Overview of AEGL Program and NAC/AEGL Committee
The concerns of the U.S. Environmental Protection Agency (EPA), other U.S. federal agencies, state and local agencies, private industry, and other organizations in the private sector regarding short-term exposures due to chemical accidents became sharply focused following the accidental release of methyl isocyanate in Bhopal, India in December of 1984. In November 1985, as part of EPA’s National Strategy for Toxic Air Pollutants, EPA developed the Chemical Emergency Preparedness Program. This voluntary program identified a list of more than 400 acutely toxic chemicals and provided this information, together with interim technical guidance, for the development of emergency response plans at the local community level. At that time, EPA adopted the National Institute for Occupational Safety and Health (NIOSH) immediately dangerous to life and health (IDLH) exposure values, or an approximation of those values in instances where IDLH values were not published, to serve as the initial airborne concentrations of concern for each chemical.
During this same period, the U.S. Chemical Manufacturers Association (CMA) (now known as American Chemistry Council) developed and implemented the Community Awareness and Emergency Response Program. This program encouraged chemical plant managers to assist community leaders in preparing for potential accidental releases of acutely toxic chemicals. The program was intended to provide local communities with information on existing chemicals and chemical processes, technical expertise to assist in
emergency planning, notification, and response, as well as in training emergency-response personnel.
In October 1986, as part of the reauthorization of Superfund, Congress wrote into law an emergency planning program under the Superfund Amendments and Reauthorization Act (SARA Title III). Under this act, states were required to have emergency response plans for chemical accidents developed at the local community level. EPA subsequently adjusted the level-of-concern values to one-tenth of the IDLH values or their equivalents as an approach to improving the safety of the levels used for the general public. Since that time, the agency and other organizations, including private industry, have been interested in adopting more rigorous methodologies for determining values that would be deemed safe for the general public. During this period, the American Industrial Hygiene Association (AIHA) established a committee, the Emergency Response Planning Guidelines (ERPG) Committee to develop ERPGs and pioneered the concept of developing three different airborne concentrations for each chemical that would reflect the thresholds for important health-effect endpoints. That committee was later renamed the Emergency Response Planning (ERP) Committee. Although constrained by limited resources, the ERP Committee has developed 1-h exposure limits for more than 70 chemicals during the past 10 years.
At a workshop hosted by EPA in 1987, it was proposed by EPA that the ERP Committee and scientists from federal and state agencies as well as scientists and clinicians from academia and public interest groups pool their technical and financial resources and form a single committee comprising scientists from both the public sector and the private sector to develop AEGL values. EPA conceived the idea to formulate general guidance for developing short-term exposure limits and together with the Agency for Toxic Substances and Diseases Registry (ATSDR) subsequently requested that the National Research Council (NRC) develop guidance on the use of procedures and methodologies to establish emergency exposure guideline levels for the general public. The NRC convened the Subcommittee on Acute Exposure Guideline Levels within the Committee on Toxicology (COT) to address the project.
Since the 1940s, COT has developed emergency exposure levels for numerous chemicals of concern to the U.S. Department of Defense (DOD). These values are referred to as “emergency exposure guidance levels” (EEGLs). Although the EEGLs were developed for use for military personnel, the NRC also developed emergency exposure levels for the general public, termed “short-term public emergency guidance levels” (SPEGLs). On the basis of this extensive experience and the high level of scientific and technical expertise continually available to the NRC, this organization was considered
the most qualified entity to develop guidance on the methodologies and procedures used to establish short-term exposure limits for acutely toxic chemicals.
The NRC guidance document Guidelines for Developing Community Emergency Exposure Levels for Hazardous Substances was published in 1993. The community emergency exposure levels (CEELs) and the acute exposure guideline levels (AEGLs) represent the identical short-term emergency exposure levels. The NRC CEELs has been replaced by AEGLs to convey the broad applications of these values for planning, response, and prevention in the community, the workplace, transportation, the military, and the remediation of Superfund sites. A discussion of how AEGLs might be used for emergency planning, response, and prevention appears later in this chapter.
The efforts to mobilize the federal and state agencies and organizations in the private sector to form the committee began shortly after the release of the NRC (1993a) report. In October 1995, the National Advisory Committee on Acute Exposure Guideline Levels for Hazardous Substances (NAC/AEGL Committee) was formally chartered and the charter filed with the U.S. Congress under the Federal Advisory Committee Act (FACA) with approval by the Office of Management and Budget (OMB) and concurrence by the General Services Administration (GSA). Due to EPA budgetary constraints, the first meeting of the NAC/AEGL Committee was not held until June 1996. This meeting represented the culmination of the efforts to solicit stakeholders, identify committee members, form the committee, obtain the technical support of the Oak Ridge National Laboratories (ORNL), and begin the development of the AEGL values.
PURPOSE AND OBJECTIVES OF THE AEGL PROGRAM AND THE NAC/AEGL COMMITTEE
The primary purpose of the AEGL program and the NAC/AEGL Committee is to develop guideline levels for once-in-a-lifetime, short-term exposures to airborne concentrations of acutely toxic, high-priority chemicals. AEGLs are needed for a wide range of applications—planning, response, and prevention. These applications may include EPA’s SARA Title III Section 302–304 emergency planning program, the U.S. Clean Air Act Amendments (CAAA) Section 112(r) accident prevention program, and the remediation of Superfund sites program; the Department of Energy’s (DOE’s) environmental restoration, waste management, waste transport, and fixed facility programs; the Department of Transportation’s (DOT’s) emergency waste response program; the Department of Defense’s (DOD’s) environmental restoration, waste manage-
ment, and fixed facility programs; ATSDR’s health consultation and risk assessment programs; NIOSH’s and the Occupational Safety and Health Administration’s (OSHA’s) regulations and guidelines for workplace exposure; state CAA Section 112(b) programs and other state programs; and private-sector programs, such as the AIHA-ERPG and the American Chemistry Council’s Chemtrec programs.
A principal objective of the NAC/AEGL Committee is to develop scientifically credible, acute (short-term) once-in-a-lifetime exposure guideline levels within the constraints of data availability, resources, and time. That objective requires highly effective and efficient efforts in data gathering, data evaluation, and data summarization; fostering the participation of a large cross-section of the relevant scientific community; and the adoption of procedures and methodologies that facilitate consensus-building for AEGL values within the committee.
Another principal objective of the committee is to develop AEGL values for approximately 400 to 500 acutely hazardous substances within the next 10 years. Therefore, the near-term objective is to increase the level of production of AEGL development to approximately 40 to 50 chemicals per year without exceeding budgetary limitations or compromising the scientific credibility of the values developed.
Further, in addition to determining AEGL values for three health-effect endpoints, it is intended to derive exposure values for the general public that are applicable to emergency (accidental), once-in-a-lifetime exposure periods ranging from 10 min to 8 h. Therefore, exposure limits will be developed for a minimum of five exposure periods (10 min, 30 min, 1 h, 4 h, and 8 h). Each AEGL tier is distinguished by varying degrees of severity of toxic effects, as initially conceived by the ERP Committee and further defined in Guidelines for Developing Community Emergency Exposure Levels for Hazardous Substances (NRC 1993a) and further defined by the NAC/AEGL Committee. The AEGL-1, AEGL-2, and AEGL-3 definitions are presented in the Introduction and in Section 2.1 of this standing operating procedures (SOP) manual.
As stated in the NRC guidelines (NRC 1993a) and described in the AEGL definitions, these exposure limits are intended to protect most individuals in the general population, including those that might be particularly susceptible to the deleterious effects of the chemicals. However, as stated in the guidelines and the definitions, it is recognized that certain individuals, subject to unique and idiosyncratic responses, could experience effects at concentrations below the corresponding AEGLs.
An important objective of the NAC/AEGL Committee is the establishment and maintenance of a comprehensive SOP manual that adheres to the NRC guidelines (NRC 1993a) and supplements, clarifies, interprets, or defines those
guidelines with regard to the specific use of certain procedures and methodologies, such as selection of no-observed-adverse-effect levels (NOAELs) or the lowest-observed-adverse-effect levels (LOAELs), and the use of uncertainty factors, modifying factors, interspecies and intraspecies extrapolation methods, time scaling, carcinogenic risk assessment, and other methods and procedures relevant to the development of AEGL values.
COMMITTEE MEMBERSHIP AND ORGANIZATIONAL STRUCTURE
The NAC/AEGL Committee comprises representatives of federal, state and local agencies and organizations in the private sector that derive programmatic or operational benefits from the AEGL values. The membership includes federal representatives from EPA, ATSDR, DOE, NIOSH, OSHA, DOT, DOD, the Centers for Disease Control and Prevention (CDC), the Food and Drug Administration (FDA), and the Federal Emergency Management Agency (FEMA). States providing committee representatives include New York, New Jersey, Texas, California, Minnesota, Illinois, Connecticut, and Vermont. Private companies with representatives include Honeywell, Inc., ExxonMobil, and Arch Chemicals, Inc. The American Industrial Hygiene Association (AIHA), the American College of Occupational and Environmental Medicine (ACOEM), the American Association of Poison Control Centers (AAPCC), and the American Federation of Labor and Congress of Industrial Organizations (AFL-CIO) are represented, as are other private-sector organizations. The committee also includes individuals from academia and a representative of environmental justice. A current list of the NAC/AEGL Committee members and their affiliations is shown on page 7. At present, the committee has 30 members.
Recently, the Organization of Economic Cooperation and Development (OECD) and various OECD member countries have expressed an interest in the AEGL program. Several OECD member countries, such as Germany and The Netherlands, have been participating in the committee’s activities and actively pursuing formal membership on the NAC/AEGL Committee. It is envisioned that the committee and the AEGL program in general will progressively expand its scope and participation to include the international community.
The program director of the AEGL program has the overall responsibility for the entire AEGL program and the NAC/AEGL Committee and its activities. A designated federal officer (DFO) is responsible for all administrative matters related to the committee to ensure that it functions properly and
efficiently. These individuals are not voting members of the committee. The NAC/AEGL committee chair is appointed by EPA and is selected from the committee members. In concert with the program director and the DFO, the chair coordinates the activities of the committee and also directs all formal meetings of the committee. From time to time, the members of the committee serve as chemical managers and chemical reviewers in a collaborative effort with assigned scientist-authors (noncommittee members) to develop AEGLs for a specific chemical. These groups of individuals are referred to as the AEGL development teams and their function is discussed in Section 4.8 of this manual.
SELECTION OF CHEMICALS FOR AEGL DEVELOPMENT
A master list of approximately 1,000 acutely toxic chemicals was initially compiled through the integration of individual priority lists of chemicals submitted by each U.S. federal agency. The master list was subsequently reviewed by individuals from certain state agencies and representatives from organizations in the private sector and modified as a result of comments and suggestions received. The various priority chemical lists were compiled separately by each federal agency on the basis of their individual assessments of the hazards, potential exposure, risk, and relevance of a chemical to their program needs. A list of approximately 400 chemicals representing the higher-priority chemicals was tentatively identified from the original master list. It was acknowledged that this list was subject to change based on the changing needs of the stakeholders.
On May 21, 1997, a list of 85 chemicals was published in the Federal Register. This list identified those chemicals from the list of approximately 400 chemicals considered to be of highest priority across all U.S. federal agencies and represented the selection of chemicals for AEGL development by the NAC/AEGL Committee for the first 2–3 years of the program. The committee has addressed these chemicals, and they are in the “draft,” “proposed,” “interim,” or “final” stages of development. Certain chemicals did not have an adequate database for AEGL development and, consequently, are on hold pending decisions regarding further toxicity testing. The initial highest-priority list of 85 chemicals is shown in Appendix A.
A second “working list” of priority chemicals is being selected from (1) the original master list, (2) the intermediate list of approximately 400 chemicals (which is a subset of the master list), and (3) the list of new, high-priority candidate chemicals submitted by U.S. agencies and organizations and OECD member countries that are planning to participate in the AEGL program. Although working lists will be published in the Federal Register and else-
where from time to time to indicate the NAC/AEGL Committee’s agenda, the priority of chemicals addressed, and, hence, the “working list,” is subject to modification if priorities of the NAC/AEGL Committee or individual stakeholder organizations, including international members, change during that period.
SCIENTIFIC CREDIBILITY OF AEGL VALUES
The scientific credibility of the AEGL values is based on adherence to the NRC guidelines (NRC 1993a) for the development of short-term exposure limits, the comprehensive nature of data collection and evaluation, the consistency of the methodologies and procedures used to develop the values, the potential of acute toxicity testing in cases of inadequate data, and the adoption of the most comprehensive peer-review process ever used to establish short-term exposure limits for acutely toxic chemicals.
The comprehensive data-gathering process involves literature searches for all relevant published data and the mobilization of all relevant unpublished data. Data and information from unpublished sources is obtained through individual companies in the private sector and the cooperation of trade associations. The completeness of the data searches is enhanced through the oversight and supplemental searches conducted by individual committee members and interested parties during the peer-review process.
Data evaluation and data selection are performed by scientists with expertise in toxicology and related disciplines from staff at the organization that drafts “technical support documents” and from assigned members of the NAC/AEGL Committee. Additionally, input on data evaluation and selection is provided by interested parties who participate in the open meetings of the committee or who formally comment on the Federal Register notices of proposed AEGL values.
The work of the NAC/AEGL Committee adheres to Guidelines for Developing Community Emergency Exposure Levels for Hazardous Substances (NRC 1993a). Since this guidance document represents a more general guidance for methodologies and procedures, the NAC/AEGL Committee interprets and develops greater detail related to the methodologies and procedures that it follows. These standing operating procedures (SOPs) are documented by the SOP Workgroup and represent a consensus or a two-thirds majority vote of the NAC/AEGL Committee. SOPs also represent concurrence of the NRC Subcommittee on Acute Exposure Guideline Levels (NRC/AEGL Subcommittee). Therefore, each step of the AEGL development process follows specific methodologies, criteria, or other guidelines to ensure consistent, scientifically sound values.
In those instances where adequate data are not available in the judgment of the NAC/AEGL Committee, AEGLs will not be developed. The AEGL program is committed to ensuring that AEGL values are derived from adequate data and information based on a consensus or a two-thirds majority vote of the NAC/AEGL Committee and concurrence of the NRC/AEGL Subcommittee.
To further assure the scientific credibility of the AEGL values and their supporting rationale, the most comprehensive peer-review process ever used in the development of short-term exposure limits has been established (see next section). This review process has been designed to effectively, yet efficiently, encourage and enable the participation of the scientific community and other interested parties from the public and private sectors in the development of AEGLs. Further, the review process utilizes the NRC/AEGL Subcommittee as the final scientific reviewer. Hence, the final judgment of scientifically credible values rests with the United States’ National Academy of Sciences’ National Research Council. A detailed summary of the AEGL development process is presented in the next section.
THE AEGL DEVELOPMENT AND PEER-REVIEW PROCESS
The process that has been established for the development of the AEGL values is the most comprehensive ever used for the determination of short-term exposure limits for acutely toxic chemicals. A summary of the overall process is presented in diagram form in Appendix B. The process consists of four basic stages in the development and status of the AEGLs, and they are identified according to the review level and concurrent status of the AEGL values. They include (1) draft AEGLs, (2) proposed AEGLs, (3) interim AEGLs, and (4) final AEGLs. The entire development process can be described by individually describing the four basic stages in the development of AEGL values.
Stage 1: Draft AEGLs
This first stage begins with a comprehensive search of the published scientific literature. Attempts are made to mobilize all relevant unpublished data through industry-trade associations and from individual companies in the private sector. A more detailed description of the published and unpublished sources of data and information utilized is provided in Section 2.3 of this document, which addresses search strategies. The data are evaluated by following the published NRC guidelines (NRC, 1993a) and this SOP manual,
and selected data are used as the basis for the derivation of the AEGL values and the supporting scientific rationale. Data evaluation, data selection, and development of a technical support document (TSD) are all performed as a collaborative effort among the staff scientists at the organization drafting the TSDs, the chemical manager, and two chemical reviewers. This group is called the AEGL Development Team. Specific NAC/AEGL Committee members are assigned to a team for each chemical under review. Hence, a separate team comprising different committee members is formed for each chemical under review. The product of this effort is a TSD that contains draft AEGLs. The draft TSD is subsequently circulated to all other NAC/AEGL Committee members for review and comment prior to a formal meeting of the committee. Revisions to the initial TSD and the draft AEGLs are made up to the time of the NAC/AEGL Committee meeting scheduled for formal presentation and discussion of the AEGL values and the documents. At the committee meeting, the committee deliberates and, if a quorum is present, attempts to reach a consensus or a two-thirds majority vote to elevate the draft AEGLs to “proposed” status. A quorum of the NAC/AEGL Committee is defined as 51% or more of the total NAC/AEGL Committee membership. If agreement cannot be reached, the committee conveys its issues and concerns to the AEGL Development Team and further work is conducted by this group. After completion of additional work, the chemical is resubmitted for consideration at a future meeting. If a consensus or a two-thirds majority vote of the committee cannot be achieved because of inadequate data, no AEGL values will be developed until adequate data become available.
Stage 2: Proposed AEGLs
Once the NAC/AEGL Committee has reached a consensus or a two-thirds majority vote on the AEGL values and supporting rationale, they are referred to as “proposed” AEGLs and are published in the Federal Register for a 30-day review and comment period. Following publication, the committee reviews the public comments, addresses and resolves relevant issues, and seeks a consensus or a two-thirds majority vote of those present on the original or modified AEGL values and the accompanying scientific rationale.
Stage 3: Interim AEGLs
Following resolution of relevant issues raised through public review and comment and subsequent approval of the committee, the AEGL values are classified as “interim.” The interim AEGL status represents the best efforts of
the NAC/AEGL Committee to establish exposure limits, and the values are available for use as deemed appropriate on an interim basis by federal and state regulatory agencies and the private sector. The interim AEGLs, the supporting scientific rationale, and the TSD, are subsequently presented to the NRC/AEGL Subcommittee for its review and concurrence. If concurrence cannot be achieved, the NRC/AEGL Subcommittee will submit its issues and concerns to the NAC/AEGL Committee for further work and resolution.
Stage 4: Final AEGLs
When concurrence by the NRC/AEGL Subcommittee is achieved, the AEGL values are considered “final” and published by the NRC. Final AEGL values may be used on a permanent basis by all federal, state and local agencies, and private organizations. It is possible that new data will become available from time to time that challenges the scientific credibility of final AEGLs. If that occurs, the chemical will be resubmitted to the NAC/AEGL Committee and recycled through the review process.
OPERATION OF THE NAC/AEGL COMMITTEE
The NAC/AEGL Committee meets formally four times each year for 2 1/2 days each. The meetings are scheduled for each quarter of the calendar year and are generally held in the months of March, June, September, and December. Because of overall cost considerations, the meetings are generally held in Washington, D.C. However, committee meetings may be held from time to time at other locations for justifiable reasons.
At least 15 days before the committee meetings, a notice of the meeting is published in the Federal Register together with a list of the chemicals and other matters to be addressed by the committee. The notice provides dates, times, and location of the meetings. The agenda is finalized and distributed to committee members approximately 1 week before the meeting. The agenda is also available to other interested parties at that time, upon request, through the DFO.
All NAC/AEGL Committee meetings are open to the public, and interested parties may schedule individual presentations of relevant data and information by contacting the DFO to establish a date and time. Relevant data and information from interested parties also may be provided to the committee through the DFO during the period of development of the draft AEGLs so that the data can be considered during the early stages of development. Data and
information also may be submitted when the document is in the proposed and interim stages.
The NAC/AEGL Committee meetings are conducted by the chair, who is appointed by EPA in accordance with the Federal Advisory Committee Act (FACA). At the time of the meeting, the chair and the other committee members will have received the initial draft and one or more revisions of the TSD and the draft, proposed, or interim AEGL values for each chemical on the agenda. Reviews, comments, and revisions are continuous up to the time of the meeting, and committee members are expected to be familiar with the draft, proposed, or interim AEGLs, the supporting rationale, and other data and information in each TSD, and to participate in the resolution of residual issues at the meeting. Procedures for the AEGL Development Teams and the other committee members regarding work on proposed or interim AEGLs are similar to those for draft AEGLs.
All decisions of the NAC/AEGL Committee related to the development of draft, proposed, interim, and final AEGLs and their supporting rationale are made by consensus or a two-thirds majority of a quorum of the committee members present.
The highlights of each meeting are recorded by the scientists who draft the TSDs, and written minutes are prepared, ratified, and maintained in the committee’s permanent records. Deliberations of each meeting are also tape-recorded when possible and stored in the committee’s permanent records by the DFO for future reference as necessary.
All proposed AEGL values and supporting scientific rationale are published in the U.S. Federal Register. Review and comment by interested parties and the general public are requested and encouraged. The committee’s response to official comments on Federal Register notices on proposed AEGL values consists of an evaluation of the comments received, discussions and deliberations that take place at committee meetings regarding the elevation of AEGLs from proposed to interim status, and changes to the TSDs as deemed appropriate by the NAC/AEGL Committee. This information is reflected on the tapes and in the minutes of the meetings and will be maintained for future reference. As previously mentioned, the SOP Workgroup, established in March 1997, documents, summarizes, and evaluates the various procedures, methodologies, and guidelines used by the committee in the gathering and evaluation of scientific data and information and the development of the AEGL values. The SOP Workgroup performs a critical function by providing the committee with detailed information on the committee’s interpretation of the NRC guidelines and the approaches the committee has taken in the derivation of each AEGL value for each chemical addressed. This documentation enables the NAC/AEGL Committee to continually document and assess the
basis for its decision-making, ensure consistency with the NRC guidelines, and maintain the scientific credibility of the AEGL values and the accompanying scientific rationale. This ongoing effort is continually documented and is identified for further revisions in the SOP manual.
VALUE OF A COLLABORATIVE EFFORT IN THE AEGL PROGRAM
The value of a collaborative effort in the AEGL program is related primarily to the pooling of substantial resources of the various stakeholders and the direct or indirect involvement of a significant portion of the relevant scientific community from both the public and private sectors. These factors, in turn, promote greater productivity, efficiency, and cost effectiveness of such an effort and greatly enhance the scientific credibility of the AEGLs that are developed by the committee.
The formation of the NAC/AEGL Committee, which consists of approximately 30 to 35 members has provided an important forum for scientists, clinicians, and others to develop AEGLs and resolve related scientific issues. The composition of the committee represents a balanced cross-section of the relevant scientific disciplines and a balance of U.S. federal and state agencies, academia, the medical community, private industry, public interest groups, and other organizations in the private sector. This mutual participation of stakeholders, including the regulators and the regulated community, in the development of the AEGLs promotes the acceptance of the AEGLs by all parties involved. Additionally, the diverse composition of the committee represents the nucleus of a broad network of scientists, clinicians, and other technical personnel that fosters information and data exchange and the resolution of relevant scientific and technical issues well beyond the committee membership. This network also facilitates the identification of national and international experts with particular expertise that may provide important data, information, or insight on a specific chemical or scientific issue.
The collaborative effort also results in greater scientific credibility of the exposure values developed. The pooling of resources enables a comprehensive gathering and evaluation effort of both published and unpublished data and information. Collaboration provides a broad base of relevant scientific knowledge and expertise that is highly focused on the chemicals and issues addressed by the committee. This approach provides sufficient scientific and technical resources for the SOP Workgroup to document and evaluate procedures and methodologies that instill rigor and consistency into the process and the resultant AEGL values. The documentation of these procedures and
methodologies is contained in this SOP manual. Finally, the collaborative effort has enabled the establishment of the most comprehensive peer-review process ever implemented for the development of short-term exposure limits.
The AEGL program has extended invitations to all OECD member countries to participate on the NAC/AEGL Committee and in program activities in general. To date, Germany and The Netherlands have commitments for participation. It is believed that expanding the scope of the AEGL program to include the international community will be of great benefit. Its participation will provide even greater resources, further broaden the base of scientific and technical expertise, provide new toxicologic data and insights, and foster the harmonization of emergency exposure limits at the international level.
In summary, the establishment of a collaborative effort, with its pooling of resources, represents the most productive, efficient, and cost-effective approach to the development of exposure guideline levels. Further, the effort results in the development of uniform values for a wide range of applications. This approach eliminates inconsistencies and confusion among individuals and organizations involved in emergency planning, response, and prevention of chemical accidents. In global terms, the NAC/AEGL Committee represents an approach to unifying the international community in the development and use of chemical emergency exposure limits. In the interests of multinational companies seeking uniform operating parameters and federal agencies having mandates to achieve international harmonization of standards and guidelines, the participation of the international community in the AEGL program represents an important goal of the AEGL program.
APPLICATIONS OF THE AEGL VALUES
It is anticipated that the AEGL values will be used for regulatory and nonregulatory purposes by U.S. federal and state agencies and possibly the international community in conjunction with chemical emergency response, planning, and prevention programs. More specifically, the AEGL values will be used for conducting various risk assessments to aid in the development of emergency preparedness and prevention plans, as well as real-time emergency response actions, for accidental chemical releases at fixed facilities and from transport earners. The AEGL values, which represent defined toxic endpoints, are used in conjunction with various chemical-release and dispersion models to determine geographical areas, or “vulnerable zones,” associated with accidental or terrorist releases of chemical substances. By determining these geographical areas and the presence of human populations and facilities within those zones, the potential risks associated with accidental chemical releases
can be estimated. For example, the release and dispersion models, which take into account the quantity and rate of release of the chemical, the volatility of the substance, the wind speed and wind stability at the time of the release, and the topographical characteristics in the area of the release, will define the geographical areas exposed and, quantitatively, the airborne concentration of the “plume” or the chemical cloud as it is dispersed. By comparing the projected airborne concentrations of the chemical substance in question with the exposed populations, human health risks associated with a chemical release can be estimated. Using these risk estimates, emergency-response personnel can make effective risk-management and risk-communication decisions to minimize the adverse impact of the release on human health. Figure 1–1 is a summary diagram that indicates the overall effects that are expected to occur above each of the three AEGL threshold tiers as well as the sensory and nonsensory or asymptomatic effects that are expected to occur below the AEGL-1 threshold. Figure 1–1 also indicates the expected increase in occurrence and severity of the various adverse health effects as the airborne concentration increases beyond each of the three AEGLs.
Because of the complex nature of chemical accidents, the populations at risk, the variable capabilities among emergency response units, and many other considerations related to a specific event, it is beyond the scope of this document to discuss or speculate on specific actions that should or could be taken at any point in time or at a given level of exposure to a specific chemical. However, emergency responders and planners know that various options are available, depending upon the circumstances, for reducing or even preventing the adverse impacts of chemical releases. In general, they include public notification and instruction, sheltering-in-place, evacuation procedures to enable or facilitate medical attention, or a combination of these options. Decisions on these options are important and are best left to local emergency planners and responders to be addressed on a case-by-case basis. Further, information regarding the applications of short-term exposure limits such as AEGLs may be obtained in Technical Guidance for Hazards Analysis (EPA 1987).