I bought my 1st ionizer air purifier in the 1970's. I hoped to get help with asthma and severe allergies. Did it actually work? Just the reverse: It brought on an asthma attack, and I'll explain why shortly. It was many years later that the Sharper Image model was removed from the airwaves for this very reason.
The washable "plates" in many air purifiers create dangerous levels of ozone.
This has caused much confusion for consumers. And most consumers don't know the difference between ions, ionizers and ozone.
Ionizers do something different from ozone. Ionizers aren't made intentionally to create ozone, they work in a different way, but some ionizers just (because of their design) happen to make ozone.
Perhaps the important major point about ionizers is that they are not particularly good for pollen and dust. The plates simply do not work well since most dust and pollen does not have very much electrical charge. The static electricity from the plates, which also creates the zapping noise often complained about, however does attract some dust.
Several "ozone-machines" are available for sale today with popular names like Ecoquest or Vollara. These ozone machines send ozone out of the machine into the room, they don't clean the air inside the purifier itself.
The same way ozone can kill mold and odor too much can also attack cells in the respiratory track and sinuses bringing irritation.
These flat "plates" have a tendency to create ozone because of the positive electrical charge. But it's not what they are trying to do.