The filter: Make sure it comes with a genuine HEPA {High Efficiency Particulate Air} filter.
The Clean Air Delivery Rate {CADR}: This is like the horsepower rating of the purifier—or the tonnage rating of an air conditioner—and this is what will help you determine the right air purifier for your space.
The larger the room, higher the CADR you need. For a 300 sq-ft room, the CADR in CFM should be at least 200 {340 in CMH or m3/h}.
Replacement filters: You will have to change your HEPA filter every few months or even every few weeks in the most polluted of times, so take a look at the replacement filter cost and availability.
Pre-filter: This is a filter that goes before the HEPA filter and filters out larger particles like dust and hair so that the HEPA filter can last longer. If the pre-filter is washable, that’s better.
Activated Carbon Filter: This is optional and its purpose is to remove smells and gases, like formaldehyde. Not everyone needs a carbon filter but it’s good to have.