It is highly recommendable to install filters in central air conditioning and heating systems. But even so, these filters will be useless if the fan is not running, and given that a lot of people only start up the system occasionally, stand-alone purification systems are the best option for filtration.
Even a high-efficiency stand-alone purifier may not prove to be a totally effective measure, since it must process a high enough volume of air. A measurement system is used to gauge the required air volume, which is expressed as a number of changes per hour. This represents the number of times that the purifier can clean the air in a room. The ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends air to be changed between 5 and 7 times an hour in offices where there are smokers.
All central filter systems and stand-alone purifiers will be ineffective when doors or windows are left open, as they draw in more air than the system can handle. They will also be ineffective if the interior is very dirty or has a source of pollutants, such as a problem of active mould growth.
