Oh yeah and also take a look at the CADR rating of your purifier to see what size room it has been measured as being sufficient for.
The AHAM seal (usually found on the back of an air cleaner's box) will list three CADR numbers - one for tobacco smoke, one for pollen and one for dust. The CADR indicates volume of filtered air delivered by an air cleaner. The higher the tobacco smoke, pollen and dust numbers, the faster the unit filters the air.
Follow our 2/3 Rule:
You'll always want a unit with a tobacco smoke CADR at least 2/3 your room's area. For example, a 10' x 12' room -120 square feet - would require an air cleaner with a tobacco smoke CADR of at least 80. If your room size is smaller, the unit will simply clean the air more often or faster. If you have ceilings higher than 8', you'll want an air cleaner rated for a larger room.
http://www.cadr.org/consumer.htm
Directory for Popular Purifiers:
http://www.ahamdir.com/dirsvc/aham.n...Air%20Cleaners