Look this question up and you get lists of purifiers to buy and product reviews. None of them show you what was in the air before and after. So we built a do-it-yourself filter box and put plates in the room.
This is a do-it-yourself test, not a lab study.
What a Corsi-Rosenthal box is
A Corsi-Rosenthal box, or Corsi box, is a do-it-yourself air purifier made from hardware store parts. Ours is the standard build: four MERV 13 AC filters taped into a box, a cardboard bottom, and a box fan on top blowing up and out, so the room air is pulled in through the filters.
How we tested it
A 160-square-foot bedroom, door closed, AC vent taped off. Same house and same room as our test kit experiment. Each plate is a dish of growth medium, opened and left in the room for exactly one hour, then sealed. Whatever settles on it grows over the next few days, one colony for every spore or cluster that landed.
- Before: an air sample from the same room, several days before the test.
- With the box running: a plate opened at 2, 4, 8, 12 and 24 hours after the box was switched on.
- The clean side of the filter: one plate taped to the outside of one of the filters for an hour, to see whether anything comes through.
What grew
Before
2 hours
4 hours
8 hours
12 hours
24 hours
Filter, clean side| Plate | Colonies after four to five days |
|---|---|
| Before the box | Under 10 |
| 2 hours running | 2–3 |
| 4 hours running | About 4 |
| 8 hours running | 1–2 |
| 12 hours running | 1 tiny |
| 24 hours running | None |
| Clean side of the filter | 1–2 small |
These are counts from the photographs, not a lab count. The direction is what matters: the longer the box ran, the less landed. By 12 hours the plate was nearly clean. By 24 hours it was clean.
Some of it still goes through the filter
The plate on the clean side of the filter grew one or two small colonies. That is not the box failing. It is what the filter rating says will happen. A MERV 13 filter is rated to catch at least 85% of particles between 1 and 3 microns, and at least 90% of particles between 3 and 10 microns. Most is not all. Some mold spores are small enough to be in the share that gets through.
The box still cleared the room, because it keeps passing the same air through the filters over and over. Each pass catches most of what is left.
It manages the air. It does not fix the mold. A filter takes out what is floating in the room, and in our test it did that well. It does nothing to the growth on a wall, the moisture feeding it, or what is inside a wall cavity. That is all still there.
Where a box like this makes sense
It is a reasonable thing to run in a room while the real problem is being found and fixed. It is not a substitute for finding it. If the moisture is still there, the growth is still there, and so is whatever it releases. Finding the source and removing what grew is the actual mold remediation.
One note for this climate: this is a separate box. It is not a reason to put a MERV 13 filter in your air conditioner. In the AC, a filter that dense can choke airflow, and an air conditioner that cannot move air cannot pull water out of the house. That is a different subject, covered in how to lower the humidity in your house.
The honest limits
One house, one room, one box, one day. Plates on a counter are a homeowner’s test and they catch what settles, not everything in the air. What they show clearly is the direction, and the direction was not close.
Common questions
Does an air purifier help with mold?
Yes, it collects spores out of the air. No, it does not fix mold growing somewhere in your house. In our test a box-fan filter took one room from a handful of colonies per one-hour plate to none within 24 hours.
What is a Corsi-Rosenthal box?
A do-it-yourself air purifier: four MERV 13 AC filters taped into a box with a box fan on top blowing up and out, so room air is pulled in through the filters. It is made from hardware store parts.
Does a Corsi box work for mold?
For spores in the air, it did in our test. By 12 hours the plate was nearly clean and by 24 hours it was clean. A plate taped to the clean side of a filter still grew one or two colonies, so some spores get through. It does not touch the mold itself.
Will an air purifier get rid of mold?
No. It filters what is floating in the air. The growth, and the moisture feeding it, stay until they are found and dealt with.
Should I put a MERV 13 filter in my air conditioner instead?
In this climate, we would not. A filter that dense can restrict airflow, and an air conditioner that cannot move air cannot pull water out of the house.
Source: EPA — What is a MERV rating?
Current as of September 2026.