Bethel Valley Watershed

The major operations at X-10 take place within the Bethel Valley Watershed. The main plant, key research facilities, primary administrative offices, as well as various forms of waste sites, are situated in Bethel Valley. Over the past 60 years, X-10 releases have contaminated the Bethel Valley Watershed. Mobile contaminants primarily leave the Bethel Valley Watershed via White Oak Creek. These contaminants travel from the Bethel Valley Watershed to the Melton Valley Watershed, where further contaminants enter White Oak Creek. Then, the contaminants that have been discharged to White Oak Creek are released over White Oak Dam and into the Clinch River (USDOE 2001b). The main remedial activities conducted in Bethel Valley are listed below. Please see Figure 10 in Section II.C.1. for a map of Bethel Valley that includes these areas.

Melton Valley Watershed

X-10 disposed of its radioactive wastes (liquid and solid) in Melton Valley, and also operated its experimental facilities within this watershed (USDOE 2002a, 2002b). Discharges from Melton Valley's waste areas have produced secondary contamination sources that include sediment, groundwater, and soil contamination. Furthermore, contaminants that are discharged from Melton Valley travel off the reservation through surface water and flow into the Clinch River (SAIC 2002). As a result, the waste sites in the Melton Valley Watershed "...are the primary contributors to off-site spread of contaminants" from the ORR (USDOE 2002b).

The main remedial activities conducted in Melton Valley are detailed below (SAIC 2002; USDOE 2001d; USEPA 2002a). Please see Figure 12 in Section II.C.2. for a map of Melton Valley that includes these areas. Also, please refer to Figure B-1 for the details concerning the completed, current, and future remediation activities in Melton Valley and see Figure B-2 for the Melton Valley projected closure schedule for the current and future activities (USDOE 2003b). The current schedule was accelerated by 9 years to have all closure activities completed by September 2006 (SAIC 2005; USDOE 2003b).

Completed, Current, and Future Remedial Activities in Melton Valley
Figure B-1. Completed, Current, and Future Remedial Activities in Melton Valley

Melton Valley Closure Schedule
Figure B-2. Melton Valley Closure Schedule


Summary of ATSDR Activities

Review of clinical information on persons living in or near Oak Ridge. Following a request by William Reid, M.D., ATSDR evaluated the medical histories and clinical data associated with 45 of Dr. Reid's patients. The objective of this review was to assess the clinical data for patients who were tested for heavy metals, and to establish if exposure to metals was related to these patients' various illnesses. ATSDR determined that the case data did not provide sufficient evidence to support an association between these diseases and low levels of metals. The TDOH, which also evaluated the information, reached the same conclusion as did ATSDR. In September 1992, ATSDR provided a copy of its review to Dr. Reid (ATSDR et al. 2000).

Clinical laboratory analysis. In June 1992, William Reid, M.D., an Oak Ridge physician, notified the ORHASP and the TDOH that he believed that about 60 of his patients had been exposed to numerous heavy metals through their occupations or through the environment. Dr. Reid believed that these exposures had caused a number of adverse health outcomes, which included immunosuppression, increased cancer incidence, neurologic diseases, bone marrow damage, chronic fatigue syndrome, autoimmune disease, and abnormal blood clots. Howard Frumkin, M.D., Dr.PH., from the Emory University School of Public Health, requested facilitated clinical laboratory support to evaluate the patients referred by Dr. Reid. As a result of Dr. Frumkin's request, ATSDR and the CDC's NCEH facilitated this laboratory support from 1992 to 1993 through the NCEH Environmental Health Laboratory (ATSDR et al. 2000; ORHASP 1999).

Because of the confidentiality among physicians, as well as the confidentiality between physicians and their patients, the findings of these clinical analyses have not been provided to public health agencies (ATSDR et al. 2000). Nevertheless, in an April 26, 1995, letter to the Commissioner of the Tennessee Department of Health, Dr. Frumkin suggested that one should "not evaluate the patients seen at Emory as if they were a cohort for whom group statistics would be meaningful. This was a self-selected group of patients, most with difficult to answer medical questions (hence their trips to Emory), and cannot in any way be taken to typify the population of Oak Ridge. For that reason, I have consistently urged Dr. Reid, each of the patients, and officials of the CDC and the Tennessee Health Department, not to attempt group analyses of these patients."

Health education. Another essential part of the public health assessment process is designing and implementing activities that promote health and providing information about hazardous substances in the environment.

Coordination with other parties. Since 1992 and continuing to the present, ATSDR has consulted regularly with representatives of other parties involved with the ORR. Specifically, ATSDR has coordinated its efforts with TDOH, TDEC, NCEH, NIOSH, and DOE. These efforts led to the establishment of the Public Health Working Group in 1999, which then led to the establishment of the ORRHES. In addition, ATSDR provided some assistance to TDOH in its study of past public health issues. ATSDR has also obtained and interpreted studies prepared by academic institutions, consulting firms, community groups, and other parties.

Establishment of the ORR Public Health Working Group and the ORRHES. In 1998, under a collaborative effort with the DOE Office of Health Studies, ATSDR and CDC embarked on a process to develop credible, coherent, and coordinated agendas for public health activities and for health studies at each DOE site. In February 1999, ATSDR was given the responsibility to lead the interagency group's efforts to improve communication at the ORR. In cooperation with other agencies, ATSDR established the ORR Public Health Working Group to gather input from local organizations and individuals regarding the creation of a public health forum. After careful consideration of the input gathered from community members, ATSDR and CDC determined that the most appropriate way to meet the needs of the community would be to establish the ORRHES.

Exposure investigations, health consultations, and other scientific evaluations. In addition to the Watts Bar Reservoir, ATSDR health scientists have addressed current public health issues and community health concerns related to other areas affected by ORR operations.

Following are summaries of other ATSDR public health activities involving EFPC:

Summary of U.S. Department of Health and Human Services Activities

U.S. Department of Health and Human Services' evaluation of data in an article from The Tennessean , September 29, 1998. In a November 2, 1998 letter, the Honorable William H. Frist, M.D., United States Senator, requested that Donna E. Shalala, Secretary of the Department of Health and Human Services (DHHS), have the CDC, ATSDR, and the National Institutes of Health (NIH) evaluate the data that the article in The Tennessean describes as reporting a pattern of illnesses among residents living near nuclear plants, including the DOE ORR.

In particular, Senator Frist requested the following:

In a letter dated February 22, 1999, Donna E. Shalala, Secretary of DHHS, responded to Senator Frist's request. DHHS evaluated the article in The Tennessean and responded to Senator Frist's five specific issues. DHHS concluded the following:

  1. The data in the article from The Tennessean were not compiled from an epidemiologic study and thus have many limitations. It is impossible to calculate rates for the reported illnesses or to determine whether rates of the illnesses were abnormal. It is also difficult to relate excess illnesses to specific nuclear plants because primary exposures differ among the plants.

  2. Epidemiologically, it is neither acceptable to tabulate data collected in an unstandardized manner, nor to assess illnesses and symptoms based on limited diagnostic information. Thus, it is not possible to determine if data in this report represent a new or unusual occurrence of symptoms in this population.

  3. DHHS has a significant number of ongoing studies that seek to analyze environmental exposure at each of the 11 sites rather than focusing on general medical evaluations of the populations near the sites. However, clinical data from the Fernald Medical Monitoring Program and the Scarboro, Tennessee, survey focus on respiratory illnesses in children and, although quite limited, are most relevant to the issues raised by the report.

  4. Sound data using standardized information is essential in order to establish increased prevalence of a disease and linkage to the nuclear plants.

  5. CDC, ATSDR, and NIH are working with DOE to plan appropriate public health follow-up activities to address the concerns of communities and workers regarding the nuclear weapons complexes. Embarking on such a comprehensive program will require considerable resource, planning, and evaluation. Please note that CDC, ATSDR, and NIH do not provide direct primary medical services to communities. However, where possible, CDC, ATSDR, and NIH will continue to support community leaders and existing medical care systems to address public health concerns of communities that are near nuclear plants.

Summary of TDOH Activities

Pilot survey. In the fall of 1983, TDOH established an interim soil mercury level to use for making environmental management decisions. CDC evaluated the methodology for this mercury level, and advised the TDOH to conduct a pilot survey to determine if populations with the greatest risk for mercury exposure had elevated mercury body burdens. Between June and July 1984, TDOH and CDC conducted a pilot survey to record the inorganic mercury levels of Oak Ridge residents who had the greatest risk of being exposed to mercury-contaminated fish and soil. In addition, the survey assessed if exposure to mercury through contaminated fish and soil represented an immediate health hazard for the Oak Ridge community. In October 1985, the findings of the pilot study were released; these results indicated that people who lived and worked in Oak Ridge, Tennessee, were unlikely to have a greater risk for significantly high mercury levels. Further, concentrations of mercury detected in hair and urine samples were lower than levels associated with known health effects (ATSDR et al. 2000).

Health statistics review. In June 1992, William Reid, M.D., an Oak Ridge physician, informed the ORHASP and the TDOH that he believed that about 60 of his patients had been exposed to numerous heavy metals through their occupation or through the environment. Dr. Reid felt that these exposures had caused a number of adverse health outcomes that included immunosuppression, increased cancer incidence, neurologic diseases, bone marrow damage, chronic fatigue syndrome, autoimmune disease, and abnormal blood clots. In 1992, TDOH conducted a health statistics review that evaluated the cancer incidence rates for the counties around the reservation between 1988 and 1990, and compared these rates to the state rates for Tennessee. The health statistics review determined that some of the counties' rates were low and some were high when compared to the state's rates; however, the review was unable to distinguish any patterns associated with the site. More detailed findings of the review can be found in a TDOH memorandum dated October 19, 1992, from Mary Layne Van Cleave to Dr. Mary Yarbrough. In addition, the handouts and minutes from Ms. Van Cleave's presentation at the ORHASP meeting on December 14, 1994, are available through TDOH (ATSDR et al. 2000).

Health statistics review. In 1994, area residents reported that there were several community members who had amyotrophic lateral sclerosis (ALS) and multiple sclerosis (MS). TDOH consulted with Peru Thapa, M.D., M.P.H., from the Vanderbilt University School of Medicine, to perform a health statistics review of mortality rates for ALS and MS within certain counties in Tennessee. TDOH also received technical support for the health statistics review from ATSDR (ATSDR et al. 2000).

Because ALS and MS are not reportable, TDOH determined that it was impossible to calculate reliable incidence rates for these diseases. Mortality rates for counties surrounding the ORR were analyzed for the time period between 1980 and 1992, and then compared with mortality rates for the state of Tennessee. The review found that the mortality rates did not differ significantly from the rates in the rest of Tennessee (ATSDR et al. 2000). The following results were reported by TDOH at the ORHASP public meeting on August 18, 1994.

Knowledge, attitudes, and beliefs study. TDOH coordinated a study to evaluate the attitudes, beliefs, and perceptions of residents living in eight counties around Oak Ridge, Tennessee. The purpose of the study was to (1) investigate public perceptions and attitudes about environmental contamination and public health problems related to the ORR, (2) ascertain the public's level of awareness and assessment of the ORHASP, and (3) make recommendations for improving public outreach programs. The report was released in August 1994 (ATSDR et al. 2000; Benson et al. 1994). Following is a summary of the findings (Benson et al. 1994):

Health assessment. The East Tennessee Region of TDOH conducted a health assessment on the eastern region of Tennessee. The purpose of this health assessment was to review the health status of the population, to evaluate the accessibility and utilization of health services, and to develop priorities for resource allocation. The East Tennessee Region released its first edition of A Health Assessment of the East Tennessee Region in December 1991; this edition generally contained data from 1986 to 1990. The second edition, which was released in 1996, generally included data from 1990 to 1995. A copy of the document can be obtained from the East Tennessee Region of TDOH (ATSDR et al. 2000).

Presentation. On February 16, 1995, Dr. Joseph Lyon of the University of Utah gave a TDOH-sponsored presentation at an ORHASP public meeting. The purpose of the presentation was to inform the public and the ORHASP that several studies had been conducted on the fallout from the Nevada Test Site, including the study of thyroid disease and leukemia (ATSDR et al. 2000).

Other Agencies

Assessment reports, environmental studies, health investigations, remedial investigation/ feasibility studies, and sampling validation studies. Other agencies have also addressed community health concerns and public health issues through studies and investigations. Two areas that have been investigated by other agencies—Scarboro and Lower East Fork Poplar Creek (LEFPC)—are discussed below.

Following are summaries of investigations related to the Scarboro community:

Following is a summary of a remedial investigation/feasibility study (RI/FS) for LEFPC:


TDOH's Phase I Dose Reconstruction Feasibility Study (PDF, 237KB)

TDOH's Task 4 Radionuclide Releases to the Clinch River From White Oak Creek on the Oak Ridge Reservation (PDF, 246KB)

TDOH's Task 7 Screening Level Evaluation of Additional Potential Materials of Concern (PDF, 200KB)

ATSDR's Health Consultation on the Lower Watts Bar Reservoir (PDF, 176KB)

ATSDR's Watts Bar Exposure Investigation (PDF, 212KB)

TDEC's Watts Bar Reservoir and Clinch River Turtle Sampling Survey (PDF, 208KB)

TDOH's Task 6 Uranium Releases From the Oak Ridge Reservation (PDF, 187KB)


Table E-1. Conservative Screening Indices for Radionuclides in the Clinch River


Exposure Pathway

Drinking Water

Fish Ingestion

External: Shoreline


External: Dredged Sediment

Ingestion of Beef

Ingestion of Milk

Ingestion of Vegetables


Cs 137

9.2 E-06

4.0 E-04

8.0 E-03

7.6 E-07

1.6 E-03

5.9 E-03

5.7 E-03

5.6 E-04

3.2 E-08

Ru 106

7.7 E-05

1.7 E-05

1.1 E-03

5.2 E-06

4.5 E-05

1.6 E-04

4.4 E-07

5.8 E-05

1.2 E-08

Sr 90

2.5 E-05

3.3 E-05

7.1 E-05

1.5 E-06

9.8 E-06

1.7 E-02

2.5 E-02

6.4 E-03

5.1 E-07

Co 60

2.8 E-06

1.9 E-05

6.0 E-03

1.7 E-07

8.5 E-04

1.1 E-03

7.6 E-04

7.5 E-05

6.2 E-09

Ce 144

4.2 E-06

2.7 E-06

2.1 E-05

2.6 E-07

7.2 E-08

1.1 E-08

7.4 E-08

3.2 E-07

2.2 E-09

Zr 95

8.1 E-07

5.3 E-06

1.8 E-04

4.3 E-07

5.1 E-09

8.8 E-11

2.7 E-10

2.1 E-12

3.1 E-12

Nb 95

4.2 E-07

2.7 E-06

5.1 E-05

2.0 E-07

3.1 E-09

1.4 E-11

9.1 E-11

1.4 E-11

3.7 E-12

I 131

4.1 E-05

6.7 E-06

7.2 E-08

4.1 E-06

3.2 E-12

6.0 E-07

3.8 E-05

1.1 E-11

9.3 E-10

U 235

1.5 E-07

3.2 E-08

5.0 E-06

9.4 E-09

7.8 E-07

2.8 E-07

2.7 E-07

4.6 E-07

1.8 E-10

U 238

1.3 E-07

2.9 E-08

8.4 E-07

8.0 E-09

1.4 E-07

2.5 E-07

2.4 E-07

4.2 E-07

1.6 E-10

Pu 239/240

9.8 E-07

6.4 E-07

1.4 E-07

5.9 E-08

1.5 E-09

3.8 E-07

2.8 E-08

3.1 E-06

2.4 E-10

Th 232

1.0 E-07

2.2 E-07

9.2 E-08

6.1 E-09

2.7 E-09

2.0 E-08

4.8 E-09

1.6 E-07

1.2 E-11

Am 241

1.0 E-07

6.7 E-08

3.8 E-06

6.2 E-09

2.0 E-07

1.7 E-08

1.6 E-08

2.8 E-07

2.5 E-11

Eu 154

4.9 E-06

5.3 E-06

3.6 E-08

1.1 E-06

5.1 E-09

1.3 E-06

1.7 E-07

1.0 E-06

4.4 E-10

La 140

4.9 E-06

2.7 E-06

1.0 E-06

1.8 E-06

2.0 E-09

1.1 E-07

1.6 E-08

7.2 E-12

3.9 E-13

Pm 147

7.4 E-07

4.8 E-07

2.6 E-08

4.4 E-08

1.1 E-11

1.7 E-08

2.8 E-09

6.0 E-10

3.6 3-11

Sm 151

2.3 E07

1.5 E-06

1.3 E-07

1.4 E-08

3.8 E-10

90 E-07

1.2 E-07

7.5 E-07

2.7 E-11

Sr 89

1.5 E-08

1.9 E-08

1.2 E-11

8.8 E-10

1.1 E-13

1.4 E-09

2.4 E-09

3.4 E-11

0.0 E+00

Ba 140

8.6 E-07

9.4 E-08

5.6 E-07

2.8 E-07

0.0 E+00

1.9 E-09

2.3 E-08

0.0 E+00

5.4 E-12

P 32

7.8 E-08

3.8 E-06

2.3 E-12

4.7 E-09

6.9 E16

4.2 E-08

3.3 E-13

3.3 E-13

1.6 E-13

Y 91

7.0 E-06

4.6 E-06

3.5 E-07

4.2 3-07

.3 E-11

7.6 E-08

2.3 E-08

1.1 E-10

2.9 E-11

Pr 143

3.5 E-06

2.3 E-06

9.6 E-09

2.1 E-07

1.5 E-12

7.6 E-08

1.1 E-08

8.3 E-12

0.0 E+00

Nd 147

3.1 E-06

2.0 E-06

1.6 E-06

2.7 E-07

3.6 E-10

6.8 E-08

1.0 E-08

6.0 E-12

0.0 E+00

Bold values represent radionuclides for each pathway that were carried into the next iteration of analysis in Task 4.
Screening indices are calculated probabilities of developing cancer.


During the public health assessment process, ATSDR uses radiation doses rather than risk

Public health assessments differ from the U.S. Environmental Protection Agency's (EPA) risk assessments, which evaluate hypothetical risk to determine safe regulatory limits and prioritize sites for cleanup. Typically, ATSDR does not incorporate risk numbers in public health assessments. Nevertheless, in response to public requests to describe the methodology used in this public health assessment to convert doses to risk numbers, ATSDR includes this supplemental risk appendix. By applying the methods described in this appendix, community members can estimate for themselves the theoretical risk from exposure to X-10 radionuclides released to the Clinch River and the Lower Watts Bar Reservoir via White Oak Creek.

Differences between Dose and Risk

Dose, as defined by ATSDR, is the "amount of a substance to which a person may be exposed, usually on a daily basis." For chemicals, dose is often referred to as the "amount of substances(s) per body weight per day" and is the basis for determining levels of exposure that might cause adverse health effects. In the case of radiation, dose is the amount of energy deposited in a specific body mass.

The Society for Risk Analysis defines risk as the "potential for realization of unwanted, adverse consequences to human life, health, property, or the environment; estimation of risk is usually based on the expected value of the conditional probability of the event occurring times the consequence of the event given that it has occurred."18 The EPA defines risk as "a measure of the probability that damage to life, health, property, and/or the environment will occur as a result of a given hazard."19

How Does a Risk Assessment Differ from a Public Health Assessment?

Again, EPA defines a risk assessment as a "qualitative and quantitative evaluation of the risk posed to human health and/or the environment by the actual or potential presence and/or use of specific pollutants." Risk assessments—useful in determining safe regulatory limits and prioritizing sites for cleanup—provide estimates of theoretical risk from possible current or future exposures and consider all contaminated media, regardless of whether exposures are occurring or are likely to occur. Quantitative risk estimates developed using the EPA risk assessment methodology include multiple safety factors and are not intended to predict the incidence of disease or measure the actual health effects in people resulting from hazardous substances at a site. By design, EPA risk estimates are conservative predictions that generally overestimate risk. Risk assessments do not provide a perspective on what the risk estimates mean in the context of the site community and do not measure the actual health effects hazardous substances have on people.

The mathematical formula used to calculate risk estimates assumes a linear (i.e., straight line) response to exposure, even though an actual effect may not be detected in an exposed population. The inability to detect an effect could result from the absence of an effect at lower levels of exposure or because the current epidemiological tools are not sufficient to demonstrate the existence of a very small excess of health effects, such as cancer incidence. The conservative approach to risk assessment, which likely overestimates the true potential impact of exposure, is appropriate for exposure prevention and prioritizing site cleanup. Please see Figure F-1 for examples of different models of low-level radiation effects, including the linear model used by governmental and nongovernmental entities to estimate radiation risks.

ATSDR recognizes that every radiation dose, action, or activity may carry an associated risk. ATSDR uses the public health assessment process to evaluate the public health implications of exposure to environmental contamination and to identify the appropriate public health actions for particular communities. A public health assessment provides conclusions about the level of the health threat (if any) posed by a site, as well as recommendations to stop or reduce exposures. Because of uncertainties regarding exposure conditions and because of adverse effects related to environmental levels of exposure, definitive answers are not possible on whether health effects actually will or will not occur. It is possible, however, for a public health assessment to provide a framework that puts site-specific exposures and the potential for harm in perspective.

Examples of Different Models of Low-Level Radiation Effects
Figure F-1. Examples of Different Models of Low-Level Radiation Effects

ATSDR uses the public health assessment process to answer site-specific questions for people potentially exposed to hazardous substances:

When answering community members' questions about impacts from past, current, and future exposures, extreme overestimations of possible effects can cause unnecessary fear and worry. Therefore, instead of using mathematical formulas to estimate theoretical harm caused by potential exposures, ATSDR provides the public with answers about health effects associated with exposures based on real observations by physicians, epidemiologists, or toxicologists. Using this information, ATSDR will make necessary recommendations to prevent and to mitigate exposures potentially occurring at levels that have been shown to cause adverse health effects. If, however, exposures were at levels below those associated with adverse health effects, further actions would not be recommend.

For more information on the intentional differences between public health assessments and risk assessments, please see ATSDR's Public Health Assessment Guidance Manual (, EPA's Risk Assessment Guidance for Superfund – Human Health Evaluation Manual ( Exiting ATSDR Website), and A Citizen's Guide to Risk Assessments and Public Health Assessments at Contaminated Sites (written jointly by ATSDR and EPA Region IV; see

Radiation Risks

Radiation risks are derived from many exposure studies that have undergone review by governmental and nongovernmental international groups, including

These reviews assist scientists, legislators, regulators, and others in estimating the risks of cancer and deaths associated with radiological exposures and radiological doses.

In its 1991 Publication 60,21 the ICRP discussed risk in terms of radiation detriment and derived probabilities of developing fatal cancers in various organs as measured by the effective dose. The commission also evaluated organ detriment by deriving tissue weighting factors. The ICRP defines a tissue weighting factor as "The factor by which the equivalent dose in a tissue or organ is weighted to represent the relative contribution of that tissue or organ to the total detriment resulting from uniform irradiation of the body." Thus weighting factors convert an organ dose equivalent to a committed effective dose for the whole body. (See Section III.A.1. in the PHA for more information on tissue weighting factors, organ dose equivalents, and effective doses). These weighting factors are applied to ensure the detriment produced is "broadly the same degree" regardless of the tissue or organ irradiated. As mentioned throughout this White Oak Creek public health assessment, the ICRP has a recommended annual radiation dose limit for the public of 100 millirem (mrem)/year. ICRP continues to state, however, that "The Commission does not yet recommend an annual [radiation] risk limit for individuals."

In 1993, the NCRP published risk estimates designed for radiation protection. The NCRP developed these estimates based on a review of studies from UNSCEAR and the National Academy of Sciences' Committee on Biological Effects of Ionizing Atomic Radiation (BEIR). These studies, which included investigations on radiation effects on the thyroid and the fetus, reported the risks associated with exposure to low doses of ionizing radiation. Given its review, the NCRP estimated the following risks for members of the public exposed to ionizing radiation: a lifetime cancer mortality risk of 0.05 per sievert (Sv) (5%); a hereditary risk of 0.01 per Sv (1%), and a risk of severe mental retardation for fetuses exposed at 8–15 weeks gestational age of 0.04 per Sv (0.4%).22

In 1994, the EPA published its methodology for estimating cancer risks from low-level radiation exposures. These estimates, derived from similar data used by the NCRP in Report 115, incorporated 1980 vital statistics to develop organ-specific risks for a stationary US population.23 In Federal Guidance Report 13, released in 1999, the EPA presented refined risk estimates for low-level radiation exposures to be used for various purposes, such as assessing individual sites and conducting general analysis for rule making. These estimates include risks from numerous radionuclides, routes of exposure, and ages of exposure.24

In 2005, the EPA released draft guidelines for carcinogenic risk assessments that discussed guidance for developing and using risk assessments.25 The EPA stated "where alternative approaches have significant biological support, and no scientific consensus favors a single approach, an assessment may present results using alternative approaches. A nonlinear approach can be used to develop a reference dose or a reference concentration." Thus, the EPA indicates that multiple approaches using linear and nonlinear methods are appropriate if more than one mode of action exists. Also, in an EPA Risk Assessment Task Force report titled An Examination of EPA Risk Assessment Principles and Practices, the agency stated that the "risk estimates are designed to ensure that risks are not underestimated, which means that a risk estimate is the upper bound on the estimated risk." Further, the EPA explicitly stated that the true cancer potency "could be as low as zero."26

In a proposed risk assessment bulletin released in 2006, the US Office of Management and Budget (OMB) issued new technical guidance to improve risk assessments prepared by the federal government.27 The bulletin emphasizes the importance of high-technical-quality risk assessments that present scientific issues in an objective manner. According to the OMB, risk assessments need to describe the basis of every critical assumption and specify how the assumptions affect the risk assessment's main findings. An assessment should also discuss the empirical data that both supports and conflicts with the assumptions. The OMB proposed bulletin stated that these discussions should include "the range of scientific opinions regarding the likelihood of plausible alternate assumptions" and "whenever possible, a quantitative evaluation of reasonable alternative assumptions should be provided. If an assessment combines multiple assumptions, the basis and rationale for combining the assumptions should be clearly explained."

To summarize, many governmental and nongovernmental agencies use a linear approach for estimating radiation risks. This linear approach, called the linear nonthreshold (LNT) model, assumes an inherent risk irrespective of the dose. Although this risk has not been seen to date, various agencies use this approach to set regulatory limits, to develop recommended exposure limits for the public, and to evaluate public health hazards (e.g., ATSDR's radiogenic cancer comparison value of 5,000 mrem over 70 years incorporates the LNT model).

Risk Limits

Table F-1 summarizes the organ-specific risk estimates developed by the ICRP (1991) and the EPA (1994 and 1999). The table expresses the results in units of equivalent (organ) dose, and the totals are expressed in terms of effective (whole-body) dose. For the purposes of this discussion, the dose units of Sv and gray (Gy) are interchangeable. The dose unit of rem is equal to 0.01 Sv or 0.01 Gy.

EPA guidance states that carcinogens should be limited to a risk range of 1 in 10,000 to 1 in 1,000,000 (1 x 10-4 to 1 x 10-6), presumably above background exposure. EPA applies this range in its baseline risk assessments to rank sites relatively (primarily) for cleanup; EPA does not, however, determine the likelihood that health effects might occur. The following risk numbers are calculated when the ICRP risk coefficients presented in Table F-1 (converted to 0.0005 per rem) are multiplied by the background radiation dose of 360 mrem/year (including radon) and ATSDR's radiation screening value of 100 mrem/year (for radiation exposure in excess of background):

Exposure to average background radiation (1.8 in 10,000), which cannot be avoided and to which everyone is exposed, exceeds the EPA risk range. The ATSDR screening value of 100 mrem is, however, equivalent to a risk of 5 in 100,000, which falls near the center of EPA's prescribed risk range.

Table F-1. Summary of Organ-Specific Risk Estimates


ICRP* (rem)

EPA (rad)

EPA FGR 13 (rad)





Bone marrow




Bone surface
















































Total (whole body) risk

5E-4 per rem per year

5.09E-04 per rad per year

5.75E-04 per rad per year

0.1 rem/y (100 mrem/y)

5.00E-05 per year



Calculation of Risk for the Oak Ridge Public Health Assessments

As previously discussed at public meetings, ATSDR does not perform risk assessments, nor does it report its findings in terms of risk. Calculating the risks from the doses reported by ATSDR in this PHA, however, only involves one additional step. To calculate the risk, multiply the doses reported by ATSDR by the appropriate organ risk factor from Table F-1, being sure to use consistent units throughout the calculations.

Using the following equation, here are some examples of how to calculate the risk from an estimated radiation dose.

Risk = Annual Dose × Risk Coefficient × Years of Exposure

Examples of Calculating Risks From Estimated Radiation Doses

Whole-body dose

Annual Dose (in rem): 100 mrem per year (0.1 rem)
Risk Coefficient: 0.0005 per rem per year
Years of Exposure: 5 years

Risk = 0.1 × 0.0005 × 5 = 0.00025 (2.5 per 10,000)

This result of 2.5 per 10,000 can then be compared to the estimated risk an individual would receive from typical exposures to background radiation during the same time period:

Risk = 0.36 × 0.0005 × 5 = 0.0009 (9 per 10,000)

Dose to the bone marrow

Annual Dose (in rem): 100 mrem per year (0.1 rem)
Risk Coefficient: 0.00005 per rem per year
Years of Exposure: 5 years

Risk = 0.1 × 0.00005 × 5 = 0.000025 (2.5 per 100,000)

Dose to the thyroid

Annual Dose (in rem): 10,000 mrem per year (10 rem)
Risk Coefficient: 0.000008 per rem per year
Years of Exposure: 5 years

Risk = 10 × 0.000008 × 5 = 0.0004 (4 per 10,000)

17 In situ vitrification (ISV) is a process that applies electrical power to contaminated soil to produce the heat needed to melt and blend the soil and waste into an immobile form (USDOE 1995b). DOE determined, however, that ISV could be problematic because of standing water in the trenches and higher than anticipated expenses related to the process. Thus, in May 2004, DOE issued a proposed plan to amend the Record of Decision by replacing ISV with in situ grouting (ISG). ISG involves a low-pressure grouting method to inject Portland cement-based grout throughout the trenches. In addition, a solution grout would be used to treat soil adjacent to the trench walls to close potential seepage pathways (ORSSAB 2004). In September 2004, the proposed requirement for the Record of Decision and the remedial action work plan for ISG of the trenches were approved (ORNL 2005).
18 [SRA] Society for Risk Analysis. 2004. Glossary of risk analysis terms. Available from: Exiting ATSDR Website. Last accessed 25 January 2006.
19 [USEPA] US Environmental Protection Agency 2006. Terms of environment: glossary, abbreviations and acronyms. Available from: Exiting ATSDR Website. Last accessed 14 April 2006.
20 [GAO] US General Accounting Office. 2000. Radiation standards. Scientific basis inconclusive, and EPA and NRC disagreement continues. Report to the Honorable Pete Domenici, US Senate. Washington, DC: US General Accounting Office. Report GAO/RCED-00-152; June. Available from: Exiting ATSDR Website. Last accessed 25 April 2006.
21 [ICRP] International Commission on Radiological Protection. 1991. 1990 Recommendations of the International Commission on Radiological Protection. New York: Pergamon Press; ICRP Publication 60.
22 [NCRP] National Council on Radiation Protection and Measurements. 1993. Risk estimates for radiation protection. NCRP Report 115. Bethesda, MD: National Council on Radiation Protection and Measurements.
23 [USEPA] US Environmental Protection Agency. 1994. Estimating radiogenic cancer risks. EPA 402-R-93-076; June. Available from: Exiting ATSDR Website. Last accessed 15 March 2006.
24 [USEPA] US Environmental Protection Agency. 1999. Cancer risk coefficients for environmental exposure to radionuclides. Federal Guidance Report 13. EPA 402-R-99-001. Washington, DC: US Environmental Protection Agency; September. Available from: Exiting ATSDR Website. Last accessed 15 March 2006.
25 [USEPA] US Environmental Protection Agency. 2005. Guidelines for carcinogen risk assessment. EPA/630/P-03/001B. Washington, DC: US Environmental Protection Agency; March. Available from: Exiting ATSDR Website. Last accessed 25 April 2006.
26 [USEPA] US Environmental Protection Agency. 2004. An examination of EPA risk assessment principles and practices. EPA/100/B-04/001. Washington, DC: US Environmental Protection Agency; March. Available from: Exiting ATSDR Website. Last accessed 20 April 2006.
27 [OMB] US Office of Management and Budget. 2006. Proposed risk assessment bulletin. Available from: Exiting ATSDR Website. Last accessed 20 April 2006.

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