It's been a year since we first heard that ionic air purifiers can actually make air quality worse,
and that finding has now been confirmed by a peer-reviewed, government-funded study. According to the study, which appears in the Journal of the Air & Waste Management Association, using an ionic ozonolysis air purifier in a sealed room can lead to ozone levels as high as 350 parts per billion, a level equivalent to a Los Angeles Stage 2 Smog Alert. The EPA has already come out against ozone-producing air purifiers, saying that they have "little potential to remove indoor air contaminants."
Update: As pointed out below in comments, the study quoted above apparently referred to both ionic air purifiers and ozone generators. However, many media outlets, including LiveScience (our source for this post), conflated the two. According to lead researcher Sergey Nizkorodov, a chemistry professor the University of California, Irvine, "ionic air purifiers do emit ozone. [But] none of the ionic air purifiers produce enough ozone when they are used properly to exceed smog alerts." LiveScience has since added a correction, which can be viewed here.