Well  to make it simple the USA military has tested 100 air purification units and they selected Austin Air.

If you want go about it in detail here are the key things to look for: 

1)      The first and most important principle is the  air purifiers ability to mix air. In other words the greater the area an air purifier can disturb air the more air it can clean, or the further the distance the unit can push air from its intake the better. Look for a unit like Austin Air Or Aller Air who have a large input area and small output area. This cause the air to be accelerated faster out of the machine than into the machine and prevent short circuiting. Short circuiting is when the exhaust air re-enters the input of the unit without mixing with the air in the room, this  not  good. It should be noted the this acceleration of air causes the unit to make noise (not motor noise but a whooshing sound). In lay terms if it doesn`t make that whooshing sound it does clean air effectively

2)      Make sure the unit has a certified HEPA filter. This means that every filter must be tested and certified to meet the HEPA standard of particle capture rate of 99.97% “ 0.3 microns. Unfortunately there are companies out there that claim their filters are HEPA but have not had each filter tested, so buyer beware.

3)      The greater the square inches of HEPA, the better. For example Austin Air has 80 square inches.

4)      Carbon, this is for gas phase toxins as well as odours. Typically the more carbon the better, however there are different types of activated carbons and other additives that expand the range of gas phase toxins it can absorb. There is no such thing as super duper activated carbon, these type of schemes  are simply marketing ploys to get your money.  For the most part the carbon offered in both the Austin Air and Aller Air are sufficient. However, for those that have multiply chemical sensitivity may need activated carbons with added Potassium permanganate  or alumina  to expand the  it chemical range. Both Austin Air and Aller Air offer different carbon filters, however Aller Air not only has a separate carbon filter, but it can be customer blend to meet the client’s needs. It should be noted that some people with multiple chemical sensitivity can have a negative response to some carbons. For these folk Aller Air offers the best option as you can order a carbon kits to determine which carbon you do NOT react to.

Ask a CIE which is the best option for you.