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Pacific Southwest, Region 9

Serving: Arizona, California, Hawaii, Nevada, Pacific Islands, Tribal Nations

Naturally Occurring Asbestos

Clear Creek Management Area

Glossary [click to open/close]

Activity-based sampling – Activity-based sampling of the air in the breathing zone of an individual while that individual participates in typical work or recreational activities.  It has been used for decades by industrial hygienists to measure personal exposures in workplace environments.  It is more representative of actual individual exposures than fixed, stationary monitors and soil sampling, and is being used by EPA to sample exposures at asbestos sites across the country.

Ambient Air – Ambient air is surrounding air that is not immediately affected by a disturbance or activity.

Amphiboles – One of two mineral families which contain asbestos minerals.  Amphibole asbestos tends to form in needle-like shapes and includes tremolite, actinolite, winchite, richterite, anthophyllite, crocidolite, and amosite asbestos.

Chrysotile – Asbestos from the serpentine family of minerals.  Chrysotile asbestos is flexible and historically accounts for about 95% of the asbestos used commercially in the United States.

Excess Lifetime Cancer Risk – An estimate of the probability that a person may develop cancer in excess of background rates sometime in his or her lifetime following exposure to a particular contaminant. 

Mesothelioma – A rare cancer which may affect the lining of the lungs (pleura) or the abdominal contents (peritoneum). Most mesotheliomas are caused by exposure to asbestos.  Most cases of mesothelioma are diagnosed 30 years or more after the first exposure to asbestos.

PCME or Phase Contrast Microscopy Equivalent – Phase Contrast Microscopy (PCM) is the analytical method for asbestos used in occupational environments.  Current health standards for asbestos are based on studies which document the adverse health effects from asbestos exposure in workers.  Since the worker exposures used PCM, the health standards are based on the fibers which are counted in PCM.  Today, EPA uses an analytical technique, Transmission Electron Microscopy or TEM, which can see much smaller and thinner fibers (20,000 magnification vs. 400 magnification).  In order to apply the current health standards to fibers which were counted using a TEM analysis, the EPA used only those fibers which would have been seen in the PCM method, and this equivalent count of fibers is called the Phase Contrast Microscopy equivalent or PCME.  For this risk assessment, only those fibers which are considered to be of PCME dimensions, greater than 5 microns in length, with at least a 3:1 length to width ratio, and a diameter between 0.25 microns and 3 microns inclusive, were used.  To give some idea of the size of a PCME fiber, the average width of a human hair is 80 microns.

95% UCL or 95% Upper Confidence Limit of the Mean – A statistical calculation of the mean concentration so that the actual mean will be less than this value 95% of the time.

Final Report Resources
CCMA Asbestos Exposure and Human Health Risk Assessment Final Report without Appendix G (PDF) May 1, 2008 (160 pp, 5.5MB)
CCMA Asbestos Exposure and Human Health Risk Assessment Final Report Fact Sheet May 1, 2008 (PDF) (10 pp, 356K)
CCMA Asbestos Exposure and Human Health Risk Assessment Frequently Asked Questions (PDF) May 1, 2008 (4 pp, 22K)
Press Release announcing CCMA Asbestos Exposure and Human Health Risk Assessment Final Report May 1, 2008
Health and Safety Plan, Appendix F (PDF) 10/22/04 (28 pp, 169K)
CCMA Asbestos Exposure and Human Health Risk Assessment Final Report, Appendix G (PDF) May 1, 2008 (250 pp, 603K)
Sampling and Analysis Plan (PDF) (60 pp, 248K)

Additional Reference Materials

Agency for Toxic Substances and Disease Registry (ATSDR) publication “Asbestos and Health:  Frequently Asked Questions” (PDF) (4 pp, 141K) Exiting EPA (disclaimer)
Technical Memo 9/5/07 Human Health Risk Assessment, Air Sampling Event (PDF) (15 pp, 112K)
Technical Memo 11/4/05 Human Health Risk Assessment, Air Sampling Event (PDF) (6 pp, 93K)
Technical Memo 9/15/04 Human Health Risk Assessment, Air Sampling Event (PDF) (6 pp, 93K)
Documents on the Atlas Asbestos Mine Superfund Site:
EPA Risk Assessment
(Aug 2004)  
EPA Air Sampling Fact Sheet
(Feb 2005)

Sampling Photos | Study Area


The Clear Creek Management Area (CCMA) in San Benito and Fresno Counties is located on one of the largest naturally occurring asbestos deposits in the world. The rugged terrain overlaying this 31,000-acre serpentine deposit is a popular and challenging riding spot for off-road motorcyclists. The naturally barren slopes, bald ridges, chaparral and rare plants are also enjoyed by rock collectors, botanists, hikers, hunters and campers, including families with children. Thousands of visitors each year use hundreds of miles of criss-crossing routes, a legacy of historic mining activities in the area.

The CCMA is managed by the Department of the Interior’s Bureau of Land Management (BLM).  Within the boundaries of the CCMA is the Atlas Asbestos Mine Superfund site.  In 1991, EPA signed the Record of Decision (ROD) selecting the cleanup remedy for the Atlas Mine.  In the ROD, EPA designated the CCMA as one of four geographic areas that comprise the site, but did not propose a cleanup action for the CCMA.  Instead, EPA stated that it would evaluate whether the BLM’s plans for management of CCMA were adequate to protect public health from exposure to asbestos found in the CCMA’s soil and air.  BLM has designated the area as hazardous, and asbestos warning signs are posted at entry points and on bulletin boards.

Map of Study Area
Map of study area.
Larger version

In 2004, as part of the evaluation of the Atlas Mine cleanup for possible delisting of the site from the federal Superfund list, EPA initiated an asbestos exposure and human health risk assessment for the CCMA. The goals of the assessment were:

  • To update the 1992 BLM Human Health Risk Assessment by using current asbestos sampling and analytical techniques, and
  • To evaluate risks to children because families are frequent visitors to CCMA.

BLM will use the information provided in the assessment to evaluate management and use alternatives in an upcoming environmental impact statement for managing the CCMA. The final Clear Creek Management Area Asbestos Exposure and Human Health Risk Assessment report was released by EPA on May 1, 2008 and is summarized on this website, please use the tabs at the top to navigate among the pages.

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