Before getting an air purifier for a basement, especially if it has a musty smell, be sure to correct the source of the odor. Then we can employ air purifiers that work best on mold – photocatalytic oxidation (PCO), HEPA, and even an ionizer.
You may also choose to fog essential oils after remediation but before starting the use of the air purifier which helps break down mold spores.
A dehumidifier helps to prevent further mustiness.
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Get to the source of the Musty Smell First
1. Water Ingress
First, if you’re basement does have a musty or moldy smell, you have to correct the source of the water damage:
- Grade land away from the foundation at a 5% or more grade
- Make sure your gutters are moving water and dumping it away from the foundation
- Repair cracks in the foundation
- You may need to install drains or repair the drains that you do have
- Don’t have plants that need watering right next to the foundation
If you have a finished or unfinished basement with mold growing on the concrete or behind materials you need a professional mold remediator to remove the mold under containment.
Most, or at least many, basements are not dry enough for finished walls and floors. The concrete is always trying to dry inside so water vapor can get stuck behind the walls or under the floor and mold can form in these locations.
If you don’t have a vapor barrier under your slab then you will likely have significant water vapor drying up.
If you have fixed everything that can be fixed and your humidity is still not under 60% RH then bring in a dehumidifier.
3. How to Finish a Basement
The most ideal way to finish a basement is to keep a breathable (vapor open) flooring like polished concrete or tile and breathable rugs. The walls can be finished with a silicate paint like Kiem which is vapor open or a plaster.
Ideally, you don’t finish the ceiling so that you can see leaks from bathrooms or kitchens above when they do happen.
Solutions for Musty Basements
1. PCO Air Purifiers
Photocatalytic oxidation (PCO) is a technology that breaks down mold, VOCs as well as some pathogens.
My interest in these air purifiers comes from first-hand accounts of this benefiting homes with low levels of mold, or even when mold from outdoors is coming in, and from the studies showing the eradication of mold and mycotoxins.
This is the best technology in my opinion for breaking down mold. It’s ideal to pair it with HEPA to capture more spores.
What is PCO?
In photocatalytic oxidation, UV light hits a catalyst, usually titanium dioxide, creating hydroxyl radicals (OH) and superoxide radical anions (O2–). These molecules bind with and break apart pollutants, including mold and mycotoxins, into harmless molecules.
Best for Mold, Not VOCs
PCO technology is best for mold reduction, not for VOC reduction. I have a different post on the best air purifiers for VOCs.
The reason is that in real-life circumstances the chemical breakdown of VOCs is not a simple linear process. Byproducts like aldehydes can be (and often are) produced as an intermediary product of PCO.
With a high amount of hydroxyl radicals in a closed chamber experiment, it will almost certainly break everything down (with enough time) into harmless molecules, but in a home with relatively high VOCs, the results could look different.
Brands Under Fire
It has not surprised me (or really concerned me) that big brands have come under fire for some of their claims. What did shock me when I did the initial research is that smaller air purifier companies get away with outrageously false claims since no regulatory body even has time to get to them.
I would say most companies are exaggerating claims, the companies that had to correct some claims are still some of the best. There are still other scamy brands out there but I’m not going to mention them by name as air purifier brands are extremely litigious.
My focus is on the technology itself, and what studies based on that technology show us, and to remember that there are only a few types of air purifier technologies out there – their claims need to fall into one of a handful of categories.
2. HEPA Air Purifiers
HEPA filters do capture some mold spores and this article does include filters that use both PCO technology and HEPA.
If you’re looking at HEPA filters that don’t include PCO, but do include high levels of carbon for VOCs, the post on filters for VOCs will be more relevant.
3. Low-Grade Ozone Producing Air Purifiers
Some PCO air purifiers produce ozone either through an ionizer or through an oxone-producing UV bulb. I prefer ozone-free units for occupied spaces as many chemically sensitive folks are highly sensitive to ozone (it’s also possible fo highly sensitive folks to be reactive to OH molecules from PCO but that’s less common in my observation). Though CARB does allow for 0.050 parts per million ozone,
The Air Oasis I talk about below uses ozone to create ions and this can be very helpful in clearing up low levels of mold. If you’re not in the basement while you use this or you run it while you are out of the house I would not have a problem with it. (And of course, many people would feel fine with it while in the room).
4. Essential Oils that Kill Mold
To help clean up after mold has been remediated, or just to help with the musty smell that cannot be traced back to visible mold, fogging with an antimicrobial/antifungal blend can help break down molds and malodors.
You can use one of the antimicrobial blends in a waterless nebulizer while people and pets are out of the room. More info on the section on that below.
If you have already fixed water ingress at the source and still have a humid basement, use a dehumidifier to bring the humidity levels under 60% RH.
The Importance of Air Exchanges Per Hour (ACH)
Air exchange per hour (ACH) is how many times you are “replacing” the air in the room per hour, this is a key measure of effectiveness with air purifiers – both for HEPA and PCO. CFM is cubic feat per minute, that’s how much air the air purifier can move per minute.
ASHRAE (The American Society of Heating, Refrigerating, and Air-Conditioning Engineers) recommends a minimum of 4 ACH for patient rooms in hospitals, 5 for intensive care units, and 25 for operating rooms. For those extremely sensitive to mold and VOCs, 5-10 air exchanges per hour is ideal. 1 ACH is the bare minimum.
To maximize how much air in the room the air purifier can process, put it in a central location.
I made two calculators to calculate ACH from CFM and the CFM needed if you have the room size and desired ACH.
1. Molekule – PCO & HEPA
If I had to go back and pick one right now, I would choose Molekule.
This is a slightly different technology called PECO.
Here is a summary of their studies – very promising results on eradicating mold. The data they showed me showed it worked better on mold than PCO.
This company has well-respected big names behind the design. The inventor of Molekule is the person who discovered PCO. He has an impressive resume.
Their newest filter option also contains HEPA and some carbon.
I will be trying this machine out as I think it is very promising. I like that this maximizes PCO (presumedly more OH molecules are produced compared to other brands), plus it has HEPA now which also capture spores.
The warranty started as a 1-year warranty but now it is a 2-year limited warranty. The company has been around since 2016.
Molekule Air Pro
The Molekule Air Pro is the biggest model, it is advertised as cleaning up to 1000 sq ft. You will get 1 ACH in a room of 1000 sq ft with 8.5 ft ceilings. (So around 133 CFM).
The Molekule Air Pro now comes with a HEPA filter for the Pro. It also has some carbon. (Amazon still has two versions, one without the HEPA and one with HEPA).
The replacement filters for the Pro are $140 which lasts about 6 months, so replacement filter costs are fairly high.
The new Molekule Mini provides 1 air exchange per hour in 250 sq feet. They offer the HEPA filter option in the Mini now as well.
It sells for under $450 (often much lower, check the price here). Note that Amazon is still offering the Mini with and without HEPA.
Which Size Should You Choose?
In a room that is 15 by 15, the biggest model would get you 4 ACH. That is a good number of air exchanges to aim for if you want a high level of air purification.
Here is my calculator that helps you size an air purifier for your room based on how many times you want to overturn the air in an hour.
If you can afford it, and if design is important to you, Molekule with HEPA is my top pick.
2. Air Oasis – PCO & Ionizer with Ozone
The Air Oasis Ionic Air
The Ionic Air has three components, a PCO component composed of a germicidal UV light and a catalyst that is made of different metals to produce hydroxyl radicals; two, a bi-polar ionizer; and three it produces ozone (either through the UV light or via the ionizer).
The Ionic Air 3000G3 model is rated for 3,000 sq ft and only moves 11 CFM of air. 3,000 sq ft at 11 CFM is 0.02 air exchanges an hour. That is very little air movement. (I am using 8 ft ceilings in my calculations of CFM to ACH). But that’s not that important here because this is a unit that is sending out ions due to the ionizer and ozone components.
Technology – PCO
The company states that the PCO process creates “ionized hydroperoxides”. I have seen other PCO air purifier companies make this claim. This seems to me a confusing way to describe PCO which primarily produces hydroxyl radicals and secondarily produces superoxide radical anions (O2–).
Technology – Biopolar Ionzation
A bipolar ionizer produces negative and positively charged ions that can cling to pollutants, dropping them to the ground where they can be cleaned and they might be able to break apart (chemically) some types of pollutants or biologicals. This type of ionizer can sometimes be advertised as cold plasma ionization (source).
I’m a little wary of bipolar ionization which Dr. Marwa Zaatari, Mechanical Engineer, claims is not necessarily effective (in a now-deleted essay, I’m guessing because she has been sued by a big air purification company). However, I’ve also seen many units that collect pollutants on the wall behind them so they are doing something.
Technology – Ozone
The air purifier also relies on low levels of ozone for air cleaning technology. You can have one made without the ozone production component but that seems to be a key component of its air cleaning abilities. It meets CARB levels of ozone which are considered safe.
I generally would not use an ozone-producing air purifier. However, in a musty basement, I would definitely consider it (especially when the basement is not in use).
It has a 3-year warranty and the replacement parts are $80 every 2 years.
You can the Ionic Air through their website and the code CHEMICALFREE will get you 10% off.
Air Oasis iAdaptAir
The newer iAdaptAir is similar but claims to produce zero parts per million (ppm) of ozone. This is something I would be more likely to try, especially if the basement was in use.
It contains a HEPA filter, a carbon filter, PCO technology, and a bipolar ionizer.
Most ionizers give off ozone but it is possible to produce ions with minuscule ozone that does not register against background levels.
It comes in three different sizes for around $400, around $600, and around $700.
This is really a great little unit if you want HEPA, carbon, PCO and you want (or don’t mind) the ionizer.
Air Oasis Versus Molekule – A Comparison
The main comparison that I can’t quantify is the strength of the PCO component – just how many hydroxyl radicals do they produce. Molekule has a good reputation for working well even before the HEPA option was added.
On the other hand, iAdaptAir has a really good value for what it provides. The only downside for me is I’m not a big fan of bipolar ionization as I have my doubts about its effectiveness and I tend to be sensitive to ionizers.
Both are good choices, however.
Here’s the comparison:
Molekule Air Pro
- Modified PCO technology
- True HEPA filtration for particulates
- Some carbon for VOCs
- No ozone
- CFM ~133
- Costs around $1000 but usually on sale for around $800
- Replacement filters $200 per year
- Modified PCO technology
- True HEPA filtration for particulates
- Some carbon for absorbtion of VOCs
- Bipolar ionizer
- No ozone
- CFM 151
- Costs around $600
- Replacement filters $100 per year
3. Vornado – PCO & HEPA for Less
They have true HEPA and activated carbon in addition to UV light and titanium dioxide. Most PCO units are much pricier or don’t include all three air purification methods.
True HEPA and activated carbon capture dust, pollen, pet dander, smoke, bacteria, mold spores, dust mites, and odors including VOCs.
PCO actually breaks down molecules including molds and mycotoxins.
Why I chose this unit:
- The PCO component has true UV and titanium dioxide
- Respected brand name
- Has been used by many highly mold-sensitive folks since I wrote this and the vast majority like it
- Has a 10-year warranty
- Replacement parts are reasonably priced ($25 bulb every year, $35 titanium dioxide screen every 5 years – for the PCO-related parts)
- Does not put out ozone
What I don’t like about it:
What I don’t like about it is that the unit itself offgasses a little bit (I am extremely sensitive). Not everyone thinks so or would notice this. After two weeks I found it to be good. I’m happy with it offgassing within two weeks. (Anyway, all air purifiers on this list offgas a little).
The other drawback is that the amount of titanium dioxide is quite minimal so it may not be producing as many hydroxyl radicals as other PCO units. I am going to go with Molekule if I try another brand of PCO air purifier.
Model 375 Versus 575
The difference between the two sizes is that the PCO575 has a lower low speed and a higher high speed. The PCO575 has 2x the HEPA and 2x the activated carbon of the PCO375.
They both have the same PCO technology – so if you want to increase the effectiveness of the OH molecules in a large space you would want two of the PCO375 instead of one PCO575.
The 375 has a 113 CFM on high, 28 on low.
The 575 moves 156 CFM and is advertised for 258 sq ft., which would get you about 5 ACH in that room.
Just want HEPA? The Vornado AC550 does not contain PCO but it’s a powerful HEPA filter. It contains two HEPA filters and two small carbon filters. In an 18 x 18 room it will get you 5.1 air exchanges per hour (so around 216 CFM, which is good). Plus it’s the best bargain for a HEPA filter under or around $250.
Vornado versus Air Doctor
The Vornado AC550 does not contain PCO so it’s the closest comparison to the Air Doctor. It contains two HEPA filters and two small carbon filters. Around 216 CFM. Cost: under or around $250.
Air Doctor does not contain PCO. It’s mainly a HEPA filter. It has a little carbon just like the Vornado, only 2.2 lbs of carbon which is not much (or double that for their biggest model). The 3500 model has 340 CFM. So you do get quite a bit more air movement, but you’re also more than doubling the price (so I would rather get two Vornados and get a total of 432 CFM). Cost $630.
If you want to have the added PCO capacity to break down mold the Vornado PCO575 contains PCO, two HEPA filters, and 2 carbon filters (a couple of pounds each). CFM 156. Cost: Around $400.
4. Essential Oils to Clean up Mold
Essential oils can also be used to clean up mold spores and reduce a musty smell in a basement. This assumes that you have already done the remediation work and corrected the source of the water which I talk about in the beginning of the article.
This is also not to be used at the same time as an air filter.
Many people like to use this post-remediation as the essential oils “digest” the mold spores and break down some odors.
You want to use a blend of antimicrobial oils like Germ Fighter by Plant Therapy which contains Cinnamon Cassia, Lemon, Eucalyptus, Clove, and Rosemary. This is a well-priced non-MLM brand that is high-quality and legit (they produce GC-MS testing by several third-party laboratories).
There are more options in my article on essential oils that kill mold.
Diffusing/nebulizing essential oils is the best way to deploy them to kill mold spores in a building. I would do this when people and pets are out of the room because diffusing creates high levels of VOCs and PM2.5 particulates.
Dr Close recommends that you place one waterless cold air nebulizer diffuser in each room (one for every 1000 sq ft) and diffuse continuously for a minimum of 24 hours. More detailed instructions can be found here.
Corinne Segura is a Building Biologist Practitioner with 8 years of experience helping others create healthy homes.
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This post was written with the technical assistance of an electrical engineer, though the opinions and conclusions are my own.