Energy & Resources
Naturally Occurring Asbestos: An Introduction
David Bieber, Bruce R. Hilton, Eric Hubbard, Bill Mitchell and David Sederquist


Bruce R. Hilton

Serpentine, the state rock of California, was once considered a valuable component in building materials. It is now recognized as a common source of potentially hazardous, naturally occurring asbestos minerals, or NOAMs. NOAMs are contained in Earth materials, but can be disturbed by construction or mining. Once airborne, these fibers could cause harm. Geologists have a key role to play in determining where NOAMs might occur so that projects can be planned around them.

Serpentine and other mafic and ultramafic source rocks are present in one form or another in at least 50 of the 58 counties in California and occur in at least 27 of our 50 U.S. states. Nor are NOAMS unique to the United States, having been reported as potential health hazards in South Africa, Turkey, Greece, Australia, Russia, Canada, Finland and China.

Asbestos manufacturing and use in construction materials began nearly 200 years ago. In the mid-1970s, asbestos was banned in the United States with the discovery of a relationship between exposure to asbestos fibers and respiratory diseases including asbestosis, lung cancer and mesothelioma (cancer of the pleural lining).

Epidemiological studies continue to explore this relationship, but in the case of mesothelioma, are severely hampered by the low frequency of less than 400 new cases per year in the United States. But one concept remains relatively uncontroversial: Few people want to wait for more case studies and most want to implement effective, practical measures to reduce the potential exposure risks today. However, in the absence of widely accepted threshold air concentrations for asbestos fibers (not unlike the absence of threshold values for hydrocarbons in the early 1980s), the extent of airborne fibers that represent a health hazard cannot be quantified.

As a result, it is even more difficult to understand the relationship between source concentrations and the level of soil and rock disturbance necessary to generate harmful doses of NOAM-bearing dust. Until widely accepted risk models become available, assessment of the risk depends on the qualitative identification (i.e. presence or absence) of NOAMs on a particular site, thereby placing this responsibility on those broad shouldered-geologists willing to accept the challenge.

Two schools of thought represent the dichotomy of concern over NOAMs: The less conservative "never been a problem" group who feel nothing needs to be done, and the more conservative "panic and epidemic" group who feel dealing with and building on NOAMs cannot be done safely, no matter what measures are taken. The application of practical, sound science seems to represent the only reasonable middle ground and a rare opportunity for geologists to contribute to the development and implementation of appropriate public process and policy. The most affected regions are those where NOAMs occur geologically and urban growth results in disturbance. Geologists from Fairfax County, Virginia, Libby, Montana, and several counties in California continue to exchange their experience with and new methods for assessment and mitigation. The result is a collaborative, effective process of assessment and management discussed herein.

Hilton is a Senior Engineering Geologist for Kleinfelder, Inc. in Sacramento, California. E-mail him at

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David Bieber and Bill Mitchell

Geologic assessment of NOAMs requires an experience-based understanding of the metamorphic environment of occurrence of NOAMs alongside knowledge of proposed disturbance scenarios for a given project. In order to perform an effective assessment, the specific NOAMs, or target constituents needs to be clear.

Asbestiform minerals and asbestiform crystal habits occur primarily, but not exclusively in mafic and ultramafic rock formations, and faults or shear zones within or near these mafic rock units. In both cases, NOAMs typically occur as very thin veins, fracture coatings, or finely disseminated mineral grains. In many cases, this mineralization is not visible in a hand specimen and therefore represents the proverbial "needle" in a haystack.

Without disturbance, there is no exposure. Understanding the exposure pathways between potential disturbance sources and human inhalation opportunities, or receptors is essential to the assessment process. The pathway between the disturbance source and receptor site is referred to as the exposure pathway and, when clearly defined for a specific project, can be assessed, monitored, and mitigated.

The geologic assessment of NOAMs is a classic application of engineering geology. Once the disturbance scenarios for the project, and the potential sources of NOAMs are recognized, the assessment process should focus on evaluation of the presence of NOAMs in the areas where the NOAMs are likely to occur and be disturbed. Based on the initial site reconnaissance, one can arrive at three possible alternatives. First, the observed conditions are not conducive to the formation of NOAMs, and work can progress without modification. Second, the outcome of the site reconnaissance is that NOAMs are either found or assumed to be present, in which case mitigation and monitoring are warranted. Third, on questionable sites where the presence of NOAMs cannot be ruled out or confirmed, a simple cost benefit analysis is appropriate to guide further actions.

Bieber is a Senior Engineering Geologist for Geocon Consultants, Inc. in Rancho Cordova, California. E-mail him at

Mitchell is a Senior Geologist for Wheeldon Geology in Placerville, California. E-mail him at

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Eric Hubbard

The additional investigation step is analogous with the Phase II Environmental Assessment process and requires field investigation involving additional surface and/or subsurface sampling (depending upon the depth of proposed site excavations) and petrographic analyses. The appropriate analytical procedures for the assessment of NOAMs are still quite controversial; largely because the test methods that have been accepted for a long period of time for asbestos construction materials and the application of the data or, in many cases, their applicability to field conditions and disturbance scenarios are poorly understood. Clearly, one of the most important considerations in selecting an applicable laboratory testing methods is the value of the resulting data as input parameters for risk assessment modeling.

The NOAM assessment hierarchy described above requires the direct input of a professional geologist, as applicable for each state. In addition, the responsible geologic professional should have local geologic knowledge and experience regarding the occurrence of NOAMs in the project area. It is also important to remember that assessment continues through the life of a construction project.

Hubbard is a Senior Engineering Geologist for Kleinfelder, Inc. in Reno, Nevada. E-mail him at

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David Sederquist

The application of engineering controls to reduce exposure to disturbed NOAMs is now very well established. Earthwork requires the application of appropriate OSHA regulations. These can all be used to minimize worker exposure and to eliminate exposure to residents neighboring construction areas and to properly communicate information indicating that the property is underlain by mitigated naturally occurring asbestos. The practice of asbestos removal from buildings has demonstrated the importance of water in dust suppression. The judicial use of water is important in protecting workers. Other exposure pathways, such as dust blowing off of storage piles, require management and mitigation tools such as covering by tarps or soil during periods of construction inactivity (when constant wetting may be impractical) such as weekends or holidays.

Engineering controls need to be designed with the potential receptor in mind. The placement of a thick cover of clean soil over asbestos containing soil provides an effective barrier against exposure. Future site improvement activities, such as utility installation, swimming pool installation, or massive landscaping, need to consider where NOAMs have been or may be exposed in the subsurface. Ultimately, the final site conditions need to be disclosed to end users of the property. Improvements that require a permit should trigger additional scrutiny by regulatory agencies to determine if more asbestos management and mitigation may be necessary.

Sederquist is a Senior Engineering Geologist for Youngdahl Associates, in El Dorado Hills, California.

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