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Naturally Occurring Asbestos Research

UNLV researchers Brenda Buck, Dirk Goossens, Rodney Metcalf, Minghua Ren, UNLV student Fritz Freudenberger and Bloomsberg University scientist Brett McLaurin recently published an article in the Soil Science Society of America Journal related to naturally occurring asbestos in Boulder City, southeast Henderson, and surrounding regions.


Naturally occurring asbestos refers to asbestos found as a natural component of rocks and soils. It occurs in rocks and soils as a result of natural geological processes and is found in many states. Previous to this study the presence of naturally occurring asbestos in Clark County, NV, was not recognized.

Asbestos is the name given to six different, regulated, fibrous minerals that occur naturally in the environment. The six fibrous minerals are divided into two categories: amphibole and serpentine. Amphibole forms include riebeckite (commercial name crocidolite), grunerite-cummingtonite (commercial name amosite), anthophyllite, tremolite, and actinolite.. The serpentine form is called chrysotile. Amphibole asbestos minerals are more toxic than the serpentine form. One of these regulated amphibole asbestos minerals (actinolite) is the asbestos type in Southern Nevada.

Other fibrous amphibole minerals that are not regulated, but are known to cause disease include winchite, magnesioriebeckite and richterite. These minerals are found in northwestern Arizona and studied by Metcalf.

The source of the naturally occurring asbestos in Southern Nevada is granite bedrock, and sediments and soils that have eroded from these rocks.

Much of the asbestos is found in soils formed on alluvial fans (deposits of sediment eroded from the granite), and in sediment found in active washes (stream drainages) and dry lakebeds. Soils and sediments represent the most likely source for dust containing naturally occurring asbestos.


The primary route of exposure for people is through air. Any activity or natural wind that generates dust from soils that contain asbestos will put the fibers in the air and increase the risk that people will be exposed. Therefore, the Agency for Toxic Substances and Disease Registry (ATDSR) and the Environmental Protection Agency (EPA) recommend avoiding activities that generate dust in order to lessen your exposure. However, asbestos fibers cannot be seen by the naked eye and therefore they can be present even without visible dust. Links for more information about how to limit exposure are listed below.

Health Information

Asbestos is a known human carcinogen with no known safe levels of exposure. Other, non-regulated minerals are also hazardous and behave similarly to asbestos when inhaled. In particular, Erionite is a carcinogenic mineral, but is non-regulated. 

The risk for adverse health effects increases with increasing exposure over time because the amphibole fibers remain in the body. This is particularly worrisome for children, as early life exposures have more time to manifest in disease (latency period).

In Libby, Mont., and other sites in the U.S., and other nations, there is significantly increased risk for disease in areas where people are being exposed to sources of asbestos in the environment.

For example, researchers at the University of California, Davis, found increased risk of malignant mesothelioma with residential proximity to naturally occurring asbestos.

American Journal of Respiratory and Critical Care Medicine article: Residential Proximity to Naturally Occurring Asbestos and Mesothelioma Risk in California

The USEPA recently released an inhalation reference concentration (RfC) value for chronic exposure for non-cancer health effects. 

The RfC is defined as an estimate of exposure that is likely to be without an appreciable risk of adverse health effects over a lifetime (70 yrs). The RfC is expressed in terms of the lifetime exposure in units of fibers per cubic centimeter of air (fibers/cc) as measured by phase contrast microscopy (PCM). The chronic RfC value for non-cancer health effects is 0.00009 f/cc.  

Results for ambient air are presented in the Final Phase I Site Characterization Report for the Boulder City Bypass Naturally occurring Asbestos (NOA) Project Phase I (Railroad Pass to Silverline Road).

Ambient air was measured in the area of Railroad Pass, for Phase I of the Boulder City Bypass (see final report).  Reported values from May 8 to August 10, 2014 vary from non-detect to 0.0014 s/cc with an average of 0.00021 s/cc. 

Hazard Quotient HQ = Ambient air concentration/Reference concentration. Using the limited data available (3 months summer 2014): HQ = 0.0002/0.00009 = 2.33. If the Hazard Quotient is calculated to be less than 1, then no adverse health effects are expected as a result of exposure. 

Note that OSHA (Occupational Safety and Health Administration) has a Permissible Exposure Limit (PEL) for asbestos that is 0.1 f/cc as an 8 hr time-weighted average. As you can see this number is much higher than the 0.00009 f/cc listed above. OSHA admits that although these levels are permissible, they do involve significantly increased health risk. In addition, the OSHA PEL only counts the 6 regulated asbestos minerals, therefore the asbestos minerals that are similar to those in Libby Montana that are found in and around Wilson Ridge in Arizona, and erionite, do not count towards the permissible exposure limit. 

Exposure to asbestos is known to cause asbestosis; pleural fibrosis; lung, ovarian, and larynx cancer; mesothelioma, and there is evidence of other health effects including: Cardiovascular disease, depressed immune function, gastrointestinal cancer, and several autoimmune diseases including rheumatoid arthritis, lupus, and scleroderma.


Buck and her research team specialize in medical geology, a field that studies how Earth materials affect health. In particular, their research focuses on the health effects of exposure to mineral dusts. This research led them to find the naturally occurring asbestos in Southern Nevada.

For their most recent research, which was published in the Soil Science Society of America Journal, the team collected 43 samples. Seventeen rock samples were collected from exposed granitoid outcrops: seven from Boulder City, four from Black Hill, and six from the McCullough Range. Seventeen soil samples were collected, 11 of which were from desert surfaces and six from dirt roads. They also collected samples from Buck’s clothing after she walked her horse on dirt roads in Boulder City. The subsequent analysis showed that all 43 samples, including rock, soil, dust, car tire, and clothing, contained fibrous amphiboles.

The sites that were found to contain the fibrous amphiboles include areas in and around Boulder City, and BLM-managed land south of Henderson.

Next Steps

Researchers are aware that there is a hazard but that air sampling and epidemiological studies are needed to learn more before speaking affirmatively about risks to human populations. Buck and Metcalf are partnering with local, state and federal agencies, and the Universities of Hawaii and Cincinnati to pursue more research on this topic.

Buck and Metcalf are also continuing to locate other deposits of naturally occurring asbestos in the region. As new data becomes available, maps showing these new locations will be uploaded here.

Helpful Links

Peer-Reviewed Publication

Naturally Occurring Asbestos: Potential for Human Exposure, Southern Nevada, USA in Soil Science Society of America Journal

Publications/News Stories

About the Researchers

Brenda Buck is a professor in UNLV’s Department of Geoscience. She is an elected Fellow in both the Geological Society of America and the Soil Science Society of America. She also was awarded the M.L. & C.M. Jackson Award, for Outstanding Contributions in Soil Mineralogy/Chemistry from the Soil Science Society of America.

Buck is the Director of the Environmental Soil Analytical Laboratory and the Clinical Toxicology and Chemistry Laboratory. Her research focuses on medical geology, in particular how geological materials impact health. Currently her work focuses on dust and hazards associated with dust exposure including those from asbestiform minerals, arsenic, and other carcinogens.

Rodney Metcalf is an associate professor in UNLV’s Department of Geosciences. His research interests center on the origin and evolution of the earth’s crust. His research tools are drawn primarily from igneous and metamorphic petrology, mineral chemistry, trace element and isotope geochemistry, and geochronology. He has served as chair of the Department of Geosciences and as Associate Dean for the College of Sciences at UNLV. He is secretary of the Cordilleran Section of the Geological Society of America and served two terms as an Associate Editor for the Bulletin of the Geological Society of America.