|Developing ASTM Standards For Monitoring Asbestos |
A Continuing Challenge
by Michael E. Beard, James R. Millette, and James S. Webber
A Transmission Electron Microscope Used in Analysis of Fine Asbestos Fibers Such as Those Found in Air and in Some Dust Samples
Asbestos is a generic term used to describe a series of naturally occurring silicate mineral fibers. The most common of these fibers are chrysotile (a serpentine mineral), and five amphibole minerals amosite, crocidolite, anthophyllite, tremolite and actinolite. These fibers have been widely incorporated into fireproofing, friction products, soundproofing, insulation, concrete reinforcement, and a variety of other uses.
While asbestos fibers have been very useful in such applications, they have also proved to be harmful to humans when breathed because the body has no mechanism for removing these durable fibers from deep in the lungs. These entrenched fibers cause a variety of diseases, including asbestosis, lung cancer and mesothelioma that may not appear until decades after inhalation. Asbestosis is a scarring of the lung tissue that is characterized by impaired lung function and shortness of breath. Incidence of lung cancer is exacerbated among smoking asbestos workers whose upper-respiratory fiber-removal mechanisms are damaged and whose fiber-filled lungs may have been overly susceptible to the carcinogens in cigarette smoke. In some cases the asbestos fibers will work their way through the lung to the outer lining of the chest cavity and cause a rare and always-fatal cancer known as mesothelioma.
The recognition of sprayed-on asbestos enormous fire-proofing efficiency during the shipbuilding years of World War II led to its subsequent increased use in schools and buildings for fireproofing, acoustical ceiling plasters, and insulation. The perplexing outbreak of mesothelioma, a previously rare disease, in the 1960s and 1970s was traced to the exposure of WW II shipyard workers to asbestos. As further studies solidified airborne asbestos as a cause of severe respiratory diseases, various government agencies established controls to protect workers from exposure. Workplace atmospheres were monitored by collecting air samples and respiratory protection was required for workers where the exposure limits were exceeded.
In the 1970s the U.S. Environmental Protection Agency and the Occupational Safety and Health Administration were formed, and formal monitoring programs were established. These agencies established standards for asbestos in the environment and named specific techniques for determining compliance with the standards. Regulated asbestos-containing materials were defined as having greater than 1 percent asbestos as determined by polarized light microscopy (PLM). Standards for workplace atmospheres were established and compliance was determined by phase contrast microscopy (PCM).
D22 and Regulations
In 1985, ASTM formed a working group in Subcommittee D22.05 on Indoor Air, part of Committee D22 on Sampling and Analysis of Atmospheres, to develop a series of analytical test methods to address asbestos monitoring in the indoor environment. This working group became Subcommittee D22.07 on Sampling and Analysis of Asbestos in 1991. Test methods were drafted for PLM, PCM, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). A guide was proposed that would describe the use of each of these techniques in detecting asbestos in the indoor environment. A Proposed Test Method for Asbestos-Containing Materials by Polarized Light Microscopy was published in 1993. Portions of this temporary standard, especially definitions for asbestos fibers, were included in subsequent methods developed by the EPA and others.
In 1986, the Asbestos Hazard Emergency Response Act (AHERA) was passed by the U.S. Congress to protect school occupants from exposure to asbestos. The EPA established abatement procedures to meet AHERA and clearance criteria were established requiring TEM and PCM to determine compliance. Analytical procedures were established describing PLM, PCM, and TEM for determining compliance with the AHERA standard. Many of these methods were influenced by the groundwork established by the participants in D22.07.
Federal and state regulations for asbestos included compliance monitoring analytical methods that all but eliminated the need for the voluntary standards being developed in ASTM D22.07. However, because the compliance monitoring methods were widely used, they were applied to a wide variety of materials not originally anticipated. A prime example was the need to analyze vinyl asbestos floor tiles and mastic. The PLM and TEM analytical capabilities are suitable for the identification of asbestos in any material, but the material must be properly prepared for the analysis and these preparation techniques were lacking or not clearly described in the compliance methods. As D22.07 members discussed these and other issues, they conducted research and shared results with government agencies to overcome these problems.
In 1989, the subcommittee began work on methods for the analysis of asbestos in settled dust. Concern for potential exposure to asbestos fibers in aerosols generated by the disturbance of settled dust seemed a legitimate area for monitoring, but interpretation of the data and estimating the risk has proved highly controversial. Three methods for asbestos in settled dust have become ASTM standards through the efforts of D22.07. They are:
D 5755, Test Method for Microvacuum Sampling and Indirect Analysis of Dust by Transmission Electron Microscopy for Asbestos Structure Number Surface Loading;
D 5756, Test Method for Microvacuum Sampling and Indirect Analysis of Dust by Transmission Electron Microscopy for Asbestos Mass Surface Loading; and
D 6480, Test Method for Wipe Sampling of Surfaces, Indirect Preparation, and Analysis for Asbestos Structure Number Concentration by Transmission Electron Microscopy.
These methods collect surface dust by vacuuming or wiping a surface and use an indirect means of sample preparation to disperse the dust particles. The samples are then analyzed by TEM. The results of the analysis are expressed in numbers or mass of structures per square centimetre of surface sampled. The number count methods were originally designed with an analytical sensitivity of about 1,000 structures per square centimeter but can achieve much better sensitivities on clean surfaces. A nominal analytical sensitivity for the mass determination is 0.24 picograms of asbestos per square centimetre.
The indirect preparation step has been questioned by some since it produces a result that is higher than a direct analysis. The indirect analysis may separate clusters of fibers or matrix materials containing fibers that were originally in a form that may not have been respirable. However, methods that attempt to use a direct sample preparation may not be able to detect asbestos fibers in a complex matrix. A direct relationship to exposure risk and the data produced by any of these methods has not been developed. However, the methods remain useful and many consultants use these methods in building surveys.
Interpretation of the data produced by the dust methods has been controversial and a subject argued in litigation. The dust methods have been widely used for assessment in such places as California following the 1994 Northridge earthquake, in Libby, Mont., in conjunction with studies of the asbestos-laden vermiculite deposits, and in New York, N.Y., following the World Trade Center disaster. While many concede that asbestos-containing dust may create an aerosol if disturbed by activity in a given environment, there is no universal agreement on how to use settled-dust data to predict exposure.
Some investigators calculate the mass of the asbestos in the dust and suggest that only levels above 1 percent are subject to government regulations. Visible dust in an area where asbestos-containing materials have been disturbed is considered asbestos-containing material as well by USEPA and OSHA, but no government regulations use data on asbestos in settled dust as determined by any of the ASTM methods. Another interpretation of the dust data is reference levels based on observations made by experienced industrial hygienists, in which a level less than 1,000 structures per square centimeter is considered background, greater than 10,000 structures per square centimeter is considered above background, and above 100,000 structures per square centimeter is considered high.
A third interpretation predicts airborne asbestos concentrations using surface loadings and an empirical relationship determined from coordinated measurements of each. The ratio of the air concentration to surface loading is called K, and K factors developed from various activities are reported by several investigators.
Members of D22.07 are working diligently to develop a guide that will provide a reasonable approach to the interpretation of dust data. Although this effort has been ongoing since 1993, recent progress has been made based on comparing asbestos dust levels in the area of interest with background levels. All interests would like to reach an agreement on this most controversial subject.
D22.07 members have also produced the following ASTM standards:
D 6281, Test Method for Airborne Asbestos Concentration in Ambient and Indoor Atmospheres as Determined by Transmission Electron Microscopy Direct Transfer (TEM); and
D 6620, Practice for Asbestos Detection Limit Based on Counts.
Another important area where D22.07 is working is related to definitions for asbestos fibers. While chemical composition and crystal structure of the regulated forms of asbestos are generally agreed upon, classifying other fibers based on their length and diameter (or width) characteristics is a matter of controversy. Literature reports asbestos fibers having a wide range of lengths and widths, but with average aspect ratios (the ratio of length to width) ranging from 20:1 to 100:1. Amphibole materials, when comminuted (pulverized), produce elongated particles called cleavage fragments that have the same chemistry and crystal structure as asbestos fibers. These cleavage fragments generally have much shorter aspect ratios, are not considered hazardous by many. Although cleavage fragments have not previously been included as asbestos fibers in government regulations, this issue is currenty being reviewed.
The role of cleavage fragments in risk assessment is a much debated subject. A source of confusion in this issue is the fact that many government regulations count particles with a 3:1 aspect ratio as asbestos. While a particle may well be defined as a fiber when it has the 3:1 ratio, it may not be an asbestos fiber. Histograms showing aspect ratios of cleavage fragments and asbestos fibers reveal an overlap of high aspect ratio cleavage fragments and low aspect ratio asbestos fibers. Unfortunately, analytical methods do not clearly define a difference between asbestos fibers and cleavage fragments.
Currently, there is no agreement on how to make a distinction between asbestos fiber and cleavage fragments, and analytical results are subject to being challenged. D22.07 members are planning a conference to discuss these issues and hopefully reach some consensus on definitions and protocols. The conference is likely to be held at Johnson State College in Johnson, Vt., in the summer of 2005.
Subcommittee D22.07 meetings are often lively exchanges of information and discussion. Membership is currently about 60 and between 10 and 20 members faithfully attend the meetings. As with any ASTM committee, visitors are always welcome. The members have put aside their differences and produced five consensus standards that are widely used. In spite of differences in opinion or approach to monitoring asbestos, the members are dedicated to the ASTM process. //
Copyright 2004, ASTM International