There are many types and sizes of air cleaners on the market, ranging from table-top models to sophisticated whole-house systems. Some air cleaners are highly effective at particle removal, while others, including most table-top models, are much less so.

The effectiveness of an air cleaner depends on how well it collects pollutants from indoor air (expressed as a percentage efficiency rate) and how much air it draws through the cleaning or filtering element. A very efficient collector with a low air-circulation rate will not be effective, nor will a cleaner with a high air-circulation rate but a less efficient collector.
http://www.epa.gov/iaq/is-imprv.html#Air%20Cleaners