From what I have seen, you want to have HEPA or ULPA Filtration, but another factor is the flow rate of the device/the refresh rate of the room. If you have an ULPA filter in the device, but it has a very low flow rate, it could actually be less effecive than other seups despite it's good filter. I think also you may want to have a well sealed device so air can't flow past it and bypass the filtration.
This Page is Worth a read: http://www.airpurifierguide.org/faq/cadr , As well as various Wikipedia Pages
I have been thinking about more large scale setups like cleanrooms with the Open Source Cleanroom Construction Set , but it may still apply somewhat here.
I think another good project would be a 3D Printed Fan + Filter + Electronics Housing Enclosure of Sorts.
Anyways just some thoughts, hope I could help.