Before buying a tabletop or room air cleaner / air purifier, pay attention to how many CFM (cubic feet per minute) of air move through the system. For each 250 square feet of space, you’ll need about 100 CFM. Room models can handle up to 400 CFM, and tabletop models can handle 175 CFM. Also be sure that the model you choose is quite and do not make much noise. Also check price of replacement filter, life of the air cleaner. Make sure that the filters are easily available so that you don’t have to scratch your head later.

The best types of filters are called ULPA and HEPA filters. A 12 x 16 x 6-inch filter will be much more effective than an 8 x 8 x 6 inch filter.

Whole house air cleaners quite and much more efficient. It has the capacity of about 1,000 CFM. But they’re relatively expensive and harder to install.

Prices for room air cleaners range from $200 to $1,000. Tabletop air cleaner units cost from $50 to $200.

Area Coverage: Before buying an air cleaner, make sure the square area coverage listed for the air purifier is about the same or slightly greater than the square area coverage of the room where you intend to use it.

Air Changes Per Hour (ACH): ACH rating tells us how frequently the air purifier can exchange all the air in a particular room. E.g. if ACH of the air cleaner has a ACH rating of 6 for a 20' x 20' room, then it is capable of exchanging all of the air in that room 6 times every hour.

Clean Air Delivery Rate (CADR) – CADR tells us not only how much air is purified, but also how well it is purified. Air purifiers that have been tested by AHAM should have the AHAM Certified seal and CADR numbers for three pollutants: tobacco smoke, pollen, and dust. The higher the CADR rating, the better will be the air purifier.

Filter Replacements: If you're buying a HEPA filter, check how much replacement filters will cost. If your air filter includes both a HEPA filter and a carbon filter or other combination of filters, they will probably need to be replaced separately. Make sure you have easy access to these filters.

Noise Level: Some air purifiers are extremely quiet, while others can be quite loud and noisy when operating at high power. Ask for a demonstration before you buy your air purifier.

Energy Usage: Different air purifiers use different amounts of energy for operation. Check for power consumption to save money on your electricity bill.

Air Pollution: Some air purifiers pollute the air with ozone which is a health hazard. Keep distance from these air cleaners that pollute the air with ozone.

Additional Features: Some additional features can make your air purifier a lot easier to use. For example, a filter change indicator light lets you know when to change the filter, and handles are important if you plan on moving your air purifier around a lot.

Manufacturer's Reputation and goodwill: A manufacturer's reputation and the experiences of past customers deserve consideration in your decision of buying an air cleaner. Check out manufacturer’s goodwill and customer reviews.

Warranty: Look out for longer warranty. Also check if motor, fan, electrical components are under warranty.