Washington Administrative Code › Title 296 - Labor and Industries, Department of › Chapter 296-62 - General occupational health standards › Part I-1 - ASBESTOS, TREMOLITE, ANTHOPHYLLITE, AND ACTINOLITE › Section 296-62-07753 - Appendix JPolarized light microscopy of asbestos-Nonmandatory

Current through Register Vol. 24-14, July 15, 2024
Method number: ID-191
Matrix: Bulk
Collection Procedure
Collect approximately 1 to 2 grams of each type of material
				  and place into separate 20 mL scintillation vials.
Analytical Procedure
A portion of each separate phase is analyzed by gross
				  examination, phase-polar examination, and central stop dispersion
				  microscopy.
Commercial manufacturers and products mentioned in this
				  method are for descriptive use only and do not constitute endorsements by
				  USDOL-WISHA. Similar products from other sources may be substituted.
(1) Introduction
This method describes the collection and analysis of asbestos
				  bulk materials by light microscopy techniques including phase-polar
				  illumination and central-stop dispersion microscopy. Some terms unique to
				  asbestos analysis are defined below:
Amphibole: A family of minerals whose crystals are formed by
				  long, thin units which have two thin ribbons of double chain silicate with a
				  brucite ribbon in between. The shape of each unit is similar to an "I beam."
				  Minerals important in asbestos analysis include cummingtonite-grunerite,
				  crocidolite, tremolite-actinolite and anthophyllite.
Asbestos: A term for naturally occurring fibrous minerals.
				  Asbestos includes chrysotile, cummingtonite-grunerite asbestos (amosite),
				  anthophyllite asbestos, tremolite asbestos, crocidolite, actinolite asbestos
				  and any of these minerals which have been chemically treated or altered. The
				  precise chemical formulation of each species varies with the location from
				  which it was mined. Nominal compositions are listed:
Chrysotile ........................
				  Mg3Si2O5(OH)4
Crocidolite (Riebeckite asbestos) ..............
				  Na2Fe32+Fe23+Si8O22(OH)
				  2
Cummingtonite-Grunerite asbestos (Amosite) ............
				  (Mg,Fe)7Si8O22(OH)2
Tremolite-Actinolite asbestos ..................
				  Ca2(Mg,Fe)5Si8O22(OH)
				  2
Anthophyllite asbestos .........
				  (Mg,Fe)7Si8O22(HO)2
Asbestos Fiber
: A fiber of asbestos meeting
				  the criteria for a fiber. (See section (3)(e).)
Aspect Ratio
: The ratio of the length of a
				  fiber to its diameter usually defined as "length: width", e.g. 3:1.
Brucite
: A sheet mineral with the
				  composition mg(OH)2.
Central Stop Dispersion Staining (microscope): This is a dark
				  field microscope technique that images particles using only light refracted by
				  the particle, excluding light that travels through the particle unrefracted.
				  This is usually accomplished with a McCrone objective or other arrangement
				  which places a circular stop with apparent aperture equal to the objective
				  aperture in the back focal plane of the microscope.
Cleavage Fragments: Mineral particles formed by the
				  comminution of minerals, especially those characterized by relatively parallel
				  sides and moderate aspect ratio.
Differential Counting: The term applied to the practice of
				  excluding certain kinds of fibers from a phase contrast asbestos count because
				  they are not asbestos.
Fiber: A particle longer than or equal to 5 microns with a
				  length to width ratio greater than or equal to 3:
1. This may include cleavage fragments. (See
				section (3)(e) of this appendix).
Phase Contrast: Contrast obtained in the microscope by
				  causing light scattered by small particles to destructively interfere with
				  unscattered light, thereby enhancing the visibility of very small particles and
				  particles with very low intrinsic contrast.
Phase Contrast Microscope: A microscope configured with a
				  phase mask pair to create phase contrast. The technique which uses this is
				  called Phase Contrast Microscopy (PCM).
Phase-Polar Analysis: This is the use of polarized light in a
				  phase contrast microscope. It is used to see the same size fibers that are
				  visible in air filter analysis. Although fibers finer than 1 micron are
				  visible, analysis of these is inferred from analysis of larger bundles that are
				  usually present.
Phase-Polar Microscope: The phase-polar microscope is a phase
				  contrast microscope which has an analyzer, a polarizer, a first order red plate
				  and a rotating phase condenser all in place so that the polarized light image
				  is enhanced by phase contrast.
Sealing Encapsulant: This is a product which can be applied,
				  preferably by spraying, onto an asbestos surface which will seal the surface so
				  that fibers cannot be released.
Serpentine: A mineral family consisting of minerals with the
				  general composition
				  Mg3(Si2O5(OH)4
				  having the magnesium in brucite layer over a silicate layer. Minerals important
				  in asbestos analysis included in this family are chrysotile, liz-ardite,
				  antigorite.
(a) History
Light microscopy has been used for well over 100 years for
				  the determination of mineral species. This analysis is carried out using
				  specialized polarizing microscopes as well as bright field microscopes. The
				  identification of minerals is an on-going process with many new minerals
				  described each year. The first recorded use of asbestos was in Finland about
				  2500 B.C. where the material was used in the mud wattle for the wooden huts the
				  people lived in as well as strengthening for pottery. Adverse health aspects of
				  the mineral were noted nearly 2000 years ago when Pliny the Younger wrote about
				  the poor health of slaves in the asbestos mines. Although known to be injurious
				  for centuries, the first modern references to its toxicity were by the British
				  Labor Inspectorate when it banned asbestos dust from the workplace in 1898.
				  Asbestosis cases were described in the literature after the turn of the
				  century. Cancer was first suspected in the mid 1930's and a causal link to
				  mesothelioma was made in 1965. Because of the public concern for worker and
				  public safety with the use of this material, several different types of
				  analysis were applied to the determination of asbestos content. Light
				  microscopy requires a great deal of experience and craft. Attempts were made to
				  apply less subjective methods to the analysis. X-ray diffraction was partially
				  successful in determining the mineral types but was unable to separate out the
				  fibrous portions from the nonfibrous portions. Also, the minimum detection
				  limit for asbestos analysis by X-ray diffraction (XRD) is about 1%.
				  Differential Thermal Analysis (DTA) was no more successful. These provide
				  useful corroborating information when the presence of asbestos has been shown
				  by microscopy; however, neither can determine the difference between fibrous
				  and nonfibrous minerals when both habits are present. The same is true of
				  Infrared Absorption (IR).
When electron microscopy was applied to asbestos analysis,
				  hundreds of fibers were discovered present too small to be visible in any light
				  microscope. There are two different types of electron microscopes used for
				  asbestos analysis: Scanning Electron Microscope (SEM) and Transmission Electron
				  Microscope (TEM). Scanning Electron Microscopy is useful in identifying
				  minerals. The SEM can provide two of the three pieces of information required
				  to identify fibers by electron microscopy: Morphology and chemistry. The third
				  is structure as determined by Selected Area Electron Diffraction-SAED which is
				  performed in the TEM. Although the resolution of the SEM is sufficient for very
				  fine fibers to be seen, accuracy of chemical analysis that can be performed on
				  the fibers varies with fiber diameter in fibers of less than 0.2 micron
				  diameter. The TEM is a powerful tool to identify fibers too small to be
				  resolved by light microscopy and should be used in conjunction with this method
				  when necessary. The TEM can provide all three pieces of information required
				  for fiber identification. Most fibers thicker than 1 micron can adequately be
				  defined in the light microscope. The light microscope remains as the best
				  instrument for the determination of mineral type. This is because the minerals
				  under investigation were first described analytically with the light
				  microscope. It is inexpensive and gives positive identification for most
				  samples analyzed. Further, when optical techniques are inadequate, there is
				  ample indication that alternative techniques should be used for complete
				  identification of the sample.
(b) Principle
Minerals consist of atoms that may be arranged in random
				  order or in a regular arrangement. Amorphous materials have atoms in random
				  order while crystalline materials have long range order. Many materials are
				  transparent to light, at least for small particles or for thin sections. The
				  properties of these materials can be investigated by the effect that the
				  material has on light passing through it. The six asbestos minerals are all
				  crystalline with particular properties that have been identified and cataloged.
				  These six minerals are aniso-tropic. They have a regular array of atoms, but
				  the arrangement is not the same in all directions. Each major direction of the
				  crystal presents a different regularity. Light photons travelling in each of
				  these main directions will encounter different electrical neighborhoods,
				  affecting the path and time of travel. The techniques outlined in this method
				  use the fact that light traveling through fibers or crystals in different
				  directions will behave differently, but predictably. The behavior of the light
				  as it travels through a crystal can be measured and compared with known or
				  determined values to identify the mineral species. Usually, Polarized Light
				  Microscopy (PLM) is performed with strain-free objectives on a bright-field
				  microscope platform. This would limit the resolution of the microscope to about
				  0.4 micron. Because WISHA requires the counting and identification of fibers
				  visible in phase contrast, the phase contrast platform is used to visualize the
				  fibers with the polarizing elements added into the light path. Polarized light
				  methods cannot identify fibers finer than about 1 micron in diameter even
				  though they are visible. The finest fibers are usually identified by inference
				  from the presence of larger, identifiable fiber bundles. When fibers are
				  present, but not identifiable by light microscopy, use either SEM or TEM to
				  determine the fiber identity.
(c)
Advantages and Disadvantages
The advantages of light microscopy are:
(i) Basic identification of the materials was
				first performed by light microscopy and gross analysis. This provides a large
				base of published information against which to check analysis and analytical
				technique.
(ii) The analysis is
				specific to fibers. The minerals present can exist in asbestiform, fibrous,
				prismatic, or massive varieties all at the same time. Therefore, bulk methods
				of analysis such as X-ray diffraction, IR analysis, DTA, etc. are inappropriate
				where the material is not known to be fibrous.
(iii) The analysis is quick, requires little
				preparation time, and can be performed on-site if a suitably equipped
				microscope is available.
The disadvantages are:
(iv) Even using phase-polar illumination, not
				all the fibers present may be seen. This is a problem for very low asbestos
				concentrations where agglomerations or large bundles of fibers may not be
				present to allow identification by inference.
(v) The method requires a great degree of
				sophistication on the part of the microscopist. An analyst is only as useful as
				his mental catalog of images. Therefore, a microscopist's accuracy is enhanced
				by experience. The mineralogical training of the analyst is very important. It
				is the basis on which subjective decisions are made.
(vi) The method uses only a tiny amount of
				material for analysis. This may lead to sampling bias and false results (high
				or low). This is especially true if the sample is severely
				inhomogeneous.
(vii) Fibers may be
				bound in a matrix and not distinguishable as fibers so identification cannot be
				made.
(d) Method
				Performance
(i) This method can be used for
				determination of asbestos content from 0 to 100% asbestos. The detection limit
				has not been adequately determined, although for selected samples, the limit is
				very low, depending on the number of particles examined. For mostly
				homogeneous, finely divided samples, with no difficult fibrous interferences,
				the detection limit is below 1%. For inhomogeneous samples (most samples), the
				detection limit remains undefined. NIST has conducted proficiency testing of
				laboratories on a national scale. Although each round is reported statistically
				with an average, control limits, etc., the results indicate a difficulty in
				establishing precision especially in the low concentration range. It is
				suspected that there is significant bias in the low range especially near 1%.
				EPA tried to remedy this by requiring a mandatory point counting scheme for
				samples less than 10%. The point counting procedure is tedious, and may
				introduce significant biases of its own. It has not been incorporated into this
				method.
(ii) The precision and
				accuracy of the quantitation tests performed in this method are unknown.
				Concentrations are easier to determine in commercial products where asbestos
				was deliberately added because the amount is usually more than a few percent.
				An analyst's results can be "calibrated" against the known amounts added by the
				manufacturer. For geological samples, the degree of homogeneity affects the
				precision.
(iii) The performance of
				the method is analyst dependent. The analyst must choose carefully and not
				necessarily randomly the portions for analysis to assure that detection of
				asbestos occurs when it is present. For this reason, the analyst must have
				adequate training in sample preparation, and experience in the location and
				identification of asbestos in samples. This is usually accomplished through
				substantial on-the-job training as well as formal education in mineralogy and
				microscopy.
(e)
				Interferences
Any material which is long, thin, and small enough to be
				  viewed under the microscope can be considered an interference for asbestos.
				  There are literally hundreds of interferences in workplaces. The techniques
				  described in this method are normally sufficient to eliminate the
				  interferences. An analyst's success in eliminating the interferences depends on
				  proper training.
Asbestos minerals belong to two mineral families: The
				  serpentines and the amphiboles. In the serpentine family, the only common
				  fibrous mineral is chrysotile. Occasionally, the mineral antigorite occurs in a
				  fibril habit with morphology similar to the amphiboles. The amphibole minerals
				  consist of a score of different minerals of which only five are regulated by
				  federal standard: Amosite, crocidolite, anthophyllite asbestos, tremolite
				  asbestos and actinolite asbestos. These are the only amphibole minerals that
				  have been commercially exploited for their fibrous properties; however, the
				  rest can and do occur occasionally in asbestiform habit.
In addition to the related mineral interferences, other
				  minerals common in building material may present a problem for some
				  microscopists: Gypsum, anhydrite, brucite, quartz fibers, talc fibers or
				  ribbons, wollastonite, perlite, attapulgite, etc. Other fibrous materials
				  commonly present in workplaces are: Fiberglass, mineral wool, ceramic wool,
				  refractory ceramic fibers, kevlar, nomex, synthetic fibers, graphite or carbon
				  fibers, cellulose (paper or wood) fibers, metal fibers, etc.
Matrix embedding material can sometimes be a negative
				  interference. The analyst may not be able to easily extract the fibers from the
				  matrix in order to use the method. Where possible, remove the matrix before the
				  analysis, taking careful note of the loss of weight. Some common matrix
				  materials are: Vinyl, rubber, tar, paint, plant fiber, cement, and epoxy. A
				  further negative interference is that the asbestos fibers themselves may be
				  either too small to be seen in Phase Contrast Microscopy (PCM) or of a very low
				  fibrous quality, having the appearance of plant fibers. The analyst's ability
				  to deal with these materials increases with experience.
(f) Uses and Occupational Exposure
Asbestos is ubiquitous in the environment. More than 40% of
				  the land area of the United States is composed of minerals which may contain
				  asbestos. Fortunately, the actual formation of great amounts of asbestos is
				  relatively rare. Nonetheless, there are locations in which environmental
				  exposure can be severe such as in the Serpentine Hills of California.
There are thousands of uses for asbestos in industry and the
				  home. Asbestos abatement workers are the most current segment of the population
				  to have occupational exposure to great amounts of asbestos. If the material is
				  undisturbed, there is no exposure. Exposure occurs when the asbestos-containing
				  material is abraded or otherwise disturbed during maintenance operations or
				  some other activity. Approximately 95% of the asbestos in place in the United
				  States is chrysotile.
Amosite and crocidolite make up nearly all the difference.
				  Tremolite and anthophyllite make up a very small percentage. Tremolite is found
				  in extremely small amounts in certain chrysotile deposits. Actinolite exposure
				  is probably greatest from environmental sources, but has been identified in
				  vermiculite containing, sprayed-on insulating materials which may have been
				  certified as asbestos-free.
(g) Physical and Chemical Properties
The nominal chemical compositions for the asbestos minerals
				  were given in subsection (1). Compared to cleavage fragments of the same
				  minerals, asbestiform fibers possess a high tensile strength along the fiber
				  axis. They are chemically inert, noncombustible, and heat resistant. Except for
				  chryso-tile, they are insoluble in Hydrochloric acid (HCl). Chrysotile is
				  slightly soluble in HCl. Asbestos has high electrical resistance and good sound
				  absorbing characteristics. It can be woven into cables, fabrics or other
				  textiles, or matted into papers, felts, and mats.
(h) Toxicology (This Section is for
				Information Only and Should Not Be Taken as WISHA Policy)
Possible physiologic results of respiratory exposure to
				  asbestos are mesothelioma of the pleura or peritoneum, interstitial fibrosis,
				  asbestosis, pneumoconiosis, or respiratory cancer. The possible consequences of
				  asbestos exposure are detailed in the NIOSH Criteria Document or in the WISHA
				  Asbestos Standards, WAC
				  296-62-077.