Picking the best air purifier is a very difficult decision since the need of each allergy or asthma situation has different needs. Adding to this tough task is different allergens and different types of home pollutions are removed in different ways. Below are some comparisons of some of the more popular type of air purifiers.
air purifiers with HEPA (High Energy Particle Arresting) filters were originally developed for the atomic energy commission and are used in commercial clean rooms to remove particles of dust and other airborne pollution. This is a paper or polymer sheet with very tiny holes that allow airflow to pass though the material but not very much else. The holes are so small you need a lot of surface area for a reasonable amount of airflow to pass though the material. It is because of this that the medium is always folded in an accordion or zigzag fashion to get a lot of surface area in a small area. There is also a mixed version of the technology that one manufacturer uses where the holes in the HEPA filter are slightly larger to promote more airflow but the air is charged with electrostatic wands. Which then attracts any of the smaller particles that get thru the holes. HEPA Filters work well in the home for trapping particulates. The disadvantage is that the filters must be changed regularly (every 6-months to 5-years depending on the manufacturer). Before purchasing you would also want to review how much air by-passes the filter, fan noise, how long the filter is good for and finally price of both original unit and replacement filters. The review of any HEPA unit depends on the efficiency (how much air by-passes the filter), fan noise, filter life and price.
UV air purifiers use ultraviolet light. Ultraviolet light in the right intensity destroys the DNA of microorganisms such as bacteria and viruses. The disadvantage of a UV air purifier is that removes very few gas pollutants and no air particles. It would be necessary then to purchase a UV air purifier along with another type or one that combines with another filter system.
Ion Generator air purifiers are small air purifier devices that put out a stream of negatively charged Ions. These ions charge particulate pollution and when the particle bumps up against something it sticks due to the electronic charge. These devices are used on US submarines and in the poultry industry to keep the feather dust down.
These air purifiers do what they say but there are some very distinct disadvantages. Particulates end up on your walls so your walls turn black near the device. Secondly, since they typically do not have a fan they only work in a localized area. They also do nothing for gas pollution and some may put off some dangerous Ozone.
You can see just with the 3 examples above there are differences in air purifiers. Research and comparison shopping is necessary to find the right purifier for your specific case. Do not allow promotional hype to sway your judgment, picking the wrong air purifier is as bad as not having one at all.