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Bay Nature magazineJanuary-March 2013

Should we be worried about asbestos in serpentine rock?

by on January 31, 2013

Serpentinite, KQED Quest
Serpentinite, KQED Quest

The answer is yes, we actually should worry about exposure to the soil created from serpentine rock, especially airborne dust. Serpentinite is a unique and beautiful rock that’s rare in most of the world. Here in California, however, we have North America’s largest exposures and we’ve made it our official state rock. Serpentine soil habitats are often home to many native species that have adapted to some of its odd properties.

The word serpentine refers to the mottled, snakelike pattern sometimes seen on the rock. The Greek physician Dioscorides suggested ground-up serpentinite as a prevention for snakebite. Not a healthy idea, as you’ll soon learn.

The source material of serpentinite, peridotite, is a rock made of upwelled magma containing large amounts of iron. Our local serpentinite formed when there was still subduction happening here (one plate diving under another) more than 30 million years ago. As the oceanic plate dove under the continent, the peridotite was subjected to intense pressure. But because this occurred near the surface underwater, temperatures stayed low. And that’s what makes this rock special: It stayed cool under pressure. (Wish I could do that!)

Early geologists in California recognized the economic resources in serpentinite. Mercury, nickel, chromium, and magnesite were often found near its outcroppings, along with naturally occurring asbestos particles–microscopic needlelike crystals of magnesium-iron silicate. There are several forms of asbestos, but the most common type here is chrysotile. Due to its unique properties (tensile strength, flexibility, and heat and chemical resistance), asbestos has a number of valuable economic uses: acoustic tiles, fireproofing, caulking, brake pads, and filters (for removing fine particles from chemicals, wine, and other liquids).

But those slender crystals have a downside: They can become lodged in a person’s lungs or abdominal cavity and, over the course of two or three decades, lead to asbestosis or peritoneal mesothelioma (irritation of the abdomen). It’s not clear how many fibers are needed to cause lung cancer or other diseases, but any exposure involves some risk of disease. Children may be at higher risk due to their higher metabolic rate and longer time for disease to develop.

To reduce dangerous exposure to the dust, leave serpentine outcroppings undisturbed. As long as the asbestos fibers remain stabilized in the rock, they pose no hazard. All the more reason to leave them alone and just admire both the beautiful rocks and the diverse native wildflowers that thrive on the soil created by this distinctly Californian rock.


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Amy Gravitch on August 24th, 2013 at 9:12 am

I am house hunting. I have two young children. Should I stay away from houses built on or nearby serpentine rocks? Please advise.

David Tharp on February 18th, 2014 at 5:51 am

Michael, could you please cite any studies that show chrysotile, the mineral that is in serpentine and is called “asbestos” that definitively shows it to be a human health concern?
I think you are confused about the differences between the mineral groups amphibole and serpentine and their individually unique properties.

doug havelcheck on February 18th, 2014 at 8:00 am

Hi Amy, this article is a bit misleading. There is no demonstrated link between naturally occuring asbestos and mesothelioma. Man made asbestos is the asbestos you hear of in the news that is bad. If naturally occurring asbestos were really a threat, we would see cancer clusters or other epidemiological evidence of the problem in local residents or farmers that have been tilling these soils for decades.

Jon on June 9th, 2014 at 2:12 pm

I worked construction in the 1980s in El Dorado County and we used serpentine as base gravel on every project. I noticed one day that there was a lot of asbestos mixed in with the rock and from then on we always sprayed it with water to keep the dust down, but really I, and countless other laborers, had been wallowing in the stuff for years. 30 years later and I am still fine, as far as I know none of the guys I worked with – those I kept in touch with anyway – have not had any problems.

Steven Johnson on March 18th, 2015 at 6:35 pm

@jon Not to rehash old threads, but I am glad that you have had no problems from the serpentinite. I am a newly employed laborer, and I’m sure that with the EPA and OSHA regulations they have nowadays, I’ll be fine. You made me feel okay with my project tomorrow! hahaha

Ethan Meagher on September 8th, 2015 at 12:19 am

If you have Serpentine rocks in your area, you’re probably exposed to asbestos fibers in dust. Better to remove it.

Brian B on September 23rd, 2015 at 11:26 am

I work with soapstone and occasionally serpentinite (serpentine is not an individual stone but a group of minerals).
The newer studies I’ve read indicate all asbestos types are carcinogenic and can lead to asbestosis.
Chrysotile does appear to be less active than amphibole asbestos but is still a concern.

However I’m not sure what Ethan Meagher advises is exactly practical or necessary. As Doug Havelcheck notes if natural, environmental serpentinite deposits were truly dangerous we could expect to see a higher incidence of mesothelioma in people living in those zones for long periods. I’m not aware of any such finding; mesothelioma still being a rare cancer linked almost exclusively to long term workplace exposure to airborne industrial uses.

Terry on January 30th, 2016 at 9:10 pm

For those who worked in El Dorado County, the serpentine is of no concern. The problem began in 1996 when a large Tremolite amphibole deposit was opened up. Then another. It never was shipped with gravel but there are now thousands of people living directly on top of it. We did animal studies which show the exposure level approaches 400 times over where lung cancer begins to occur. Andfrom teh places in teh world where we know there are epidemics, it is the highest level in animal lungs ever seen, right there in El Dorado County.

Anyway, Tremolite is not green rock, it need not be associated with serpentine, it is naturally white in color adn can appear brown when it is dirty.

95% of all asbestos is Chrysotile but amphiboles cause 99% of all disease. This author is all messed up!!!

K.L. Kincaid on December 19th, 2016 at 7:52 pm

After years of collecting grinding and shaping this rock into scaled pieces of art rock sculptures, I am now at risk of developing lung cancer associated with asbestos dust!? However, many google search sites I’ve come across are confusing and contradictory as to what a few sites have stated. Guess it’s up in the air so to speak to get solid facts or the truth behind the hyped up articles that are published. Cheers


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