Ginger L. Gist, JeAnne R. Burg, and Timothy M. Radtke
Agency for Toxic Substances and Disease Registry
Public Health Service
U.S. Department of Health and Human Services
Atlanta, Georgia 30333
Introduction
An estimated 14,000 chemical substances are released into the environment each year (see Table 1). According to Fields (1991), 41 million people currently live within 4 miles of at least one of the 1,306 sites on the National Priorities List (NPL). By the year 2010, the number of sites on the NPL is estimated to increase to approximately 4000, thereby increasing the number of persons potentially exposed to hazardous substances in the environment.
Because of the potential for public health problems associated with environmental exposures to hazardous substances from waste sites and chemical spills, Congress enacted the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) in December 1980 (Burg 1989). CERCLA provided for the establishment of the Agency for Toxic Substances and Disease Registry (ATSDR), a new agency within the Public Health Service of the Department of Health and Human Services. One of the mandates in CERCLA requires that ATSDR create a registry of persons exposed to hazardous substances, the National Exposure Registry (NER). In addition, under the Superfund Amendments and Reauthorization Act of 1986 (SARA), ATSDR must consider a registry as a follow-up to a public health assessment when the results indicate a potentially significant risk to human health (ATSDR 1988).
The purpose of the NER is to facilitate the development of new scientific knowledge of long-term health consequences of environmental exposures to specific hazardous substances (ATSDR 1988) known as primary contaminants. Such hazardous substances have been or will be designated by ATSDR as having data gaps that the Registry data could help close. In addition, the hazardous substances have been designated to be of major concern by ATSDR for unique reasons and no adequate data are available to assess the health effects of these substances (ATSDR 1988).
The large database of information obtained by the NER will aid in assessing long-term health consequences by facilitating epidemiologic or health studies. The NER can serve an important role in ensuring the uniformity and quality of the collected data. In addition, the NER will minimize duplicative data collection. The mechanism by which subregistries are established, conducted, and maintained can be found in Policies and Procedures for Establishing a National Registry of Persons Exposed to Hazardous Substances (ATSDR 1988) as outlined in Figure 1.
The objective of this paper is to provide an explicit description of the process used to find, validate, and select sites for the NER.
Exposure Assessment
Estimation of the potential for adverse health outcomes associated with environmental contamination is not only complicated but also full of uncertainties and assumptions (Gochfeld 1991). It is composed of two primary activities: (1) exposure assessment and (2) effects assessment (Sexton et al. 1992). The process of selecting sites for inclusion in the NER focuses on exposure assessment.
Exposure to a contaminant is defined as an event that occurs when there is contact at a boundary between a human and the contaminant or a mixture of contaminants located within environmental media (Sexton et al. 1992, Ott 1982).
Although this expression represents the optimum, it should be noted that there is no uniform, well-established procedure for expressing exposure. Magnitude, usually expressed as concentration, is perhaps the most commonly reported parameter (Sexton et al. 1992). Other possibilities for expressing magnitude include quantities available for absorption, rates of intake, concentrations in body tissues, body burden, and organ dose (Brown 1987).
According to Michaud et al. (1991), the objective of an exposure assessment is to measure or estimate the intensity, frequency, and duration of human exposure to an agent, such as a hazardous substance, present in the environment. In its most complete form, an exposure assessment should describe the source, magnitude, duration, schedule, and route of exposure; the populations exposed; and the uncertainties in assessments. The site selection process for the NER essentially parallels the goals of exposure assessments. These goals include:
Identifying the sources of a specific substance and quantifying releases into the environment.
Determining the movement of the substance from its sources to the point of contact with the population and identifying the pathways of exposure.
Identifying important compounds and determining the concentration of the substance in the media at the point of contact with the population, in other words, determining exposure.
Determining the applied dose of the substance or its metabolite(s) within the individual (via dose reconstruction) (Akland 1991).
The actual site selection process for the NER is outlined in Figure 3 and discussed in the following section.
Phase I - Identification of Potential Sites
The first step in Phase I of site identification is to locate sites that are reported to have the primary substance present in the environmental media of concern. Identification of potential sites is initiated by reviewing data from available national databases. It should be noted that, despite their obvious importance, human exposure data typically are not collected in a systematic or comprehensive manner (Sexton et al. 1992, Burke et al. 1992, Stevens and Swackhamer 1989); therefore, the mere presence of the primary contaminant in a media of concern qualifies a site for initial review.
ATSDR has developed a database, HazDat (ATSDR 1993), in which all the information available on NPL sites, ATSDR public health assessment sites, emergency response sites, sites for which a health consultation has been conducted, and federal facility sites is stored. This database, as well as such others as the Federal Reporting Data System (FRDS) (USEPA 1991a) and the Storage and Retrieval of U.S. Waterways Parametric Data (STORET) (USEPA 1991b), is queried for a list of sites with the substance of concern in the media of concern. Finally, notices are sent to ATSDR regional representatives and placed on the Public Health Network electronic bulletin board requesting information for additional non-NPL sites at which potential exposure to the primary contaminant has been indicated by quantitative laboratory analysis of the media of concern. It should be noted that all potential sites, whether NPL or non-NPL, are given equal consideration for inclusion in the NER.
After a list of potential sites has been developed, the applicable ATSDR public health assessment, available for each NPL site and ATSDR-petitioned site, is reviewed. (Note: An ATSDR public health assessment is an evaluation of data and information on the release of hazardous substances into the environment designed to assess any current or future effects on public health, develop public health advisories or other health recommendations, and identify studies or actions needed to evaluate and mitigate or prevent human health effects (ATSDR 1992). The following criteria, in alphabetical order, are used to assess the candidacy of each site.
Completed exposure pathways. It can be confirmed that the population was exposed to the environmental media contaminated with the primary contaminant.
Contaminant concentration(s). The concentration(s) at which the primary contaminant was found in the environmental media during the period of exposure must be documented.
Documented contamination of media. Environmental samples indicate that the media of concern was contaminated with the primary contaminant; samples were taken from representative locations; and samples were collected and transported appropriately to be analyzed by a certified laboratory.
Duration of exposure. The duration of the exposure; the current exposure status of the population; and the length of time since the exposure ceased must be determined.
Size of the potentially exposed population. The number of people who may have been exposed to the substance from the site must be determined (ATSDR 1988).
All sites that are reviewed are logged into an in-house database so that disposition of the site considered can be tracked in an orderly fashion. Sites not meeting the preceding criteria are removed from further consideration. All information collected and retained for a site is entered into a tracking system, which is maintained for historical purposes.
Phase II - Data Collection
In the second screening of candidate sites, all ATSDR site files are reviewed. These files, compiled and maintained by ATSDR, typically contain the remedial investigation/feasibility study reports, the hazard review scoring packages, the remedial action master plans, and risk assessments (for NPL sites), as well as sampling information and reports of other health studies that might have been conducted at the site. In addition to the criteria listed previously, the site files are reviewed for information pertaining to the following secondary criteria (ATSDR 1988).
Assessment of participation. Information, such as news articles or records of public meetings, exists regarding community interest and/or prior community participation in site-related activities. The level(s) of support from state and local officials for the registry can be determined.
Existing biomonitoring data. Information is available indicating the body burden of the primary contaminant in the population associated with this site.
Number of secondary and/or potential confounding contaminants.
The presence or absence of other contaminants has been analytically confirmed in the environmental media of concern at this site. If contaminants are present, the levels have been determined. Evidence exists to indicate whether exposure to this substance(s) is likely to result in health effects that would mask those potentially attributable to the primary contaminant.
Particularly sensitive subpopulations. It has been determined whether sensitive subpopulations, such as children or the elderly, have been exposed to the contaminants at the site. The size of the exposed subpopulation(s) has been determined.
Reported health problems. Documentation exists that indicates the population has reported experiencing health problems potentially related to the site (ATSDR 1988).
As always, personal knowledge of a site is important. For that reason, each ATSDR regional representative is an invaluable source of site-specific information and contacts. At this point in the site selection process, additional documentation of site criteria is solicited from the regional representative for the region in which the site is located. The regional representative is also the liaison for contact with state and local officials who may have specific, in-depth knowledge pertinent to the site that might affect its selection for the subregistry. Other people who might be contacted for site-specific data are U.S. Environmental Protection Agency personnel, including the remedial project manager, and U.S. Geological Survey representatives, who might have additional sampling data or information about the physical characteristics of the site.
After review, sites are either retained for or removed from further consideration. Sites that are retained are ranked using the NER site-scoring protocol. This allows subjective ranking of a site using a scoring system which quantifies the requirements listed in Phases I and II. An example of a scoring form for chromium is included as an appendix.
Phase III - Site Visit/Data Validation
Site visits are conducted only for the highest ranking sites. A typical site visit consists of meeting with interested officials from the departments of health and/or environment for the state in order to discuss the NER and how the candidate site might be included in a subregistry. Permission is sought to review all state files for the candidate site. The same process is repeated with the county and local officials. The types of documentation typically reviewed during a site visit are media specific; for example, if the medium of concern is drinking water, the operating records and chemical analysis records for the water system are reviewed. Of particular interest are minutes from previous public meetings at the site and news articles written about the site, which indicate the level of citizen interest in site-related activities.
The neighborhoods affected by the site are inspected to discover any specific characteristics that might affect the site's position in the final ranking of candidate sites. The types of information that might affect a site's rank include previously unreported evidence of the presence of specifically susceptible populations or evidence of a particularly transient population.
Finally, environmental sampling data are reviewed for adherence to quality assurance and quality control (QA/QC) procedures. Data without verifiable QA/QC measures for sampling and analytical procedures are not accepted for consideration. Some procedures that are considered in the review include use of appropriate sample selection and collection procedures; chain of custody; sample storage, packaging, and transport; sample identification; split and duplicate sample analysis; analytical procedures (calibration verification, holding time, sample preparation, and blanks); and data reporting.
Phase IV - Final Selection
Upon completion of the site assessment and site visit, a rationale for selecting the site is prepared. The rationale includes information indicating how the site meets the criteria for site selection and which of the criteria, if any, the site does not meet. The rationale paper is reviewed by the administration within ATSDR. Based on the resources available, the site is either approved or disapproved for inclusion in an exposure subregistry.
Following inclusion of a site in the NER, the environmental data used in the selection process then serve as the basis for quantifying each registrant's exposure and accumulated dose. Because the environmental data do not fully characterize exposure throughout the defined exposure period, mathematical models and other methods of estimating exposure and dose are used. Through this process of dose reconstruction, each individual's exposure and/or the concentration of contaminants (or metabolites) in tissues are quantified. It is imperative to recognize, however, the uncertainty of the particular estimation technique when relating exposure/dose to adverse health outcomes.
Problems Associated With the Site Selection Process
A common problem when conducting exposure assessments for sites is there is typically little or no information on which to base estimates of exposure. Very often no actual environmental measurements, or far too few to quantify the exposure to a given individual or population, have been collected (Shalat 1990). Most of the environmental sampling data for a site are data that were collected after the site had been identified and exposures to contaminants had ceased. In addition, the available data tend to be anecdotal, fragmented, and focused narrowly on specific pollutants, media, and routes of exposure (Sexton et al. 1992). Although valuable for characterization of a site, this information does not provide an adequate basis for determining exposure to receptor populations.
Historical data, which are typically what is available for exposure assessment, can be inadequate for a variety of reasons.
Some common problems encountered by ATSDR staff with the use of historical data include:
No identified source(s) of contamination.
No information available to confirm individual levels of exposure or potential levels of exposure.
No information available to determine the duration of the exposure.
Samples taken in an inappropriate manner or analyses lacking the proper QA/QC measures.
Gaps in essential records or data; for example, lack of operation records or distribution samples.
Data problems of this type can and frequently do result in the elimination of what could possibly be a viable site for inclusion in the NER.
When data gaps exist, biologic markers can sometimes be used to verify that human contact with hazardous substances has occurred (Gift et al. 1991). A biologic marker of exposure is (1) an exogenous substance or its metabolite(s), (2) the product of an interaction between a xenobiotic agent and some target molecule or cell that is measured within a compartment of an organism, or (3) other events in an organism related to exposure (National Research Council 1987). As biologic markers become available for field use, they will be incorporated into the NER data collection process, thus overcoming many of the problems listed previously.
Although considerable information exists on NPL sites and some state priority sites that have proceeded through the remedial investigation phase, little or no information exists for thousands of sites that have been reported to local, state, or federal authorities. In some cases, although adequate information exists for a site, the site is not listed in any database or on any list of recognized sites. Additionally, access to the data for these sites is often limited, the data may be scattered among multiple files in multiple offices, or the data might even have been lost.
Summary
In summary, through the process of elimination, the number of potential sites for inclusion in a given subregistry is gradually reduced, resulting in the identification of sites with the most complete exposure assessment information for the contaminant of concern. Although these data never fully characterize personal exposure, they serve as the basis for dose reconstruction which is necessary for determining dose/response relationships.
To date, over 3,000 sites have been reviewed and 22 have been approved for inclusion in the five subregistries of the NER (Table 2). These subregistries include (in the order of establishment) the Trichloroethylene (TCE), Dioxin, and Benzene Subregistries (all with established sites), the Radioactive Substances Subregistry (for which one site has been approved), and the Chromium Subregistry (currently in the site selection phase).
As the site selection process continues, further refinements to the methods used will be needed to integrate improved environmental data into the process of exposure assessment. The process described within this paper has been structured to be flexible so that updating and validating the methods used will be relatively simple. By continually assessing and improving the site selection process, and thus improving exposure assessment, the health information collected for the NER should bring us closer to understanding the relationships between environmental exposures to contaminants and potential health effects related to those exposures.
REFERENCES
Agency for Toxic Substances and Disease Registry (ATSDR). HazDat Database, Version 1.5. Atlanta, Georgia: U.S. Department of Health and Human Services, Public Health Service, 1993.
Agency for Toxic Substances and Disease Registry (ATSDR). Public Health Assessment Guidance Manual. Atlanta, Georgia: U.S. Department of Health and Human Services, Public Health Service, 1992.
Agency for Toxic Substances and Disease Registry (ATSDR). Policies and Procedures for Establishing a National Registry of Persons Exposed to Hazardous Substances: The National Exposure Registry. Atlanta, Georgia: U.S. Department of Health and Human Services, Public Health Service, 1988.
Akland GG. Organizational components and structural features of EPA's new human exposure research program. Journal of Exposure Analysis and Environmental Epidemiology 1991; 1(2):129-141.
Brown SL. Exposure assessment. In: Tardiff RG, Rodricks JV, eds. Toxic Substances and Human Risk: Principles of Data Interpretation. New York: Plenum Press, 1987: 377-390.
Burg JR. Policies and procedures for establishing the National Exposure Registry. J. Am. Col. Tox. 1989; 8(5):949-954.
Burke T, Anderson H, Beach N, Colome S, Drew RT, Firestone M, Hauchman FS, Miller TO, Wagener DK, Zeise L, Tran N. Role of exposure databases in risk management. Arch. Environ. Health 1992; 47(6):421-429.
Fields T. Magnitude of toxic substances pollution problem: Superfund sites in regions, states, and localities close to HBCU/MI institutions. Presented at the Conference on Health Effects of Toxic Substances and Contingency Planning; September 26, 1991; Atlanta, Georgia.
Gift JS, Grissom RE, Straight JM. Biological markers: Monitoring populations exposed to hazardous substances. J. Environ. Health 1991; 54(2):22-26.
Gochfeld M. Setting the research agenda for chromium risk assessment. Environ. Health Perspect. 1991; 92:3-5.
Lioy PJ. Assessing total human exposure to contaminants: A multidisciplinary approach. Environ. Sci. Technol. 1990; 24:938-945.
Michaud JM, Ripple SR, Paustenbach DJ. Human health risks associated with contaminated sites: Critical factors in the exposure assessment. In Kostecki PT, Calabrese EJ, eds. Hydrocarbon Contaminated Soils and Groundwater. Chelsea, Michigan: Lewis Publishers, 1991: 283-292.
National Research Council. Biological markers in environ-mental health research. Environ. Health Perspect. 1987; 74:3-9.
Ott W. Concepts of human exposure to air pollution. Environ. Inter. 1982; 7:179-196.
Sexton K, Selevan SG, Wagener DK, Lybarger JA. Estimating human exposures to environmental pollutants: Availability and utility of existing databases. Arch. Environ. Health 1992; 47(6):398-407.
Shalat SL. Can we use distance to study disease? In Andrews JS, Askew LO, Bucsela JA, Hoffman DA, Johnson BL, Xinteras C, eds. Proceedings of the Fourth National Environmental Health Conference - Environmental Issues: Today's Challenge for the Future. San Antonio: Association of State and Territorial Health Officials, Agency for Toxic Substances and Disease Registry, Center for Environmental Health and Injury Control - Centers for Disease Control 1990:33-38.
Stevens JB, Swackhamer DL. Environmental pollution. A multimedia approach to modeling human exposure. Environ. Sci. Technol. 1989; 23:1180-1186.
U.S. Environmental Protection Agency (USEPA). FRDS: Federal Reporting Data System. Washington, DC: Office of Groundwater and Drinking Water, 1991a.
U.S. Environmental Protection Agency (USEPA). STORET: Storage and Retrieval of U.S. Waterways Parametric Data. Washington, DC: Office of Water, 1991b.
Table 1.--Number of hazardous substances releases in the United States, 1987-1991 (Fields 1991).
| Event | Year | ||||
|---|---|---|---|---|---|
| 1987 | 1988 | 1989 | 1990 | 1991* | |
| Substances released | 12,120 | 13,425 | 16,367 | 14,554 | 10,094 |
| Oil Spills | 20,436 | 20,316 | 21,216 | 23,224 | 17,688 |
* Through September 1991.
Figure 1.--Outline for creating a subregistry of the National Exposure Registry (ATSDR 1988).
Selection of Primary Contaminant(s)
Site Selection
Population Selection
Data Collection
Data Storage
Follow-up Activities Termination Other Activities
of Subregistry
Figure 2.--Common pathways for human exposure to environmental pollutants (Sexton et al. 1992).
Figure 3.--Outline of the site selection process for the National Exposure Registry.
PHASE 1 - IDENTIFICATION OF POTENTIAL SITES
Step 1. Potential sites identified (primary contaminantpresent)
1. VIEW database
2. HazDat database
3. ATSDR regional representatives
4. Other federal and state and local officials
5. Other databases (STORET, FRDS, etc.)
Step 2. ATSDR public health assessments for potential sites reviewed(Primary criteria for review applied)
1. Documented contamination of media (one or more)
2. Level of contamination
3. Size of population potentially exposed
4. Current potential exposure
5. Past potential exposure (duration and time elapsed since last exposure)
6. Susceptible subpopulations exposed
7. Existing biomonitoring data
8. Number of secondary contaminants
Step 3. Candidate sites identifiedPHASE II - DATA COLLECTION
Step 4. ATSDR site files and additional information for candi-date sites reviewed(Secondary criteria for review applied)
1. Presence of sensitive subpopulations
2. Availability of biomonitoring data
3. Number of secondary or potentially confounding contaminants
4. Reported health problems
5. Assessment of participation
Step 5. Candidate sites ranked using the NER site-scoring protocol.PHASE III - SITE VISIT/DATA VALIDATION
Step 6. Site visit1. Meetings with state, county, and local officials
2. Review of state, county, and local files
3. Tour of site and affected neighborhoods
Step 7. All data collected are reviewed for appropriate quality assurance/quality control proceduresPHASE IV - FINAL SELECTION
Step 8. Preparation and review of rationale for selection of candidate site(s) Step 9. Site(s) selected Step 10. Environmental dose reconstruction implementedTable 2.--Sites included in the National Exposure Registry by chemical-specific subregistry.
| Subregistry | State | Site Name |
|---|---|---|
| Trichloroethylene | Michigan
Indiana Illinois Pennsylvania Arizona |
Verona Wells McGraw-Edison Dowagiac Superior Street Central Area Gemeinhardt Piccolo Company Conrail Rail Yard Belloit Corporation Byron Johnson Salvage Yard Southeast Rockford Groundwater Contamination Warner Electric Brake and Clutch Acme Solvents Reclamation Frinks Industrial Wastes Crossley Farms Tucson International Airport Site |
| Dioxin | Missouri | Minker/Stout/Romaine Creek
Quail Run Mobile Park Shenandoah Stables Times Beach |
| Benzene | Texas
New York |
Three Lakes Municipal Utilities District
Vestal Water Supply 1-1 |
| Radioactive Substances | Washington | Hanford Nuclear Reservation |
| Chromium | TBD* | TBD* |
*To be determined
Appendix. Site scoring protocol for the chromium subregistry of the National Exposure Registry.
Agency for Toxic Substances and Disease Registry, 1825 Century Blvd, Atlanta, GA 30345
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