Several people have recommend you air purifiers. I would add one thing, look at the price of replacement filters. Air filters are similar to printers in a way. You become tied to a company and have to buy replacement pieces every year or so. Some have bad track record (quick device depreciation because of out of stock filters, etc.)
Just a plug for Coway Airmega. I bought the 400, which is expensive, but it was for someone with health issues. I absolutely love it. Super quiet on low speeds and has been about as low maintenance as one could expect. Filters seem to be of exceptionally high quality and overall build is great.
I purchased several stand-alone Air Purifier appliances for something like $100-200 each [0]. They have two kinds of filters that work in tandem, you replace one every month and the other every three months. It gets expensive to replace all the filters when you have several of them, but they pull a ton of junk out of the air.
> I started putting HEPA air filters all over my house
Can you very briefly say how you accomplished this? Was it with modified box fans? Thank you!