Potting Mixtures

Asbestos in Potting Mixtures

Potting mixtures are premixed soil sold to gardeners for use in growing vegetables and flowers. Asbestos has never been intentionally added to potting mixtures, as it is an inert mineral with nothing that would amend soil or serve as a nutrient. However, many potting mixtures became contaminated with a particularly deadly form of asbestos by accident.

Vermiculite is a unique mineral that expands when heat is applied, forming a soft material that can be used as a soil conditioner, among other things. Today, vermiculite is mined in several countries around the world, including Russia, South Africa, China and Brazil. Until 1990 however, 90% of the world's vermiculite came from the W.R. Grace Company’s mining operation in Libby, Montana.

As it turns out, most of the vermiculite mined in Libby – which was marketed under the brand name Zonolite – was contaminated with deadly amphibole varieties of asbestos, namely tremolite. In addition, two other asbestiform minerals – winchite and richterite – were present in Zonolite.

None of those varieties of asbestos were ever commercially exploited, but they share characteristics in common with two very similar asbestos varieties: crocidolite (blue asbestos) and amosite (brown asbestos). The fibers of these varieties are quite toxic and known to cause cancer when ingested.

By itself, vermiculite is harmless. However, in Libby, it was often found in proximity to tremolite deposits, and as a result for many years potting mixtures made with Libby vermiculite – a significant fraction of the total sold – were contaminated with tremolite, winchite, and richterite.

Vermiculite mined and marketed today is normally subject to inspection and testing, however the safety of this material cannot be taken for granted. A commercial vermiculite mine was still in operation in Virginia at the turn of the century, and it was reported in the Seattle Post-Intelligencer that the ore contained "dangerously high levels of asbestos."

Hazards Associated with Potting Mixture Products

The users of contaminated potting mixtures may have received significant exposure to asbestos. In addition, the vermiculite miners and people working in the facilities producing the potting mixes would have been exposed to an even higher level of tremolite. Tremolite is an exceptionally dangerous form of asbestos and inhalation of tremolite fibers has been conclusively linked to the development of extremely serious health problems, including malignant mesothelioma.

Sources

Bowker, Michael. Fatal Deception: The Untold Story of Asbestos (New York: Touchstone, 2003)

N/A. "Libby Site Background." Environmental Protection Agency Website (http://www.epa.gov/region8/superfund/libby/background.html). Retrieved 6 January 2011.

Schneider, Andrew. "Virginia Miners At Risk From Asbestos." Seattle Post-Intelligencer, 4 October 2000.