So what should you buy? When it comes to air purifiers, it really comes down to square footage. Most dorm rooms are in the 150 to 200 square foot range (there are exceptions!), and you want an air purifier that exchanges the air 4 to 5 times per hour. I know the Minis are tempting - the price point is attractive, and some come in really cute colors - but a Mini simply cannot keep up with that exchange rate. You really need to look for one that is rated to handle your actual room size at that exc