Researchers have invented a prototype new form of high-performance air purifier that promises zero harmful waste and is 99.999% efficient in removing common bacteria and viruses::New purifier and filtering foam invented by University of Bath engineers is ready for commercialisation, seeking partners
This is very good advice. I’ve made a couple - far from beautiful or quiet, but very effective and very cheap.
This is a good page from an early DIY promoter: How to Make a DIY Air Purifier