Pre-filter Most air purifiers have a pre-filter which is usually a carbon filter designed to remove the coarser particles which would otherwise clog up the main filters or other processes within the purifier. It may remove some odor but shouldn’t be considered for serious odor removal. HEPA filter There are now replaceable true HEPA filters and permanent true HEPA filters which are designed to be cleanable but will eventually need replacing when efficiency starts to degrade. There are also HEPA-type filters which are cheaper but aren’t as efficient in removing the smaller particles. True HEPA filters capture at least 99.97% of all particles passing through the filter of 0.3 microns and less. Electrostatic There are two types of electrostatic process but both work on similar principles. The airborne particles are negatively charged within the unit and then collected either on a positively   charged collection plate in the first method or in a positively charged filter in the second method. I think the first method is more common. You should check that these devices don’t produce more ozone than is healthy. Ionization Ionization creates negative ions or in some cases negative and positive ions. The problem with this process is that it doesn’t actually remove pollutants from the room but causes them to stick to surfaces in the room or to stick together and fall to the ground. This can lead to soiling of surfaces like curtains and carpets. It can also product ozone as a by-product. So, as with electrostatic devices, make sure the level of ozone is within limits.  

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