Workplace Exposure to Asbestos:
Review and Recommendations

ASBESTOS NOMENCLATURE/
DEFINITIONS

Memorandum on Asbestos Update and Recommended Occupational Standard
I. Asbestos Nomenclature/Definitions
II. Asbestos Sampling and Analysis
III. Biologic Effects of Exposure to Asbestos in Animals
IV. Biologic Effects of Exposure to Asbestos in Humans
V. Smoking and Asbestos
VI. Exposure to Asbestiform Minerals other than Commerically Mined Asbestos
VII. Non-Occupational Exposure to Commerical Sources of Asbestos
VIII. Dose-Response Relationships
References

Review

There is considerable controversy as to which mineral particles should be considered "asbestos" insofar as demonstrated health effects are concerned (Campbell et al., 1977; Ampian, 1978; Zoltai, 1978; Langer, 1979). Until recently, most environmental and epidemiological studies concerning mineral fibers were focused on occupational cohorts exposed to asbestos fibers from commercial deposits. However, there is currently much interest in exposures to other minerals such as submicroscopic amphibole fibers and cleavage fragments and related health effects. Mineralogists have voiced concern that mineralogical terms have been used improperly, potentially classifying many non-asbestos materials as asbestos (Ampian, 1978; Campbell et al., 1977; Zoltai, 1978).

An important problem which exists is the basic definition of what minerals should be called "asbestos." Various definitions have been proposed or used:

National Academy of Sciences (1977):

"The name for a group of naturally occurring hydrated silicate minerals possessing fibrous morphology and commercial utility."

Bureau of Mines (1977):

"(1) A collective mineralogical term encompassing the asbestiform varieties of various minerals;

"(2) an industrial product obtained by mining and processing asbestiform minerals." Asbestiform minerals were further defined to be "a specific type of mineral fibrosity in which the fibers and fibrils possess high tensile strength and flexibility."

IARC (1977):

"Asbestos is the generic name used for a group of naturally occurring mineral silicate fibers of the serpentine and amphibole series."

Zoltai (1978):

"...a collective term referring to the unusual crystallization of certain minerals in the form of long, strong, and flexible fibers, aggregated in parallel or radiating bundles from which fibers can easily be separated."

The above definitions demonstrate an important problem. That is, the condition of fibers in nature as a result of crystal growth is the only criteria for distinguishing asbestos from other silicates (Langer, 1979). Most properties mentioned above can only be measured on bulk samples (megascopic properties). However, airborne fibers in the occupational setting are only observable on the microscopic level, thus not allowing measurements of such properties as tensile strength and flexibility. Langer et al. (1979) have pointed out that, using strict mineralogical nomenclature, isolated submicroscopic single fibers derived from known asbestos sources could not be termed "asbestos." In fact, among the many minerals demonstrating a fibrous habit in nature, only six minerals are commercially exploited and thus considered "asbestos." These include the serpentine mineral chrysotile and the amphiboles cummingtonite-grunerite including amosite, anthophyllite asbestos, tremolite asbestos, actinolite asbestos, and crocidolite.

In addition to problems relative to exposures to mineral fibers other than one of the six mentioned above, acicular cleavage fragments are frequently indistinguishable from mineral fibers derived from commercial asbestos sources, especially on the submicroscopic scale. Cleavage plays an important role during commutation for some amphibole minerals. Submicroscopic amphibole mineral fragments often demonstrate structural and chemical properties indistinguishable from asbestos homologues. Airborne size characteristics such as length and diameter are often similar to asbestos.

General Definition

The foregoing considerations present a fundamental question of how broad a new or revised regulation should be. Arguments can be made for inclusion of all fibrous minerals posing risks comparable to commercially exploited fibrous minerals called asbestos. On the other hand, the fibrous minerals generally called asbestos appear to form the most pressing hazards to the largest number of current workers. This effort has not attempted to recommend coverage of all fibrous minerals or analogs, but has focused on commercial materials generally considered asbestos and asbestiform contaminants common to commercially exploited materials. We recommended the following definition for regulatory purposes, pending thorough and complete regulation of the hazards of all fibrous materials.

l. Asbestos is defined to be chrysotile, crocidolite, and fibrous cummingtonite-grunerite including amosite, fibrous temolite, fibrous actinolite, and fibrous anthophyllite.

2. The fibrosity of the above minerals is ascertained on a microscopic level with fibers defined to be particles with an aspect ratio of 3 to 1 or larger.

We also expressly adopt the following approach articulated by the recent British Advisory Committee on Asbestos (Vol.1, p.11):

Asbestos is a generic term for the fibrous forms of several mineral silicates. These occur naturally in seams or veins, generally between about 1 and 20 millimetres (mm) in width in many igneous or metamorphic rocks and belong to one of two large groups of rock-forming minerals: the serpentines and amphiboles.

We recognize the mineralogical complexities associated with the definition and identification of asbestos and asbestos fibre, but for the purposes of this report we concentrate on the fibre types with which people are most likely to come into contact as a result of their use in industry. The serpentine group contains the type of asbestos known as chrysotile ('white asbestos'), which is the only asbestiform member of this group of minerals and by far the commonest and commercially the most important type of asbestos. The amphibole group contains crocidolite ('blue asbestos'), amosite ('brown asbestos'), anthophyllite, actinolite and tremolite.

Amosite is an acronym for Asbestos Mines of South Africa and is mineralogically known as cummingtonite-grunerite asbestos. Tremolite may occur as a contaminant with chrysotile and with other minerals such as talc. Crocidolite, amosite and anthophyllite have all been exploited commercially, although anthophyllite is no longer in significant quantities.

The above definitions of asbestos should not be taken to mean that fibers or mineral fragments of other minerals are without biological significance (IARC, 1977). Although epidemiologic data for other "mineral fibers" are limited at this time, prudence dictates that such substances be handled with caution.

Memorandum on Asbestos Update and Recommended Occupational Standard
I. Asbestos Nomenclature/Definitions
II. Asbestos Sampling and Analysis
III. Biologic Effects of Exposure to Asbestos in Animals
IV. Biologic Effects of Exposure to Asbestos in Humans
V. Smoking and Asbestos
VI. Exposure to Asbestiform Minerals other than Commerically Mined Asbestos
VII. Non-Occupational Exposure to Commerical Sources of Asbestos
VIII. Dose-Response Relationships
References