, not a health meter. It is genuinely good at showing that something in the room started emitting, and genuinely poor at telling you whether the air is safe, because one summed number cannot separate a harmless citrus terpene from formaldehyde. If your monitor reports an index, remember it is scaled to your own room over the past day, so the shape of the curve matters and the absolute value mostly does not.

The practical use is simple. Watch what makes it spike, and watch how long it takes to come back down. A reading that falls quickly after you open a window is telling you the ventilation works. One that stays up for weeks is telling you something in the room is still emitting.

References

1. United States Environmental Protection Agency. Volatile Organic Compounds' Impact on Indoor Air Quality. epa.gov
2. United States Environmental Protection Agency. Wildfire Smoke, A Complex Mixture. epa.gov
3. German Committee on Indoor Air Guide Values, Umweltbundesamt. Indoor air guide values and TVOC assessment. umweltbundesamt.de
4. European Collaborative Action on Indoor Air Quality and Its Impact on Man, Working Group 13. Total Volatile Organic Compounds (TVOC) in Indoor Air Quality Investigations, Report No 19, 1997.
5. Indoor Air Quality Management Group, Government of the Hong Kong Special Administrative Region. Guidance Notes for the Management of Indoor Air Quality in Offices and Public Places, January 2019. iaq.gov.hk
6. Sensirion AG. What is Sensirion's VOC Index? Information note. sensirion.com