A reader recently asked me whether it was true that vermiculite contains asbestos and therefore should be avoided. It just goes to show that horticultural myths have an extended life, as this one was debunked ages ago and I thought it was long dead.
First, an explanation.
Vermiculite is a naturally occurring mineral composed of shiny flakes resembling mica. When heated to outrageously hot temperatures, about 1000°C, it puffs up like popcorn, giving a light-weight expanded rock useful not only in horticulture and also in insulation and fireproofing.
Vermiculite itself is not a health hazard. It’s an inert mineral widely employed in gardening circles, practically omnipresent in packaged growing mixes, and accepted in organic gardening.
So far, so good.
But vermiculite wasn’t always quite so innocuous. Back in the mid-twentieth century, much of the vermiculite produced in North America was taken from the Libby Mine in Montana. Sold under the trade name Zonolite, this vermiculite was contaminated with asbestos fibers, as the two are naturally copresent in the same rock formation, although this went unrecognized for some 60 years. From about 1920 to 1990, Zonolite was widely used in the construction industry throughout North America, notably as attic insulation. It was also employed, to a lesser extent, in horticulture.
By the way, Zonolite never was sold in Europe or other continents: it was strictly a North American commodity.
Following the discovery of the presence of asbestos fibers in the vermiculite it produced, the Libby Mine closed in 1990 (in fact, little Zonolite was distributed after 1980). To date, no other vermiculite mines have been found to contain asbestos and periodic tests are done to make sure it stays that way.
As a result, you can safely use any growing mix presently sold that contains vermiculite and be sure it is asbestos-free. That is, unless you have bags of potting soil dating back to before 1990 and which come from Libby, Montana… and that strikes me as being unlikely.
What About Perlite?
Concerns about vermiculite contaminated with asbestos fibers prompted people to wonder whether if that wasn’t also the case with perlite, another expanded rock used to lighten gardening soils. But perlite, essentially a form of volcanic glass, is not formed in the same way as vermiculite and is never found in the company of asbestos. So there was never any risk of it becoming contaminated.
Avoid Inhaling Dust of Any Kind
Any text presenting the safe use of vermiculite and perlite has to include one caveat: don’t breathe in their dust.
When working with any product that gives off dust, be it vermiculite, perlite, potting soil or even flour, it is always important to protect your eyes and lungs from the dust they can release by wearing safety goggles and a dust mask, as dust can be an irritant. This is especially true if you have lung problems.
There are millions of homes insulated with vermiculite containing asbestos fibers in North America. Currently, the usual recommendation is not to touch this insulation, quite simply. To quote the Canadian Center for Occupational Health and Safety (CCOHS): “As long as this kind of vermiculite-based insulation remains undisturbed behind intact walls or in attic spaces and does not become airborne, it should not be a concern.”
On the other hand, if for any reason you have to disturb old vermiculite insulation, you should hire a professional who is trained and certified in handling asbestos to do the job.
Obviously, the latter aspect completely exceeds my field of knowledge: I know horticulture, not construction. If you are concerned that your home is possibly insulated with contaminated vermiculite, I suggest you contact the EPA in the United States, the CCOHS in Canada or the government body currently manages health issues in construction in your country.