@jeff1047 You brought up another good point. My house is low VOC. We worked with a builder and did everything we could to reduce formaldehyde that is common to a lot of building materials. We used formaldehyde free insulation and put in prefinished hardwood floors everywhere except the foyer and bathrooms that are tile. That was over plywood which did have glue containing some VOC, but I seal coated the plywood floors with Safecoat (consistency of painting with milk) before the hardwood was laid. I wasn't able to coat the bottom side of the plywood that formed the floor. The builder wouldn't allow seal coating the plywood before installation, and they wanted an unreasonable amount of money to do this extra step. We had a flooring company come in after we purchased the house to install the hardwood over existing plywood. The raw wood staircase and banister were stained and varnished with polyurethane. We have no carpet.

Twenty years ago, this was a new neighborhood under construction, and I would get a headache if I visited inside a neighbor's house within an hour, and I was fine in my own home. It lessened my allergies a lot. I do have other allergies here, because we have a river with wetlands behind the house, so I think mold counts are always higher in the area, and I can get sinus and asthma issues on days when mold counts are high outside. The water levels fluctuate with rain fall, leaving a lot of plant surfaces to grow mold.

If you talk to Foust, you can order bulk coconut carbon with the permanganate to try. Why not put that in your existing filter systems to see if the permanganate helps? I think it does help. When it is spent, the little permanganate spheres turn from purple to brown. I talked to them when a motor on a smaller unit died, and they suggested looking for some used units because they don't fix them and replace the motor. I bought 2 that I found on ebay, and Foust didn't mind selling me media for those. They have always been helpful and answered questions. I guess I haven't been paying attention to changing the media tray. Perhaps after 4 years, I should do that. The air coming out of the 400 unit still smells fresh, so I haven't thought about it. FYI, the size of the media and HEPA filter box for the 400 is 12" x 12". I'm not sure how deep it is without taking the unit apart. Here is the page about carbon choices from Foust. https://foustco.com/carbon-choices