1.0 Introduction

The Agency for Toxic Substances and Disease Registry (ATSDR) invited seven expert panelists to a meeting to discuss the current understanding of health effects related to asbestos and synthetic vitreous fibers (SVF) less than 5 micrometers (µm) in length—an issue that is related to the agency's ongoing work at many sites. The expert panel review took place in a meeting open to the public on October 29-30, 2002, in New York City. Discussions at the meeting focused on three specific issues: the physiological fate of fibers less than 5 µm in length, health effects of fibers less than 5 µm in length, and data gaps.

This report summarizes the technical discussions among the expert panelists and documents comments provided by observers. The remainder of this introductory section reviews the background on ATSDR's concern about fibers less than 5 µm in length (Section 1.1), the scope of this expert panel review (Section 1.2), and the organization of this report (Section 1.3).

1.1 Background

ATSDR conducts public health assessments to evaluate the public health implications of exposure to contaminants from hazardous waste sites and other environmental releases. A crucial part of these evaluations is understanding the toxicologic implications of environmental exposures. Recent events have highlighted a need for ATSDR to explore the potential of exposure to biopersistent fibers—specifically asbestos and some SVF—to cause health effects. For instance, ATSDR is currently assessing the implications of residential and community exposures to fibers from past industrial operations (e.g., vermiculite processing plants across the country), contaminants at hazardous waste sites, and dust in Lower Manhattan generated from the collapse of the World Trade Center (WTC) buildings. These sites are distinct in that contaminants have been found, or are suspected of being present, in residents' homes. Moreover, ATSDR has received concerns specifically about the public health implications of exposure to shorter fibers, particularly for materials found in Lower Manhattan.

ATSDR has therefore identified a need to understand the potential of fibers less than 5 µm in length to contribute to adverse health effects. As one part of addressing this need, ATSDR convened an expert panel to discuss and review the current state of the science regarding the influence of fiber length on health effects of asbestos and SVF. ATSDR will use the panel's findings to help develop scientifically sound public health evaluations for human exposures to small fibers.

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1.2 Scope of the Expert Panel Review

The expert panel review involved many activities before the meeting (see Section 1.2.1), at the meeting (see Section 1.2.2), and after the meeting (see Section 1.2.3). The following subsections describe what each of these tasks entailed.

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1.2.1 Activities Prior to the Expert Panel Meeting

ATSDR selected seven experts in toxicology, epidemiology, pathology, pulmonology, hygiene, and medicine to serve as panelists for the meeting. Every panelist is either a senior scientist, physician, or researcher with extensive experience in the aforementioned fields, as demonstrated by peer-reviewed publications, awards, and service to relevant professional societies. ATSDR selected panelists with a broad range of affiliations (e.g., academia, consulting, other federal agencies) in hope that the expert panel would offer a balanced perspective on the meeting topics. Furthermore, during its search for expert panelists, ATSDR asked all candidates to disclose real or perceived conflicts of interest. Appendix A lists the names and affiliations of the seven expert panelists selected for this meeting, and Appendix B includes brief biographies that summarize the panelists' areas of expertise.

To focus the discussions at the meeting, ATSDR prepared written guidelines (commonly called a "charge") for the expert panelists. The charge included several questions that the expert panelists discussed during the meeting. These questions addressed the physiological fate of fibers less than 5 µm in length, the health effects associated with these fibers, and data gaps. A copy of the charge is included in Appendix B. Several weeks prior to the expert panel meeting, every panelist received a copy of the charge, logistical information for the meeting, a preliminary bibliography of publications on asbestos and SVF, and copies of six publications relevant to the meeting topics (Bourdes et al. 2000; Churg et al. 2000; Davis 1994; Kinnula 1999; Morgan 1995; Ohyama et al. 2001).

In the weeks after the panelists received these materials, the panelists were asked to prepare their initial responses to the charge questions. Booklets of the premeeting comments were distributed the expert panelists, and made available to observers who registered in advance to attend the expert panel review. These initial comments are included in this report, without modification, as Appendix B. It should be noted that the premeeting comments are preliminary in nature. Some panelists' technical findings may have changed after the premeeting comments were submitted.

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1.2.2 Activities at the Expert Panel Review Meeting

The seven panelists and approximately 50 observers attended the expert panel meeting, which was held at the Jacob K. Javitz Federal Building in New York City, New York, on October 29-30, 2002. The meeting was open to the public, and the meeting dates and times were announced in the Federal Register. Appendix C lists the observers who confirmed their attendance at the meeting registration desk. The schedule of the expert panel meeting generally followed the agenda, presented here as Appendix D. The remainder of this section describes the introductory presentations given at the meeting.

• Introductory remarks from ATSDR. The meeting began with Rear Admiral (RADM) Robert Williams (Director of ATSDR's Division of Health Assessment and Consultation and Chief Engineer for the United States Public Health Service) explaining why ATSDR had convened the expert panel. He first reviewed ATSDR's site-specific experiences with asbestos contamination since 1980: assessing roughly 150 sites at which asbestos was a contaminant of concern, evaluating approximately 50 sites at which completed or potential exposure pathways were found for asbestos, and issuing public health advisories for sites where the public might come into contact with elevated levels of asbestos-contaminated materials. RADM Williams indicated that the available environmental data for these previous evaluations were typically the percent of asbestos in a waste material, as quantified by measurement methods that count fibers longer than 5 µm. For most of these sites, detailed information on fiber size distributions is not available.

  More recent work on sites with asbestos contamination, RADM Williams explained, has led to a greater need to understand the role of fiber length on asbestos toxicity. He reviewed ATSDR's activities at two sites with public health concerns regarding asbestos exposure. First, RADM Williams presented findings from medical testing that ATSDR conducted on residents of Libby, Montana, where vermiculite mining and exfoliation operations occurred for more than 50 years. ATSDR found that 18% of the residents tested (which included workers at the former mine and exfoliation plant) had pleural abnormalities, which were most prevalent among people who had lived in the area longest and who had completed exposure pathways for asbestos. RADM Williams described ATSDR's ongoing public health actions to address asbestos exposure issues in Libby. Second, RADM Williams described ATSDR's recent activities evaluating asbestos and SVF in dust generated during the WTC collapse. Activities included reviewing results of asbestos samples, conducting limited sampling in residential properties, evaluating whether buildings could be entered for occupational purposes, and assessing the need for maintaining the "exclusion zone" in Lower Manhattan.

  RADM Williams indicated that ATSDR's experiences with the Libby, WTC, and other sites have raised unique challenges regarding asbestos and SVF. At these sites, for example, fibers are being found in homes, rather than at waste sites and in the environment; children are being exposed; and analytical methods are now quantifying amounts of shorter fibers (less than 5 µm) than were typically characterized previously. As one step in helping the agency respond to these challenges, RADM Williams indicated, ATSDR convened the expert panel to review the current state of the science on health effects of asbestos and SVF, focusing on the role of fiber length. RADM Williams explained that ATSDR often uses the expert panel forum to seek scientific input on priority issues the agency is evaluating. He noted that the panelists were invited to present their individual opinions and were not asked to reach consensus on any issue, and representatives from ATSDR were present strictly to observe the proceedings.

• Introductory remarks from the meeting chair. Dr. Morton Lippmann, the chair of the expert panel meeting, provided additional introductory remarks. After reviewing the charge to the panelists and the meeting agenda, Dr. Lippmann indicated that the goal of the expert panel meeting was to review health effects associated with asbestos and SVF, with a special emphasis on fibers shorter than 5 µm. He explained that the focus on fibers less than 5 µm emerged from conventions previously used to evaluate asbestos exposures. Specifically, risk assessment decisions related to asbestos, Dr. Lippmann noted, have typically been based on optical measurements of fibers longer than 5 µm, and one goal of the expert panel was to evaluate the toxicity of the shorter fibers that are not counted by the optical analytical methods. Dr. Lippmann also emphasized that the expert panel's discussions should have a public health focus, such that ATSDR could apply the findings from the expert panel to sites where community members are concerned about exposure to asbestos and SVF.

  To illustrate recent concerns about asbestos and SVF, Dr. Lippmann described ongoing research being conducted to evaluate contamination by WTC dust in Lower Manhattan. He indicated, for example, that his research group and colleagues have collected and analyzed numerous settled dust samples and ambient air samples following the WTC collapse and are evaluating health effects among approximately 300 firefighters and several thousand residents of Lower Manhattan. These dust samples reportedly were composed almost entirely of particles larger than 10 µm in aerodynamic diameter, with only 1% of fine particles less than 2.5 µm in aerodynamic diameter. Dr. Lippmann also noted that asbestos fibers detected in the dust samples were primarily small (less than 5 µm), because building materials were crushed by the force of the WTC collapse. He indicated that the purpose of the expert panel review was to help ATSDR interpret the public health significance of short fibers, like those detected in the WTC dust.

Following these opening presentations, Dr. Lippmann asked the panelists to introduce themselves by stating their names, affiliations, areas of expertise, and past research experience. For the remainder of the meeting, the panelists gave individual presentations and engaged in free-flowing discussions when answering the charge questions and addressing additional topics not specified in the charge. Observers were given the opportunity to provide verbal comments throughout the expert panel meeting. Representatives from ATSDR were observers at the meeting and did not engage in or direct the panelists' discussions.

Are structures less than 5 µm in length fibers or particles? The expert panel meeting was convened to address the health effects of fibers less than 5 µm, but some panelists questioned the appropriateness of the relevant terminology. One panelist, for instance, noted that many scientists would classify structures smaller than 5 µm as particles, regardless of the structures' aspect ratios (see Dr. Case's premeeting comments in Appendix B). During his introductory remarks, Dr. Lippmann reviewed these concerns and noted that mineralogists, geologists, health scientists, and individuals in other disciplines may use different definitions of fibers and these definitions may be based on size, aspect ratio, and other properties. Section 2.4 presents more detailed information on the panelists' opinions on the most appropriate terminology. This issue is raised here to inform readers that this entire report uses the term "fibers less than 5 µm," while acknowledging that some panelists had reservations about suggesting that structures less than 5 µm are fibers.

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1.2.3 Activities Following the Expert Panel Meeting

The primary activity following the expert panel meeting was preparing this summary report. A technical writer who attended the meeting prepared a draft of this report. The expert panelists were asked to review and comment on the draft report, ensuring that its contents accurately reflect the tone and content of the discussions at the expert panel meeting. The draft report was revised based on the panelists' comments. The panelists were then given the opportunity to review the revised report; and the final expert panel review report (i.e., this report) was submitted to ATSDR. Some panelists submitted written comments after the meeting; these are included in this report, without modification, as Appendix E. ATSDR was not involved in the preparation of this report.

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1.3 Report Organization

The structure of this report follows the order of the panelists' discussions during the meeting. For instance, Section 2 summarizes the discussions on the first agenda topic (physiological fate of asbestos and SVF less than 5 µm in length), Section 3 summarizes comments on the second topic (health effects of these fibers). Section 4 presents overall conclusions and recommendations. These report sections document comments raised both by the panelists and the observers. Finally, Section 5 provides references for all documents cited in the text.

The appendices to this report include extensive background information on the expert panel review. This information includes items made available to all meeting attendees, as well as items generated since the expert panel meeting (e.g., a final list of attendees). The appendices contain the following information:

• List of the expert panelists (Appendix A).
  
  
• The panelists' premeeting comments, the charge to the reviewers, and brief bios of the expert panelists (Appendix B).
  
  
• List of registered observers of the expert panel meeting (Appendix C).
  
  
• Agenda for the expert panel meeting (Appendix D).
  
  
• Written comments that panelists submitted after the meeting (Appendix E).
  

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