Authors: Ginger L. Gist and JeAnne R. Burg
Affiliation: Agency for Toxic Substances and Disease Registry
Public Health Service
U.S. Department of Health and Human Services
Atlanta, Georgia
Corresponding author: Ginger L. Gist, Ph.D.
Senior Environmental Health Scientist
Agency for Toxic Substances and Disease Registry
1600 Clifton Road, Mailstop E-31
Atlanta, Georgia 30333
(404) 639-6202
Abbreviations: Agency for Toxic Substances and Disease Registry (ATSDR); Centers for Disease Control and Prevention (CDC); Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA); National Exposure Registry (NER); National Priorities List (NPL); National Toxicology Program (NTP); Reportable Quantity (RQ); Trichloroethane (TCA); Trichloroethylene (TCE); Superfund Amendments and Reauthorization Act (SARA); United States Environmental Protection Agency (EPA); Volatile organic compounds (VOCs)
ABSTRACT
The National Exposure Registry was created in response to the pervasiveness of chemical contamination at the nation's waste sites and the relative lack of information on human health outcomes associated with long-term, low-level exposure to most of these substances. A ranking scheme was developed by the Agency for Toxic Substances and Disease Registry (ATSDR) to select the substances for which substance-specific subregistries of the National Exposure Registry would be developed. This scheme uses a general decision analysis approach that incorporates the most relevant and up-to-date data available on the substances found at sites of which ATSDR has knowledge. There are currently four general subregistries (volatile organic compounds, dioxins, heavy metals, and radioactive substances) made up of persons exposed to specific primary contaminants as selected by means of this ranking scheme.
INTRODUCTION
The Agency for Toxic Substances and Disease Registry (ATSDR) created a registry of people exposed to hazardous substances in the environment, the National Exposure Registry (NER), in response to a mandate given in the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of 1980 (Federal Register 1980) and reiterated in the Superfund Amendments and Reauthorization Act (SARA) of 1986 (Federal Register 1986). The NER contains several subregistries on populations exposed to specific hazardous substances, known as primary contaminants. The purpose of the NER, as described in the Policies and Procedures Manual (ATSDR 1994a), is to facilitate the collection of new scientific knowledge on health effects by identifying and subsequently following people exposed to a defined substance at selected sites. The NER policies and procedures have been reviewed by scientists from ATSDR's Board of Scientific Counselors, as well as by scientists on multiple external review panels.
The first step in the process of developing a subregistry (Figure 1) is to select the primary contaminant, that is, the hazardous substance to which all members of that subregistry population have been exposed. The subsequent steps, from selection of sites to termination of a given subregistry, are discussed elsewhere (Gist et al. 1994, Burg 1989). The purpose of this paper is to describe the process of selecting a primary contaminant.
The selection of the primary contaminant(s) is based on the decision tree outlined in Figure 2. This method uses a general decision analysis approach (Petitti 1994) in that it follows a structured process that provides a framework for incorporating subjective information and interdisciplinary expertise into the decision process. This approach also incorporates a rating scale that assesses the relative value of components of the process. A discussion of the steps in this decision process follows.
DECISION LOGIC
Is the Candidate Substance in the Top 100 of the Priority List of Hazardous Substances?
CERCLA (Federal Register 1980), as amended by SARA (Federal Register 1986), requires ATSDR and the U.S. Environmental Protection Agency (EPA) to prepare a list of hazardous substances, ranked by priority, that are most commonly found at facilities on the National Priorities List (NPL) and that have been determined to pose the most significant potential threat to human health. This list of substances, known as the Priority List of Hazardous Substances, was developed by an interagency workgroup of scientists from ATSDR, EPA, and the Centers for Disease Control and Prevention (CDC). The methodology developed and used by the workgroup was peer reviewed by scientists external to the agencies and made available for public comment.
In the substance list ranking process, the hazard potential of each candidate substance was ranked according to the following algorithm:
Total score = NPL frequency + Toxicity + Potential for human exposure
where:
NPL frequency = The number of sites at which the candidate substance is found in some environmental media. This component was assigned a maximum score of 600 points, distributed between the maximum and minimum frequencies.
Toxicity = The established Reportable Quantity (RQ) for the candidate substance or a determination for substances without final CERCLA RQs, based on the RQ methodology. Each substance was assigned to one of five tiered RQ categories (1, 10, 100, 1,000, and 5,000) based on acute toxicity, chronic toxicity, carcinogenicity, aquatic toxicity, and ignitability and reactivity.
Potential for human exposure = The relative source contribution as well as the exposure status of the populations surrounding the sites, if any. The two parts of this component were assigned a maximum of 300 points each; if no concentration data were available for the substance, no points were assigned.
The listing process is discussed in detail elsewhere (ATSDR 1994b). The list currently ranks 275 substances. The first 100 of these, which form the pool of candidate substances for the NER, are listed in Table 1.
Has a Toxicological Profile Been Written for the Candidate Substance?
CERCLA (Federal Register 1980), as amended by SARA (Federal Register 1986), further directs ATSDR to prepare toxicological profiles for hazardous substances that are most commonly found at facilities on the NPL and that pose the most significant potential threat to human health, as determined by means of the Priority List of Hazardous Substances (discussed previously). Each profile contains, but is not limited to, the following information.
A review and interpretation of relevant toxicological information and epidemiological evaluations on the substance, which in turn are used to determine levels of human exposure for the substance as well as the associated acute, subacute, and chronic health effects.
A determination of gaps in available information on the health effects of each substance in order to determine levels of exposure that present a risk to human health.
The original guidelines for preparation of the toxicological profiles were published in 1987 (ATSDR 1987). Each profile is composed of nine sections, which include the public health statement; a health effects summary; chemical and physical information; toxicological data; information on the manufacture, import, use, and disposal of the profiled substance; environmental fate; potential for human exposure; analytical methods; and regulatory and advisory status. It should be noted, however, that the primary focus of the profiles is on the health and toxicological information. Table 2 contains a list of substances that had been profiled by ATSDR as of March 1995; these profiles are available in either draft for public comment or final versions.
The toxicological profiles are invaluable to the substance selection process in that they provide a concise and verified source of the most recent information pertinent to the goals of the NER. Further, the toxicological profiles identify existing gaps in the knowledge base concerning potential adverse human health effects following exposure to the substance in specified media or via a specified route. This information is crucial to selection of a substance for which NER-collected data could contribute to the knowledge base.
As seen in Figure 2, there are currently 2 substances in the first 100 on the Priority List of Hazardous Substances for which there are no toxicological profiles. These substances have been removed from consideration as priority contaminants for a subregistry at this time; however, should profiles be written for these substances, they will become eligible for further consideration.
Is the Candidate Substance Already a Priority Substance for a Subregistry?
Currently, four general subregistries are in place. These subregistries include: the Volatile Organic Compounds (VOCs) Subregistry, which includes individuals with documented exposure to trichloroethylene (TCE), benzene, and trichloroethane (TCA); the Dioxins Subregistry, which includes individuals exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin; the Heavy Metals Subregistry, which has one site selected based on exposures to chromium III and VI; and the Radioactive Substances Subregistry, which has one site selected based on exposures to iodine-131 and eight reactor cooling-water contaminants. Therefore, although there are only 4 subregistries, a total of 15 substances have been removed from eligibility as a primary contaminant (Figure 2). An existing subregistry can be expanded if the results of analyses of the subregistry health effects data indicate a need for additional information that an increased population will provide; however, any new sites and additional registrants will be considered a part of the original subregistry and not a new subregistry.
Does the Candidate Substance Appear at More than 100 Sites in HazDat?
As seen in Figure 1, the second step in the development of a specific substance subregistry is to select sites at which there has been documented exposure to the priority contaminant. The site selection process has been discussed elsewhere (Gist et al. 1994); however, it should be noted that it is difficult to find sites that meet the stringent requirements of the NER. To ensure that qualified sites can be identified for a specific subregistry, a minimum of 100 potential candidate sites for that potential subregistry must be identified. The source of this information is the HazDat database (ATSDR 1992).
The HazDat database is the scientific and administrative database developed by ATSDR as a repository for information on hazardous substances found at NPL and non-NPL waste sites or emergency events, and on the potential health effects of hazardous substances on human populations. It is designed for the user to retrieve site-specific information ( such as media, concentrations, etc.) for all substances found at these locations. HazDat also provides limited information on human exposure to these substances in specific media at a given site.
Of the 83 profiled substances in the top 100 substances on the Priority List of Hazardous Substances and for which a subregistry has not been developed, only 33 are found at fewer than 100 sites in the current HazDat listings. These substances have been removed from eligibility; however, should updated site information indicate one of these substances appears at more than 100 sites, then that substance will be returned to the candidate pool.
Does the Candidate Substance Have Data Gaps NER-Collected Data Could Fill?
The most important step in selecting a substance as a primary contaminant for the NER is determining whether a data gap exists in the available data that NER-collected information could fill. CERCLA (Federal Register 1980) directs the Administrator of ATSDR (in consultation with the Administrator of EPA and other agencies and programs of the Public Health Service) to assess whether adequate information is available on the health effects of hazardous substances that are the subjects of toxicological profiles. These data needs are reviewed by scientists from ATSDR, EPA, CDC, and the National Toxicology Program (NTP), peer reviewed by an external review panel, and made available for public comment. The data gaps for inhalation, oral, and dermal exposure routes are indicated in each toxicological profile. Three substances (lead, arsenic, and cadmium) currently have no data gaps appropriate for NER information and have, therefore, been removed from the final pool of candidate substances (Figure 2).
Does the Candidate Substance Meet the NER Criteria for Selection?
The remaining substances are scored with the following algorithm, which has been modified from the one used in the Priority List of Hazardous Substances ranking process to adjust for potential subregistry populations:
Total score = Existing data gaps + NER potential for filling data gap + Extent of contamination + Toxicity + Potential for human exposure
where:
Existing data gaps = A point score based on the extent of information available about health outcomes related to exposure to the candidate substance. The point ranges (Table 3) for the categories of existing data gaps are assigned overlapping ranges of points to account for the lack of specificity in respective studies and to allow for subjective evaluations based on a given rater's expertise.
NER potential for filling data gap = A point score based on the NER potential for filling the data gaps identified for the candidate substance. Again, the point ranges (Table 4) for the categories of existing data gaps are assigned overlapping ranges of points to allow for subjective evaluations based on a given rater's expertise. In all cases, point values are assigned proportionally by rank.
Extent of contamination = The frequency of occurrence (from HazDat) of each candidate substance in the environmental media most likely for exposure of a subregistry population (as defined in the Policies and Procedures Manual [ATSDR 1994a]). These media include crops, fish, shellfish, air, hard surface (wipe), surface or top soil, groundwater (public), surface water, groundwater (private), and dust. The extent of contamination component of the algorithm is assigned a maximum score of 200 points. The substance that is found in highest frequency in the given media is given 200 points; points are spread evenly between the maximum and minimum frequencies.
Toxicity = One-third of the point score for toxicity from the Priority List of Hazardous Substances. This scoring prevents the algorithm from being driven by the toxicity of a substance, which is not of paramount importance at this point.
Potential for human exposure = The point score for the candidate substance as it appears in the Priority List of Hazardous Substances score for exposure status of the population.
Typically, a minimum of six raters are selected to evaluate the candidate pool. First, each person individually rates the substances based on his or her judgement; then, the group reviews the ratings as a whole. Any discrepancies between scorers are discussed and resolved. Based on the final rating and discussions, one substance is then selected to become the primary contaminant.
Once a substance has been selected as a primary contaminant for the NER, a document is prepared describing the selection process and the information available to justify the selection of the substance as a primary contaminant. This document is called the rationale statement and is composed of seven sections--the introduction; the purpose of the National Exposure Registry; the selection of the substance as a primary contaminant, which includes extent of contamination and potential for human exposure, biomarkers, toxicity, priority health conditions, adequacy and uncertainties of existing data, and standards; the selection of sites; the selection of populations; the conclusion; and the references.
The rationale statement is then routed through the ATSDR administration for comment. If no major issues with respect to selection of the substance are identified, the substance becomes a primary contaminant. The time required from initiation of the process through routing of the rationale statement is typically 3 to 6 months.
DISCUSSION
The data collected for each member of the NER include environmental data, demographic information, smoking and occupational history, and self-reported responses to 25 general health status questions. The VOCs Subregistry contains information on 4,281 TCE-exposed persons (4,042 living at the time of baseline data collection and 239 deceased), 1,142 benzene-exposed persons (1,126 living at the time of baseline data collection and 16 deceased), and 3,666 TCA-exposed persons (3,205 living at the time of baseline data collection and 461 deceased). The Dioxins Subregistry contains information on 250 persons (246 living at the time of baseline data collection and 4 deceased).
Health outcomes rates are calculated for members of the each subregistry and compared with morbidity data in the National Health Interview Survey (NHIS) and with the National Cancer Institute's Surveillance, Epidemiology, and End Results Program data. Baseline information is currently available for the TCE (ATSDR 1994c) and benzene (ATSDR 1995a) exposures; information on TCA- and dioxin- exposed persons will soon be available. In addition, the data collected for TCE-exposed persons, without personal identifiers, is available on CD-ROM (ATSDR 1995b). Similar data for other exposure sets are forthcoming.
The scoring system presented in this paper is a relatively simple system designed to function largely on the basis of objective criteria, although professional judgement is an important element, especially when data are inconclusive or unavailable (O'Bryan and Ross 1988). This lower tiered screening allows for hypothesis generation, thus leading to higher tier assessments (studies) based on more precise (and less uncertain) information. The major strength of this system is that it is a team approach, incorporating both peer review and public comment; it is reviewed and updated on an ongoing basis. It should be noted that, contrary to EPA's use of similar data, this is NOT a ranking of risk, but rather a priority setting for optimal use of resources as well as to answer gaps in knowledge of health affects attributable to hazardous wastes at low levels to general populations. As with any ranking system, however, the uncertainties inherent to the components, as well as the system itself, should be acknowledged.
In establishing the Priority List of Hazardous Substances, the workgroup utilized values and methodologies already established, for example, RQs or the RQ methodology was used for toxicity data. As such, the uncertainties in toxicity, such as extrapolation from high dose effects to low dose effects, extrapolation of animal data to humans, or the assumption of a linear dose-response relationship should be taken into account. In addition, this ranking scheme does not account for either the potential for synergism or antagonism between the candidate chemical and coexisting chemicals or agents or the susceptibility of individuals to any substance or mixture of substances. There is no temporal factor in the ranking system, which is a major drawback not only for the toxicity component but also for the exposure component. Where use of default data was necessary, one must assume the uncertainties inherent to the data have been introduced. In most cases, such as with exposure levels, maximum quantities were used to assist in controlling variability.
SUMMARY AND CONCLUSIONS
Since the original Priority List of Hazardous Substances was published in 1988, there have been five additions or revisions to the list. As a result, the pool of eligible substances has also changed considerably as the information available on and algorithm for selection of these substances have improved. There are four subregistries; however, there are currently registrants enrolled in only the VOCs Subregistry (for the primary contaminants TCE, benzene, and TCA) and the Dioxins Subregistry (for the primary contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin). These subregistries had a total of 9,892 registrants at 18 sites during baseline data collection. Two sites have subsequently been added to the VOC Subregistry for exposure to TCE. Sites are currently being selected for the Heavy Metals Subregistry, based on exposures to the primary contaminant chromium; data collection is predicted to begin at the first site in 1996. One site has been identified for inclusion in the Radioactive Substances Subregistry, based on exposures to iodine-131 and eight reactor cooling-water contaminants; data collection is also predicted to begin in 1996.
It is anticipated that there will be no more than 5 active subregistries at any given point in time; however, since most exposures are to more than one substance, it will be possible to combine various subregistries in order to look at additional chemical exposures. As with all other NER data, reports will be available on these exposures and research will be encouraged on the appropriate data sets.
REFERENCES
AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY (ATSDR). (1995a). National Exposure Registry Benzene Subregistry Baseline Technical Report. Atlanta: US Department of Health and Human Services, Public Health Service.
AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY (ATSDR). (1995b). National Exposure Registry Trichloroethylene (TCE) Subregistry. CD-ROM Series: TCE, Volume: Baseline, No. 1. Atlanta: US Department of Health and Human Services, Public Health Service.
AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY (ATSDR). (1994a). National Exposure Registry Policies and Procedures Manual (Revised). Atlanta: US Department of Health and Human Services, Public Health Service.
AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY (ATSDR). (1994b). 1993 CERCLA Priority List of Hazardous Substances That Will be the Subject of Toxicological Profiles and Support Document. Atlanta: US Department of Health and Human Services, Public Health Service.
AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY (ATSDR). (1994c). National Exposure Registry Trichloroethylene (TCE) Subregistry Baseline Technical Report (Revised). Atlanta: US Department of Health and Human Services, Public Health Service
AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY (ATSDR). (1992). ATSDR HazDat Version 1.5. User's Manual. Atlanta: US Department of Health and Human Services, Public Health Service.
AGENCY FOR TOXIC SUBSTANCES AND DISEASE REGISTRY (ATSDR). (1987). Guideline for Toxicological Profiles, Federal Register 52:12866.
BURG, J.R. (1989). "Policies and procedures for establishing the National Exposure Registry." J Am Coll Toxicol 8:949-954.
FEDERAL REGISTER. (1980). Comprehensive Environmental Response, Compensation, and Liability Act of 1980, Public Law 95-510, 42 USC 9601 et seq.
FEDERAL REGISTER. (1986). The Superfund Amendments and Reauthorization Act of 1986, Public Law (PL) 99-499, 42 USC 9601 et seq.
GIST, G.L., BURG, J.R., and RADTKE, T.M. (1994). "The site selection process for the National Exposure Registry." J Environ Health 56:7-12.
O'BRYAN, T.R. and ROSS, R.H. (1988). "Chemical scoring system for hazard and exposure identification." J Toxicol Environ Health 1:119-134.
PETITTI, D.B. (1994). Meta-Analysis Decision Analysis and Cost-Effectiveness Analysis. Methods for Quantitative Synthesis in Medicine. Monographs in Epidemiology and Biostatistics, Vol. 24. Oxford University Press, New York, NY. 246 p.
Table 1. 1993 CERCLA Priority List of Hazardous Substances (top 100 only) (ATSDR 1994b).
| 1.
2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. |
Leadd
Arsenicd Metallic mercury Benzeneb Vinyl chloride Cadmiumd Polychlorinated biphenyls Benzo(a)pyrene Chloroform Benzo(b)fluoranthene Trichloroethyleneb p,p'-DDT Aroclor 1254 Hexachlorobutadienec Aroclor 1260 p,p'-DDE Aroclor 1242 Dibenzo(a,h)anthracene Hexavalent chromiumb Dieldrin Chlordane Gamma-hexachlorocyclohexane Tetrachloroethylene Aroclor 1248 p,p'-DDD Creosotec Aldrin Beta-hexachlorocyclohexane Heptachlor Di-n-butyl phthalate Cyanide Toxaphenec Alpha endosulfanc |
34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. |
Phosphorusc
Benzo(a)anthracene Disulfotonc Delta-hexachlorocyclohexane Aroclor 1221c Beryllium Endosulfan sulfatec Beta endosulfanc Aroclor 1016c Carbon tetrachloride Heptachlor epoxide Endrin 1,2-Dibromoethanec Nickel Cis-chlordanec Benzidinec Manganese Pentachlorophenol 2,3,7,8-tetrachlorodibenzo-p-dioxinb Toluene Trans-chlordanec Methoxychlorc Methanea Dibromochloropropanec 3,3'-Dichlorobenzidinec Endosulfanc Benzo(a)fluoranthenec Chromiumb Aroclor 1232c Di(2-ethylhexyl)phthalate Zinc Methylene chloride Endrin ketonec |
67.
68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. |
Benzo(e)pyrenec
2-Hexanone 1,1-Dichloroethene Naphthalene 1,2-Dichloroethane 1,1,1-Trichloroethane Ethyl benzene Asbestosc Alpha radiationa Uraniumb Bis(2-chloroethyl) etherc Radiumb Total xylenes 2,4-Dinitrophenolc Radium-226b Thoriumb Dimethylarsinic acidc 2,4,6-Trichlorophenolc Chlorobenzene Diazinonc Radonb Thorium-230b Radium-228b 2,4,6-Trinitrotoluenec Thorium-228b Hexachlorobenzenec Chrysotile asbestosc Methylmercuryc Radon-222b Thoron (Radon-220)b 4,6-Dinitro-o-cresolc Copper Alpha-hexachlorocyclohexane Fluoranthene |
b Substance is already a primary contaminant for a subregistry.
c Substance appears at fewer than 100 sites as listed in HazDat.
d Substance does not have a data gap that subregistry-derived information could fill.
| Table 2. ATSDR Toxicological Profiles (Draft for Public Comment and Final Versions), March 1995. | |||||
|---|---|---|---|---|---|
| Acetone Acrolein Acrylonitrile Aldrin/Dieldrin Aluminum Ammonia Antimony Arsenic Asbestos Barium Benzene Benzidine 2,3-Benzofuran Benzo(a)anthracene Benzo(a)pyrene Benzo(b)fluoranthene Beryllium Bis(2-chloroethyl)ether Bis(chloromethyl)ether Boron Bromodichloromethane Bromoform Bromomethane 1,3-Butadiene 2-Butanone Cadmium Carbon disulfide Carbon tetrachloride Chlordane Chlorobenzene Chlorodibenzofurans Chlorodibromomethane Chloroethane Chloroform Chloromethane Chlorophenols Chromium Chrysene Cobalt Copper Creosote Cresols Cyanide 4,4'-DDT, DDE, DDD Diazinon 3,3'-Dichlorobenzidine Diethyl phthalate Di-n-butyl phthalate Dinitrocresols | Dinitrophenols Di-n-octyl phthalate 2,4- and 2,6-Dinitrotoluene Di(2-ethylhexyl)phthalate Dibenzo(a,h)anthracene Dibromochloropropane 1,2-Dibromoethane 1,4-Dichlorobenzene 1,1-Dichloroethane 1,2-Dichloroethane 1,1-Dichloroethene cis-, trans- 1,2-Dichloroethene 2,4-Dichlorophenol 1,2-Dichloropropane 1,3-Dichloropropene 1,2-Diphenylhydrazine Disulfoton Endosulfan Endrin/Endrin aldehyde Ethylbenzene Ethylene and propylene glycols Ethylene oxide Fluoride, Hydrogen Fluoride, and Fluorine Fuel oils Gasoline Heptachlor/Heptachlor epoxide Hexachlorobenzene Hexachlorobutadiene Hexachlorocyclohexanes Hexachlorocyclopentadiene Hexachloroethane 2-Hexanone HMX Hydraulic fluids Hydrazines Isophorone Jet fuels JP-4 and JP-7 Lead MBOCA Manganese Mercury Methoxychlor Methyl mercaptan Methyl parathion Methyl t-butyl ether Methylene chloride 2-Methylnaphthalene Mineral-based crankcase oil Mirex | Mustard gas N-Nitrosodimethylamine N-Nitrosodi-n-propylamine N-Nitrosodiphenylamine Naphthalene Nickel Nitrobenzene Nitrophenol Otto fuel II Pentachlorophenol Phenol Plutonium Polybrominated biphenyls Polychlorinated biphenyls Polycyclic aromatic hydrocarbons Pyridine Radium Radon RDX Selenium Silver Stoddard solvent Styrene 2,3,7,8-Tetrachlorodibenzo-p-dioxin 1,1,2,2-Tetrachloroethane Tetrachloroethylene Tetryl Thallium Thorium Tin Titanium tetrachloride Toluene Toxaphene 1,1,1-Trichloroethane 1,1,2-Trichloroethane Trichloroethylene 2,4,6-Trichlorophenol 1,2,3-Trichloropropane Trinitrobenzene/1,3-Dinitrobenzene Trinitrotoluene Uranium Vanadium Vinyl acetate Vinyl chloride White phosphorus Total Xylenes Zinc | |||
Table 3. Scoring for Extent of Data Gaps.
| Extent of Data Gap | Point Range Assignment |
| Adequate animal data and some human data exist for relevant exposure routes | 1-125 |
| Animal data are adequate, but no human data exist for relevant exposure routes | 100-175 |
| Inadequate animal and limited human data exist for relevant exposure routes | 150-225 |
| Very limited animal data and limited or no human data exist for relevant exposure routes | 200-300 |
Table 4. National Exposure Registry Potential for Filling Data Gaps Scoring.
| NER Potential for Filling Data Gap | Point Range Assignment |
| Information might be relevant but might not be attainable via NER-based studies | 1-125 |
| Information would be relevant, but adequate data not attainable via NER-based studies | 100-175 |
| Information would be relevant, with some certainty of attaining adequate data via NER-based studies | 150-225 |
| End points (e.g., disease or adverse health outcome) could be identified via NER-based study (i.e., information is relevant and adequate data are attainable) | 200-300 |
Figure 1. Outline of Steps for Creating a Subregistry of the National Exposure Registry.
Figure 2. Outline of Steps for Selecting a Primary Contaminant for Establishing a Subregistry of the National Exposure Registry.
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