Area Coverage:
Always be sure that the square footage listed for a particular model is slightly greater than the square footage of the area in which it will be sued.

Air Changes per Hour (ACH):
How many times per hour the unit can change the air in the room can be very important, especially if you suffer from severe allergies or asthma. The most effective unit will sport a ACH rating of 4, 6, or preferably 8.

Clean Air Delivery Rate (CADR):
Tells you how much air the unit is capable of actually purifying. The higher the rating, the more effective the air purifier is against battling indoor air pollution.

Filter Replacements:
All filter systems need to have their filters replaced on a regular basis, which can become costly. Be sure to take replacement costs into consideration when choosing a model. Also, watch for systems that offer duel filter technology, since these filters usually need to be replaced at separate intervals that may be a hassle for some consumers.

Noise Level:
Be sure to test the unit you are considering since some models can be very noisy while others are extremely quiet.

Energy Usage:
Like all appliances, air cleaners use different amounts of electricity. Also consider whether or not your unit will be run on low, medium or high for most of the day to determine the correct energy usage and cost.

Air Pollution:
Believe it or not, some air purifiers (especially various ionic models) may actually emit enough ozone into the room to rival the air in one of the smoggiest cities in the nation - Los Angeles. Make sure that your model isn't one of them!

Extra Features:
Like any appliance, air purifiers come in basic standard models, and those that contain a myriad of bells and whistles. Remember, you pay for those extras, so be sure you really need them before agreeing to purchase them.

Price:
It doesn't matter whether you have $50 to spend or $5,000 t spend on your new air purifier, there's a model out there to meet every consumer's need and budget, so don't let an anxious salesperson convince you otherwise.

Warranty and reputation:
With so many different manufacture's and models of air purifiers on the market, it isn't easy to know who's are the best. Be sure to ask friends and neighbors what they like and don't like about the system they use before buying your own. Check the Internet for customer reviews, and make sure that the manufactures you choose offers a good service agreements and/or warranty on all the products they sell.