What to Look For in an Air Purifier for VOCs and Mold

Eight questions worth asking

  • 1. Does it act on gases, or only on particles?

    The first fork, and it decides everything else. A HEPA filter is a fine mesh — excellent at dust, pollen, dander and smoke particles, and structurally incapable of catching a single molecule.

    The EPA is direct about the split: to handle both, many air cleaners contain two filters, one for particles and another for gases, while others have only one, usually for particles. Check which you are buying.

    The EPA on choosing an air cleaner 

  • 2. If it handles gases with carbon, how much carbon?

    Activated carbon genuinely adsorbs VOCs. The catch is quantity. A thin carbon-dusted mat behind a HEPA panel holds very little; meaningful carbon is measured in pounds.

    The EPA’s own qualifier is the one to hold a manufacturer to: activated carbon filters can be effective “provided that there is a large amount of material used in the filter”.

    A 2022 study in Chemosphere put numbers on “a large amount”. Modeling 45 carbon–VOC pairs, it estimated that for half of them a filter needs roughly 7 to 13 ounces of carbon to last 30 days in clean indoor air, up to about 2½ pounds in a typical indoor environment, and 7 to 33 pounds or more during a wildfire or heavy cooking. Many consumer purifiers carry a small fraction of a kilogram.

    Two things to know about that paper, because this page is about checking evidence: three of its four authors were employed by Molekule, a company selling a competing non-carbon technology, and it is a modeling study built on published thermophysical data rather than a test of commercial filters. It is still the clearest published attempt to quantify the question.

    So ask for the weight, and ask the replacement interval. And note what this does not say: carbon reaching capacity means it stops taking up more, which is different from releasing what it already holds.

    Maximoff, S. N., Mittal, R., Kaushik, A. & Dhau, J. S. (2022). Performance evaluation of activated carbon sorbents for indoor air purification during normal and wildfire events. Chemosphere, 304, 135314 — doi:10.1016/j.chemosphere.2022.135314.

  • 3. Does it treat surfaces, or only the air that reaches the intake?

    VOCs keep coming off their sources — flooring, adhesives, paint, furniture, the mold in a wall cavity. A fan-and-filter unit only ever treats the air that travels to it, and only while it is running.

    Ask whether the technology acts on the room, or on the cubic foot in front of it. For an odor that has settled into fabric and drywall, that distinction is the whole question.

  • 4. What is its measured ozone output, and who measured it?

    Ask every device this, and be wary of a phrase instead of a number. California caps an air cleaner intended for an occupied space at 0.05 ppm of ozone, tested to the UL 867 method.

    “Ozone free” is marketing. A figure, a laboratory and a report number are evidence. The California Air Resources Board also flags “activated oxygen” as a way of making ozone sound healthy.

    CARB on ozone-generating air cleaners 

  • 5. On what compound was it tested, at what concentration, for how long, and by whom?

    “Removes 99% of VOCs” means nothing by itself. VOC is a category of thousands of compounds that behave differently from one another.

    This matters more than buyers realize, because the gas side of this industry is unregulated. In the EPA’s words, there are no widely used performance rating systems for air cleaners or filters designed to remove gases — no equivalent of CADR, and no certification program. A real result names the compound, the starting concentration, the time, the laboratory, and ideally a control running alongside.

  • 6. Is the coverage figure for particles or for gases?

    Square-footage ratings are almost always derived from particle clearance, because that is what the standard test measures. The EPA states plainly that the CADR rating system is for particles only.

    Gas removal does not scale the same way. Treat any room-size number as a rough guide to the space a device is built for, not a promise about odors.

  • 7. How loud is it on the setting you would actually use?

    Purifiers are usually quoted at maximum fan speed, and most people run them low, or switch them off at night. Effective performance is whatever you will tolerate leaving on.

    A unit that only works on a setting you find irritating is a unit that spends most of its life not working.

  • 8. For mold: does it claim to prevent mold growth?

    If it does, be suspicious. Mold grows because something is damp, and no device you plug in can dry a wall. The EPA’s order of operations is source control first, then ventilation, then air cleaning — and it calls source control “usually the most effective”.

    A device can reduce what mold releases into a room. That is a different claim, and a far more defensible one. Anything promising prevention is selling past its evidence.

    Our own answer to this question 

When you should buy a HEPA purifier instead

If your problem is particles, buy a filter. Dust, pollen, dander and smoke particles are physical objects, and the right way to deal with a physical object is to catch it. WellisAir is not a filter and will not do that job. If someone in your home has a particle allergy, a HEPA unit is the correct purchase, and plenty of homes run both because the two solve different problems.

We would not point you to a filter for mold, though. A filter only ever captures what reaches it, and it holds what it captures until someone changes it. In most homes that happens later than it should.

Buying questions, answered

What features should I look for in an air purifier for VOCs?

Start by checking whether it treats gases at all. Most air purifiers are built for particles, and the specification that sells them — CADR — is, in the EPA’s words, “for particles only”. A unit with no activated carbon and no gas-phase technology is not going to do much about off-gassing or odors however high its CADR is.

Then ask four things: if it uses carbon, how much by weight and how often it is replaced; whether it treats the room and its surfaces or only the air pulled through it; what its measured ozone output is and who measured it; and on which specific compound, at what concentration and over what time, its performance was tested. The gas side of this industry has no standard rating, so a named compound and a named laboratory are the only real evidence available.

Are HEPA filters good for VOCs and mold removal?

For VOCs, no. A HEPA filter is a particle filter, and VOCs are gases — single molecules that pass straight through. Gas removal in a filter-based unit comes from activated carbon, not from the HEPA element, and the EPA notes carbon works only when there is a large amount of it.

For mold, it depends what you mean. Mold spores are particles, so a HEPA filter does capture the spores that reach it. The musty smell is not a particle — it is a group of gases including 1-octen-3-ol, geosmin and 2-methylisoborneol — so a filter does not touch the smell, which is why it survives a thorough clean. And a filter holds what it captures until someone changes it.

HEPA or activated carbon for VOCs — which is better?

They are not alternatives for the same job. HEPA captures particles and does nothing to gases. Activated carbon adsorbs gases and does comparatively little to particles. A unit that handles both usually contains both, as two separate filters.

For VOCs specifically, carbon is the element doing the work, with two caveats worth knowing. The EPA says carbon is effective only when a large quantity is used, so a light carbon coating on a particle filter is close to decorative. How large? A 2022 modeling study in Chemosphere estimated that a filter needs on the order of 7 to 13 ounces of carbon to last 30 days in clean indoor air, up to about 2½ pounds in a typical home, and 7 to 33 pounds or more during a wildfire. Three of that paper’s four authors worked for Molekule, which sells a competing non-carbon technology, and it models published data rather than testing real filters — but it is the clearest published attempt at the question (doi:10.1016/j.chemosphere.2022.135314).

The second caveat: adsorption means holding the molecule, not breaking it, so capacity is finite and the filter has to be replaced on schedule.

Hydroxyl chemistry takes a third route: it breaks the molecule apart rather than trapping it, which is why there is no filter to saturate.

How do air purifiers filter out VOCs and mold?

Three different mechanisms, often sold under one word. Filtration pulls air through a mesh and catches particles — effective for spores, dust and smoke particles, no effect on gases. Adsorption uses activated carbon, where gas molecules stick to a porous surface and stay there until the carbon is full. Oxidation breaks the molecule apart chemically instead of capturing it; hydroxyl radicals do this by taking a hydrogen atom from the pollutant and becoming water in the process.

The practical difference is what happens to what was removed. A filter and a carbon bed both hold it until someone takes it out of the house. Oxidation leaves smaller fragments and water vapor, and nothing to empty.

What are common maintenance practices for air purifiers targeting VOCs?

For a filter-based unit it comes down to replacing filters on schedule, and the gas-phase element usually needs changing more often than the particle filter. The EPA’s guidance is blunt about what happens otherwise: all filters need regular replacement, and an overloaded filter will not work well. Gas-phase filter life can be short, and since there is no standard rating for gas removal there is no easy way to tell from the outside how much capacity is left.

That is the part most people get wrong, and not through carelessness — a filter gives no sign that it has stopped working.

WellisAir has no filter to replace. The only maintenance is swapping one cartridge every 90 days, and the unit tells you when. There is no UV lamp to change and nothing to wash.