HEPA Filter Air Purifiers for Mold Spores: What to Look For

If you’ve had a major mold problem in your house, or someone in your household has allergies that make them especially sensitive to mold spores, an air purifier is a good option for keeping the air as free of mold spores as possible. HEPA filters are among the most popular air purifiers on the market, but they are often combined with other air purifications technologies, so it can be difficult to know what kind is the best for you.

We will look at how effective HEPA filters are at removing mold spores from the air in your home, and then we will examine the different anti-mold technologies found in HEPA filters to see which ones will do the best job at maintaining your indoor air quality and keeping your house mold-free.

Mold characteristics

To understand how to minimize the mold spores in your house, we need to learn a little bit about mold. Mold is a fungus, and it spreads by releasing spores that get dispersed  everywhere, both outdoors and in your house. It is impossible to completely rid your home of mold spores, but fortunately they are mostly found at low concentrations that do not cause any harm unless you have very sensitive mold spore allergies.

However, you might live in an area with higher than average mold spore concentrations, or where the type of mold spore to which you are sensitive happens to be especially prevalent. The science is unclear on how much mold spore counts vary geographically, but there is evidence that some molds are more endemic to some areas than others (Vesper et al, 2011). Outdoor mold concentrations also increase in the autumn months (Reponen et al., 1994), but a more common problem is mold infestations in homes. When there is mold growing in your home, it releases spores. This means you will have far greater concentrations of mold spores in the air, enough to irritate the respiratory system of people who do not even have mold-specific allergies, according to the CDC.

Under these circumstances, you will want to take steps to clear the mold spores from the air, reducing their concentrations as much as possible. It is very important to first clean up the mold in your home. If you do not, the mold will continue to emit spores and continue to grow and spread. Mold requires moisture to grow, so once you have cleaned up the mold itself, remove whatever leak, spills, or humidity source is causing the mold so it does not recur.

When are HEPA filters used for mold removal?

If someone in your household has mold allergies, you can use a HEPA filter air purifier whenever they are present, especially in the room they sleep in, to keep mold spore concentrations at a minimum. Some mold spores will still enter your house because they enter from outdoors, so you cannot get rid of all of them, but your goal is to keep their numbers low enough to avoid allergy attacks.

If you are in the process of cleaning up a mold problem, it is a good idea to run a HEPA filter air purifier in the area where the clean-up is going on. Because the clean-up tends to cause a large amount of mold spores to get blown into the air, a HEPA filter can remove them before they get a chance to spread or land somewhere and grow more mold.

If there is an area in your house where mold is a stubborn problem—like a poorly ventilated bathroom, kitchen or basement—a HEPA filter air purifier running in that area can remove mold spores from the air and reduce their spread and possibly slow mold growth. However, you will ultimately need to address the moisture problem, otherwise mold will be a recurring problem.

How do HEPA filters work on mold?

A HEPA filter is a mesh of fabric designed to trap particles that attempt to pass through it. They are built to meet a U.S. Department of Energy standard requiring that they stop 99.97 percent of all particles that are 0.3 microns in size from passing through them. Mold spores can vary tremendously in size, but the size range is generally between 4 microns up to about 20 microns (Li et al., 2011). This means even the smallest mold spores fall well within the HEPA specification, allowing a HEPA filter to trap most mold spores that pass through them. Actual performance can depend on how frequently the HEPA filter is changed and how airtight the filter frame is. Any gaps will allow mold spores through.

However, one concern is that a dirty filter that has trapped a lot of mold spores could actually start to grow mold, which could then emit mold spores into the room. Mold spores require moisture to grow into mold, so it is not likely that this could happen, but if the air passing through the filter is extremely humid, it is possible mold could begin to grow. Preventing this requires regular changing of dirty HEPA filters. Note that we do not recommend washing and reusing HEPA filters, even ones that are labeled as reusable, as washing likely damages the fiber mesh and reduces the filter’s ability to trap particles. Plus, if you are concerned about mold, you will not be able to remove every mold spore from the dense HEPA mesh when washing it.

HEPA tech add-ons

Some HEPA filter air purifiers add other tech or air purification methods to complement its mold spore removal abilities.

Plain HEPA – A simple HEPA filter with no additional technology is an excellent mold spore removal device. Regular replacing of the filter will mitigate any potential problems with mold growing on the filter itself.

HEPA with UV-C light – There is some evidence that ultraviolet radiation of a certain wavelength (known as UV-C light) can kill mold spores. However, to be effective, the spores need to be exposed to the light for a long enough time, and the light source must be more powerful than what you will find in most commercial UV-C air purifiers. In addition, the UV light can emit ozone as a by-product. Ozone is a respiratory irritant, and the EPA does not recommend using air purifiers that could cause indoor ozone concentrations to reach unsafe levels. A UV-C light could reduce the chances of mold growing on the filter, and it might be useful if you can turn the UV-C light on only when needed, but the benefits probably do not outweigh the potential drawbacks.

HEPA with antimicrobial layer – These filters have been coated with a chemical that kills microbes, including mold spores. This should have no detrimental effect on the filter’s function, and could make it even less likely for mold to grow on the filter.

HEPA with carbon – Carbon, or activated charcoal, acts in a completely different way than HEPA filters, removing gaseous substances and odors from the air by trapping the molecules via “adsorption.” A carbon add-on will have no effect on mold spores, but could reduce musty, mildewy smells.

HEPA with ionizer – Ionizers impart an electrical charge to particles passing through it, making them stick to each other or to a nearby object (such as a HEPA filter). But there is little evidence that they would improve a HEPA filter’s performance with regard to mold spores. Ionization produces ozone as a by-product, and we have already discussed the potential dangers of ozone build-up. Air purifiers that indicate they ionize the air for “freshness” are referring to the odor of ozone, which can mask less pleasant odors. However, this does not mean pollutants are actually being removed or neutralized by the ionizer.

Other considerations

To get the most out of a HEPA filter for mold spore removal, it should be allowed to run 24/7. This means you should check to see if the air purifier you buy is Energy Star certified, so it does not use too much electricity. Noise is another concern, especially if it will be running in an area where you sleep or work. An air purifier’s CADR number (which stands for “Clean Air Delivery Rate”), though not a great guide to efficiency, can give you a general idea of how powerful its fan is—a very high CADR purifier might be extra loud. You can also check online reviews to get an idea of how noisy an air purifier is.

Finally, remember to change the HEPA filter regularly. This is an added expense, but it keeps the air purifier running at maximum efficiency and prevents mold growth on the filter.