If you need an air purifier for micro-allergens such as pollen, mold spores, dust mite debris, smoke, pet dander, dust, etc., and don't want to spend hundreds of dollars on an expensive machine like the ones on HSN, you can do what I did: For about $25 - $35 dollars, you can make your own using materials easily found at most hardware stores. All you need is a standard plastic 20"X20" box fan, typically sold for under 20 dollars, and a high quality air cleaning filter, also 20"X20", that is meant for furnaces. There are many sizes and strengths of air filters, for different purposes, and the ones that are considered equivilent to HEPA standards will start at about $10 to $15, more if you want extra protection against airborne diseases. I bought a 3M Filtrate "MicroAllergen Reduction Filter" for about 14 bucks. You just tape it up to the front of the fan, with the arrows on the cardboard filter case pointing away from the fan blades, turn it on and enjoy clean air. I put a cheap $1.00 Ace Hardware furnace filter on the intake (opposite/rear) side of the fan, just to filter out the big particles before they reach the fan blades and front filter, figuring that it might help to make the front filter last longer - but using only one good filter on the front by itself works just as well, and the only maintenance is replacing a dirty filter(s) every few months. There are several good videos demonstrating how to do this, on YouTube.
Yup I own 2 air purifiers (didn't make them myself, bought them) and while they set me back about $500 total, I rarely have allergy issues in my home anymore And they get rid of the smell of weed really quickly too