Hey, Cecil: My wife and I had a very unpleasant experience selling our house after our buyer’s inspector came back with a report of vermiculite in our attic. After a week of stress and an $80 test, what we had was not vermiculite at all. A second inspector reported the same. All the advice I could find was the standard “Leave it alone and you’ll be fine.” So what’s the Straight Dope on vermiculite? — Sparky in Ottawa

There are certain words — many, alas, are medical terms — that typically don’t make it into your vocabulary unless you or someone you know has to deal with them at close hand. Such is the case with vermiculite: it’s easy to remain oblivious until the day you learn your house may be stuffed with it.

Vermiculite is a mineral that comes out of the ground in the form of thin sheets, like mica; when you heat it up it expands into a tangle of wormlike fibers. (The name comes from the Latin vermiculari, meaning “to be full of worms.”) These fibers are fireproof, lightweight, and absorbent, and various bright types figured out a while back that they could be used as packing material, as a soil additive, to soak up unwanted chemicals, and as insulation. The U.S. is home to several major vermiculite deposits.

The trouble with vermiculite has nothing to do with the substance, but with a contaminant sometimes found in vermiculite ores: asbestos. Asbestos, for those too young to remember its preregulation heyday, is something you don’t want to mess with: inhaling its fibers can lead to various fatal diseases, including some aggressive cancers of the cardiopulmonary system that may not show up until years down the road.

And here we arrive at the hapless town of Libby, Montana, where the massive Zonolite vermiculite mine was operated for 60-plus years before its 1990 shutdown. Libby residents, downwind from the mine, were exposed to about ten times as much asbestos as OSHA has determined to be acceptable, while the mine workers got more like 1,300 times the established safe limit. The upshot, according to investigative work by the Seattle Post-Intelligencer: at least 200 asbestos-related deaths so far with more diagnoses cropping up every month and an incidence of lung abnormalities in Libby that’s conservatively 15 times the rate seen elsewhere.

In 2005 the Justice Department indicted several officers of the W.R. Grace Company — which ran the Libby mine from 1963 on — charging that they knowingly risked the health of Libby citizens by keeping quiet about asbestos risks; the case may go to the Supreme Court this year. Meanwhile, in March Grace agreed to pay the feds $250 million for investigation and cleanup efforts in Libby — a record sum for a Superfund case, but it may not even cover the EPA’s expenses.

So how many homes were insulated with vermiculite? No one seems to know; U.S. estimates range from 2.5 million to 35 million. Before fretting, check with someone who actually knows what vermiculite looks like — many home owners panic over insulation that turns out to be harmless. If vermiculite really is what you’ve got, you can have it tested for asbestos, but the EPA warns that the testing process has some technical problems. Given that, and given how much vermiculite out there came from the Libby mine, it might be simplest just to figure that any vermiculite is likely to be the bad kind.

Opinions vary on how aggressively to deal with asbestos risk, but a lot of experts tell you to just (1) make sure insulation fibers have no route into the living areas of the house and (2) forget any activity — renovations, using the attic for storage, etc — that might disturb the insulation. A 2001 government study found no airborne asbestos or asbestos dust in homes whose attics were insulated with contaminated vermiculite as long as nothing was done to stir it up. The bottom line is: Leave it alone and you’ll probably be fine.

Technically, it’s not just insulation that poses such potential threat, as there are plenty of vermiculite-bearing gardening products around too — it’s added to potting soils to help them hold more water. In 2000 the EPA tested 54 of these: 22 contained asbestos, with 8 containing significant amounts and 1 actually releasing asbestos fibers into the air when used. Overall, though, the exposure level for the average gardener was found to be quite low — you’d need to be planting peonies on a pretty epic scale to have much to worry about.