The Straight Dope 

A friend of mine is selling these "corona discharge" air filters--she has severe asthma and swears this little gizmo is better than pooping gold bricks. Is this truly the miracle we asthma sufferers are looking for? This unit claims to be a "miniature miracle" made possible by "a revolutionary discovery in electrodynamic negative corona discharge purification technology." Supposedly it's "more advanced than any filter/fan air purifier or HEPA filtration device" and cleans air "naturally....Similar to a thunderstorm, a corona discharge of negative high energy is passed through the pollutants." In layman's terms it's supposed to zap any particle large enough to be an allergen that passes through it. Or at least that's how it was marketed to me. How about it, Cecil? Is it the answer to my prednisone prayers or a crock of shit?

--Breathlessly awaiting your answer, L. Hayes, Oregon

Maybe it's just me, but when I hear somebody go on about revolutionary discoveries, negative high energy, or anything to do with ions (which are what a corona discharge device produces), I make sure I've got a firm grip on my wallet. Still, one never likes to dismiss things out of hand. We even tested laundry balls, remember? So I sent Jane to rummage through the archives. The assembled research presents the usual inconclusive muddle. Some observations:

Air ionization is to today's scientists what magnetism was to medical types in the 19th century. In other words, a possible cure for just about anything. A Web search turned up nearly 400 scientific articles on air ionization, claiming that negative ions do everything from reduce asthma symptoms to improve your mood and enhance children's learning ability. Yet the methodology of these studies is sometimes suspect. For example, a couple papers claim that daily sessions with a high-output negative ionizer will alleviate seasonal affective disorder (winter blues). But one study measured the efficacy of "bright-light" therapy (which is pretty much what it sounds like) in alleviating SAD while using negative-ion therapy as a control, that is, as a presumably ineffective therapy intended to provide a baseline comparison. Bright-light therapy alleviated SAD symptoms, but no more than negative-ion therapy did. Normally you'd consider this evidence of the placebo effect--the participants thought the therapy would do some good so they persuaded themselves it did. In this case, though, the investigators stood this conclusion on its head: bright lights and air ionization aren't equally bad, they're equally good! Whatever you say, guys.

Air ionizers may improve some things, but they don't improve asthma. I'm not saying ionizers are useless. Studies found that they reduced transmission of airborne disease in poultry, cut down on microbial pollution in a dentist's office, etc. But they didn't help asthma and arguably made things worse (Warner et al, 1993).

HEPA filters don't do much for asthmatics either. HEPA filters are often recommended to screen allergens out of the air and alleviate asthma, but Antonicelli et al (1991) concluded the filters didn't do squat. Reisman et al (1990) say "the overall impression" is that HEPA filters reduce asthma, but once you look closely at the study, your overall impression is that Reisman and company are kidding themselves. One possible explanation, which may also account for the ineffectiveness of ionizers, is that asthma is caused less by airborne particles than by contact with objects, e.g., an allergen-infested mattress.

Nobody knows what the "HEPA" in "HEPA filter" stands for. Writers in the scientific journals almost universally believe HEPA stands for "high efficiency particulate air," whatever that is. Consumer Reports, however, says HEPA stands for "high efficiency particulate absorbing." I vote for CR.

Negative ions do something to the learning ability of kids, but God knows what. Morton and Kershner (1990) report, "On a dichotic listening task using consonant-vowel combinations, both [learning disabled and normal-achieving children] showed an ion-induced increase in the normal right ear advantage." The normal right ear advantage?

To cure asthma, we must turn to the mysterious East. Studies claim that asthma symptoms can be reduced by yoga (Nagendra and Nagarathna, 1986) and by transcendental meditation (Wilson et al, 1975). Sure. My advice: don't get rid of the prednisone yet.

Art accompanying story in printed newspaper (not available in this archive): illustration by Slug Signorino.


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