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Why does the air smell and feel so good after a spring rain shower? It was that way this morning, so I thought to ask the person who has the answers to the really tough questions of life. --Thomas Vastine, Villa Park, Illinois

Tough is right, buddyroo--what you're innocently asking me to do is jump feet first into the dread negative ion controversy, which has been raging off and on for more than 30 years. According to true believers--Ionists, we call them--the reason the air feels so good after a rain shower is that there's an abundance of negative ions floating around. Negative ions are air particles (mostly CO2) that have an extra electron and thus a negative electrical charge. They're also created by waterfalls, another well-known source of good vibes. But how or even if they can really brighten your mood has never been satisfactorily determined.

Positive ions, which are missing an electron, are thought to have the reverse effect of the negative kind--they're depressants. At one time positive ions were thought to be produced by hot, dry winds such as the sirocco, chinook, and Santa Ana. Supposedly that's why the winds made people so cranky. But more recent research suggests that the correlation between winds and ions has been greatly exaggerated.

Another place that allegedly suffers from a surplus of positive ions is an office with a lot of computer display terminals. VDTs emit positive voltages that neutralize negative ions, allegedly causing workers in the vicinity to get headaches, dizziness, and nausea. Hence the brisk sales of portable negative ion generators, which you can get by mail order for anywhere from $50 to $250.

A lot of scientists, however, say ion generators are a waste of money. They recall the 1950s, when unscrupulous promoters claimed negative ion generators would cure cancer, emphysema, and just about everything except the heartbreak of psoriasis. There was no evidence to support these claims and the Food and Drug Administration eventually forced most of the sharpies out of business.

Still, a few reputable scientists have persisted in trying to prove that negative ions do offer benefits--with some success. For starters, it's now generally agreed that negative ion generators help cleanse the air of smoke and dust particles (although a few holdouts still argue the point). The negative ions attach themselves to the particles and then, acting as a sort of electrical glue, cause them to stick to a wall or some other grounded surface. That's why negative ion generators are popular in offices polluted by smokers.

One scientist found that workers using VDTs had fewer on-the-job health complaints when surreptitiously doused with negative ions. Negative ions also appeared to improve scores on coordination and reaction-time tests. But sensitivity to ions seems to vary widely among individuals. Two-thirds of the population may be immune to the ill effects of the positive ones, for example.

Today there seems to be an emerging consensus that ions do have definite if limited biological effects. But how they work remains mysterious. Efforts to show they have some direct effect on the nervous system have met with little success. Early experiments suggested a connection between negative ions and serotonin, a brain chemical that in excessive amounts has been linked with irritability. But these proved difficult to replicate, and it's starting to look like the brain chemistry angle is a dead end.

The simplest explanation is that negative ions work because they clean the air. (Although, you'd have to think, so does falling water--in which case why resort to negative ions to explain why a rain shower makes the air feel good?) Should this sound too mundane, one researcher I spoke to held out hope for--get ready for this--a "bioelectric" hypothesis. I didn't quite follow the details, but he did mutter something about a link to acupuncture. Two guesses what state this guy lived in. Hint: it wasn't Rhode Island.

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

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