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Ionic air cleaners are any kind that includes as part of its system an "ionizer," which is a device used to help the filtration system purify the air. An ionizer charges particles in the air, whether they're dust, pollen, allergens, or whatever. The particles get a negative electric charge. If you remember fiddling around with magnets as a kid in science class, you'll remember that opposites attract - and those particles will stick to anything that has a positive charge. The air cleaner takes advantage of this by sending the air past a metal plate once it's been charged. Those particles will then stick to the plate on the way out, rather than going out with the air. Usually you'll have to periodically wipe down the plate with a paper towel to clean off all the gunk that accumulates.
Most cleaners that use an ionizer don't use it on its own - they will have a "layered" approach, where multiple different cleaning systems are in a single machine. The theory is that if dirt gets past one, it won't get past the other. Another reason for this is that the most effective filters are also the most expensive to replace. So the ideal system uses a cheap filter (like an ionizer) first, to keep dirt from clogging up the more expensive filter as quickly. You'll find them in many air purifiers for that reason.
Pros: You don't have to replace the plates, so that saves you a bit of money. If the purifier uses another filter as well, less dirt will get into it (so you won't have to replace it as often). When used in conjunction with other filtration systems, it can be very effective.
One other benefit is that some studies have suggested that ionizers kill off certain kinds of bacteria (and reduced infection rates when installed in a hospital).
Cons: You have to clean them pretty frequently. Also, a lot of times it's not just the plate these particles will stick to. Anything that gets past will stick to other surfaces, like the floor or a wall. This can mean that in the area immediately around the air purifier, you'll see dust accumulate and you'll have to be on guard to clean it up. If your air purifier only has an ionizer, and nothing else, it may not work that well (most have multiple filtration systems). If it only has an ionizer, it can't do anything about odors.
For a negative article on ionizers (probably too harsh in my opinion), you can go here. The other risk it points out is that some create ozone, a potentially harmful chemical, as a byproduct. I'd read this article as a counterpoint, however - it has a lengthy correction to similar statements, and concludes that ionizers don't produce harmful levels of ozone (and that tests showing that were referring to a different kind of air purifier).
Verdict: You want your air purifier to have an ionizer, but you want it to have other kinds of filters, too.
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