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PUBLIC HEALTH ASSESSMENT

Community Exposures to the 1965 and 1970 Accidental Tritium Releases

LAWRENCE LIVERMORE NATIONAL LABORATORY, MAIN SITE (USDOE)
LIVERMORE, ALAMEDA COUNTY, CALIFORNIA


SECTION 4: PUBLIC HEALTH IMPLICATIONS OF ESTIMATED TRITIUM DOSES

Toxicology of Tritium

The final step in our analysis was to compare our estimated tritium doses with radiation doses that cause adverse health effects. In comparing our results to information in the medical literature, it is important to note that there have been no direct observations of human health effects from exposures to tritium; therefore, it is necessary to extrapolate from other data. We looked at studies of animals and tissue cultures exposed to tritium, low-energy external gamma radiation, or x-rays. We also looked at studies of humans exposed to low-energy external gamma radiation or x-rays.

Radiation effects from tritium in the body are similar to effects from low-energy gamma radiation and x-rays (Johnson et al. 1995). Therefore, we considered studies of humans exposed to gamma radiation and x-rays for information on cancer and developmental effects (exposures to the fetus in utero) that might also result from tritium exposures. We also reviewed studies of animals and tissue cultures for information on developmental effects, cancer effects, and genetic effects (effects on offspring because of exposures to either parent before pregnancy).

The lowest tritium (or low-level gamma radiation or x-rays) dose capable of causing adverse health effects is 3 rad, or 0.03 gray [Gy](13) (UNSCEAR 1986). This dose (3 rad or 3,000 mrem) resulted in abnormal developmental effects when administered (via x-rays) to mice in utero during the development of the central nervous systems. Our maximum predicted dose from tritium is ~149 mrem (1.49 millisievert [mSv]), which is more than 20 times lower than 3 rad (3,000 mrem).

The kind and severity of developmental abnormalities appear to be related to the amount of exposure and the stage of fetal development at the time of exposure. Central nervous system development in humans occurs between 8 and 15 weeks of gestation (UNSCEAR 1986). In humans, mental retardation, intelligence, and school performance were studied in Hiroshima and Nagasaki atom bomb survivors who were exposed to radiation in utero. Exposure to 70 rads (0.7 Gy) resulted in 25% of the children being severely mentally retarded (Straume 1993). The dose-response relationship appears to have a threshold at about 25 rads (25,000 mrem; 0.25 Gy). Intelligence tests administered to prenatally exposed atom bomb survivors at 10 to 11 years of age demonstrated a substantial decrease in test scores for those exposed between 8 and 15 weeks of gestation (Straume 1993). These data suggest a linear dose-response curve without a threshold and a decrease in intelligence of approximately 25 points per 100 rads (25 points per 100,000 mrem). School performance was also investigated, and the data also suggest a linear dose-response curve without a threshold (Straume 1993). Collectively, the data on central nervous system development in humans and animals are consistent with either a linear dose response with no threshold, or a dose response that appears to have a threshold at low doses (Straume 1993).

At our predicted dose during this period of gestation, the effects on intelligence and school performance would not be measurable. Therefore, we do not expect there to have been an increase in developmental effects for children born to people living near the LLNL facility.

Another adverse health effect we considered was cancer. High doses (100 to 300 rad, or 100,000 to 300,000 mrem) of x-rays to humans (radiologists) and of tritium to mice have been linked to significantly higher rates of leukemia, but the doses were 700 times higher than the maximum predicted dose in Table 3 (Johnson et al. 1995; UNSCEAR 1988). The atomic bomb survivor studies showed the lowest leukemogenic dose was in the range of 20 to 40 rads (20,000 to 40,000 mrem); however, these doses (to people in Hiroshima) included a large neutron component. In Nagasaki, where people were not exposed to the large neutron component, an increased incidence of leukemia was not seen in people who received less than 100 rad (100,000 mrem; 1 Gy) (Cember 1988).

Studies of children who received x-rays in utero indicate there is a threshold dose for radiogenic leukemia that lies in the range of 10 to 50 rad (10,000 to 50,000 mrem; 0.1 to 0.5 Gy; Cember 1988). By analogy, it appears that the lowest tritium dose associated with cancer effects in humans might be approximately 10 rem (10,000 mrem; 0.1 Sv) received by an unborn child(14). This dose is approximately 67 times greater than the maximum predicted child dose in Table 3.

Data from a United Nations Scientific Committee on the Effects of Atomic Radiation 1972 review, reported in UNSCEAR (1988), suggested that in utero fetal doses on the order of 1 to 4 rad (1,000 to 4,000 mrem), particularly during the first trimester of pregnancy, will increase the incidence of leukemia, but other data in the same report do not support this contention. These levels are still 14 to 56 times higher than the maximum in utero doses7 from LLNL exposures; therefore, we do not expect an increase in cancer rates because of exposure to tritium.

Another health effect not previously mentioned is the reduction in average life span. The measure of average life span in an irradiated population compared with a population that was not irradiated includes all radiation injuries that contribute to death. Russian researchers found that when they administered tritiated water to animals in small amounts over a prolonged period with total doses of 24 rad (24,000 mrem), the animals' life spans increased by 12%. At 200 rad (200,000 mrem), the life span of irradiated animals was the same as that for controls that were not irradiated. But increasing the total dose to 1,250 rad (12.5 Gy) and 2,500 rad (25 Gy) reduced the animal's life span by 18% and 33%, respectively. The reduction in life span at the higher doses was the same for rats that were carriers of malignant tumors as for rats that did not have tumors. Therefore, the researchers concluded that there is a common mechanism for natural aging and its acceleration that is independent of the presence of cancer, particularly with high total radiation doses (Balanov et al. 1993). It appears that animals' life spans were not affected at 200 rad (200,000 mrem) total; this level is more than 1300 times greater than the maximum child dose in Table 3. Therefore, we do not expect that tritium exposures from the LLNL releases would decrease anyone's life span.

Special Consideration of Women and Children

In addition to the above health protective exposure and dose estimation assumptions, special consideration was given to evaluate potential exposures to women and children in cases where they might be especially vulnerable to health effects from such exposures. In this case, the increased breathing rate and lower body weight of children creates potentially higher tritium doses relative to adults exposed to the same amount of tritium in the air (as listed in Table 2).

In addition to higher potential inhalation doses for children, estimates of tritium incorporation, dose, and risk become more complicated when exposure to tritium occurs in utero, or during early prenatal life. This particular consideration was explicitly evaluated for the 1970 release by the expert panel report published as an attachment to the previously referenced ATSDR health consultation (ATSDR 2002A). The expert panel concluded that there was no increased risk of a severe hereditary effect due to prenatal tritium exposure following the 1970 tritium release.

From cellular studies, we know that tritium can be incorporated into DNA and RNA molecules during cell proliferation. Radioactive decay in or near the DNA molecules might cause molecular breaks and rearrangements of the gene code, which could lead to mutagenic effects. For genetic effects (passed on to offspring), damage must occur during the maturation stages of the sperm and oocytes (Straume and Carsten 1993). In the sexually mature male, sperm cells are continuously replenished. The cells go through several stages with different radio-sensitivities and potential for cell death.

Although we know little about the lowest radiation levels required to produce genetic damage in human sperm cells, we do know that 20 doses of 25 rad (25,000 mrem) produce a more rapid drop in the number of sperm cells and the sperm cell populations require greater recovery time than if they are exposed to a single dose of 500 rad (500,000 mrem). Also, total doses of 50 to 100 rad (50,000 to 100,000 mrem) administered in increments produce temporary low sperm counts after about 3 months of exposure (UNSCEAR 1988). These doses are more than 300 times greater than the maximum total dose estimated in Table 3.

In females, oocytes are produced in the ovary before birth; therefore, oocytes in the fetus before birth would be the most radio-sensitive. When pregnant squirrel monkeys received tritiated water throughout their pregnancy that resulted in mean body water concentrations of tritium ranging from 50 to 3,100 microcuries per liter (┬ÁCi/L) (1.85 to 114.7 kilobecquerel per milliliter [kBq/mL]), their newborns showed no discernible effects except that the number of primary oocytes in the female offspring decreased markedly with increasing levels of tritium in the maternal drinking water (Jones et al. 1980). However, these tritium concentrations are more than 27 to 1700 times higher than the maximum body water concentrations predicted by the doses in Table 3. Therefore, we do not expect genetic effects in the children of pregnant women who were exposed to tritium from LLNL releases.

The next group of adverse health effects we considered is genetic effects. These are effects passed on to children from abnormal sperm or egg cells in parents. The genetic effects of concern are adverse health effects in children that arise from tritium exposures to parents. We have not identified human populations that show genetic damage from either external radiation or internally deposited radioactive materials, including tritium, that lead to adverse health effects in offspring (UNSCEAR 1988; NCRP 1979).

Tritium Doses of Public Health Concern

In order to determine whether the accidental releases of tritium from LLNL present a public health hazard, ATSDR compared the estimated tritium doses with benchmarks or screening doses that are derived from dose levels known to produce adverse health effects. For ionizing radiation, which includes tritium, ATSDR has developed minimal risk levels (MRLs) that cover brief exposures (acute, or less than 14 days) and longer term exposures (chronic, or more than a year).

An MRL is an estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse non-cancer health effects over a specified duration of exposure. The MRL is derived from exposure levels observed to produce adverse effects, with uncertainties (or safety factors) incorporated into the value. Thus, MRLs are intended only to serve as a screening tool to help public health professionals decide which release situations require more extensive evaluation. While estimated exposure dose levels below an MRL are not likely to produce non-cancer adverse effects, exposure estimates above an MRL do not mean that adverse effects will occur. ATSDR then evaluates the potential for adverse health effects in an exposed community by comparing levels known to produce adverse effects to the estimated site-related doses. This margin of exposure (MOE) approach, along with an evaluation of available epidemiologic, toxicologic, and medical data, is used by health assessors as part of the public health determination to reach qualitative (rather than quantitative), risk-based decisions about hazards posed by site-specific conditions of exposure.

On the basis of an extensive review of the health studies and documented health effects from radiological exposures, ATSDR established an MRL of 400 mrem for acute duration (14 days or less) of external exposure(15) (ATSDR 1999) to ionizing radiation. The acute MRL is based on external dose levels that did not produce behavioral and/or neurological effects on the developing human embryo and fetus.  Similarly, a chronic duration (a year or more) of external exposure MRL of 100 mrem/year (above background) has been established based on radiation doses that have not produced observable detrimental health effects in humans. Thus, the ATSDR acute and chronic MRLs for ionizing radiation (including tritium) are based on doses with No Observed Adverse Effect Levels (NOAELs). While ATSDR MRLs typically include only non-cancer health effects, all the studies on which the chronic MRL for ionizing radiation are based included cancer as the specific end-point.  Consequently, the chronic MRL for ionizing radiation is considered protective for both cancer and non-cancer health effects.

Similarly, ATSDR also evaluates the potential for cancer risk by first comparing the estimated dose levels to a theoretical risk level, usually the dose level associated with a 10-6 risk (one in a million) as defined by other governmental agencies. ATSDR designates these screening levels as Cancer Risk Evaluation Guides (CREGs). As with the non-cancer approach, levels below the 10-6 require no further evaluation, while estimated dose levels that exceed the comparison value are evaluated further.  The potential for observing adverse effects is made on the basis of dose evaluation (a MOE approach), rather than on the basis of theoretical risk calculations. (See below discussion on dose-based qualitative approaches for health assessment versus risk-based, quantiative approaches used by regulatory agencies).

In contrast to the dose-based health assessments conducted by ATSDR, the United States Environmental Protection Agency (EPA) develops regulations based on risk and policy decisions. To accommodate proper evaluation of the dose and risk issues associated with radiation exposure, it is necessary to define clear the terms dose and risk. The International Society for Risk Analysis (www.sra.org) 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." As defined, risk is a statistical concept, and the threshold for acceptable risk, which is not based on observable adverse health effects, is simply a policy statement. Risk Assessments are useful in determining safe regulatory limits. The regulatory limits have extra safety factors built into them and may in fact be orders of magnitude below levels at which adverse effects have been documented to occur in humans. Risk assessments are useful in providing comparative risk discussions for purposes of prioritizing cleanup.

ATSDR defines dose as "The amount of a substance to which a person may be exposed, usually on a daily basis. Dose is often explained as "amount of substance(s) per body weight per day". Doses are the basis for determining levels of exposure that may cause adverse health effects and may be directly related to the assessment of public health. As noted, ATSDR uses risk assessment procedures as a screening tool in its evaluation, including MOE approaches along with the consideration of health effects data (epidemiologic, toxicologic, and medical) to reach conclusions about the potential for adverse effects being observed in the community.

More specifically, ATSDR uses radiation doses instead of risk in its public health documents for various reasons. Among these are the facts that dose coefficients are based on a more exact science; that is, the doses are based on physical constants and primary principles of physics such as energy absorption, and health effects resulting from radiation doses are based on a "weight-of-evidence" approach. ATSDR, in preparing its public health documents, also relies on site-specific parameters such as demographics, realistic land uses, and other pertinent data related to the site. Using dose coefficients and modifying the coefficients for chemical forms and particle sizes, which are not typically done for risk assessments, allows ATSDR to develop health-protective, albeit realistic, values for the dose assessments as they pertain to public health documents.

Similarly, radiation health studies use dose because there is a long history of research in which health outcomes were evaluated relative to the radiation dose and not on the numerical estimation of risk. ATSDR also recognizes there are uncertainties in these dose coefficients; however, those uncertainties are addressed by the use of health protective safety factors. Risk calculations include those uncertainties plus additional uncertainty associated with the risk estimation model. Consequently, the derivation of quantitative risk is much more uncertain that the underlying dose-based assessment.

The uncertainty in the dose effects lies within the middle ranges of exposure. Adverse health effects have been conclusively demonstrated for exposures greater than 10,000 mrem/year (ATSDR 1999). Numerous studies have also demonstrated that no adverse health effects have been documented for doses less than 360 mrem/year (ATSDR 1999). The ATSDR minimal risk level (MRL) for ionizing radiation is based on numerous evaluations of health effects from exposures to background and occupational levels of radiation. "The annual dose of 3.6 mSv per year (360 mrem/year) has not been associated with adverse health effects or increases in the incidence of cancers in humans or animals." (ATSDR 1999). Consequently, 360 mrem/year is defined as a "No Observed Adverse Effect Level" (NOAEL). The derived MRL, which is further reduced by a factor of 3 to account for human variability (and conservatively rounded down from 120 mrem/year to 100 mrem/year), is protective of human health.

The science associated with risk is based on a model that, at low doses typically associated with small multiples of background, cannot be proven. ATSDR also realizes that every action, radiation dose, or activity has an associated risk. However, because no adverse health effects have been observed at doses considerably higher than 100 mrem/year (above background), there is no public health basis for using highly uncertain, risk-based screening values. Acute exposures to tritium via the inhalation, ingestion, and dermal pathways described in this health assessment that resulted in cumulative doses of less than 400 mrem or in chronic exposures less than 100 mrem/year (above background and averaged over 5 years) are unlikely to produce any adverse health effects and therefore are below levels of public health concern.

Health Outcome Data Evaluation

Health outcome data are records of the incidence of diseases. Typically, medical diagnoses of specific diseases and birth and death records are routinely reported to health agencies that compile and evaluate the patterns of disease incidence and frequency. For example, the California Cancer Registry (CCR 2002) collects data on the incidence of cancer, vital statistics bureaus collect data on mortality, and the California Birth Defect Monitoring Program collected data on birth defects.

The incidence of specific diseases or health outcomes might help determine whether the rate of any specific outcome is higher in one area than in others. For this assessment, which is focusing on short-term tritium exposures, the health outcome data would need to be queried to determine if the incidence of those diseases that might be related to tritium exposure are elevated in the population exposed to the tritium plume. While tritium, as ionizing radiation, is theoretically capable of inducing hair loss, birth defects, cancer, mental retardation, and death (ATSDR 1999), to date there have been no direct observations of cancer, hereditary and developmental effects, or in utero effects from tritium exposures to humans (ATSDR 2002A).

Because there have been no direct health outcomes associated with tritium exposures, presumed health outcomes must be extrapolated from various models. In addition to the problem of identifying specific disease outcomes related to tritium exposure, the evaluation of health outcome data includes significant limitations. First, health outcome data are only collected for select and limited outcomes. A second limitation on health outcome data evaluation is related to the size of the population and geographic area for which the data are collected. The population of concern (within the areas of potential exposure) adjacent to the LLNL facility represents a very small geographic area and a total exposed population of less than 55 people (16).

When diseases occur in such a small population, there are no appropriate statistical measures to determine whether the disease occurred by the normal chance distribution of the disease or whether the occurrence is specifically related to tritium exposures from the LLNL facility. Recognizing these inherent limitations of health outcome data analysis and that the estimated tritium doses from the accidental LLNL tritium releases are below levels expected to produce adverse health outcomes, this public health assessment will not attempt an independent evaluation of the available health outcome data.

However, several community and LLNL worker health studies have been completed to evaluate disease incidence rates in the Livermore community. These studies are reviewed to determine if they have identified any excess disease incidences that might be related to the accidental tritium releases and community exposures. Those studies of disease incidence among residents of the Livermore community include:

  • A 1995 study of cancer incidence among children and young adults conducted by the California Department of Health Services (CDHS 1995). The analysis specifically examined the cancer incidence rates for all individuals born in Alameda County during the 1960 to 1990 period. This study, conducted in cooperation with the Centers for Disease Control and Prevention (CDC), found that the overall rates of cancer among children and young adults, for the years 1960-1991, in the city of Livermore do not appear to significantly differ from those of the remainder of Alameda County. However, the study did find a higher rate of malignant melanomas in the Livermore population. An apparent elevation of brain cancer among Livermore children during the 1960s was not sustained in more recent decades, and was not evident for the birth subjects in the study.


  • A 1996 CDHS study of the incidence of invasive cancer (for the years 1988-1993) among residents of eight census tracts surrounding the LLNL facility (CDHS 1996). The results of this study indicated that the frequency of those invasive cancers was not elevated compared to the rates for the entire San Francisco Bay area. The incidence of melanoma was elevated in one census tract located west of the LLNL facility, although the increase was not statistically significant.


  • A March 1999 study by the California Birth Defects Monitoring Program (CBDMP). The study examined the incidence of birth defects in two Livermore zip codes (94550 and 94551) for the years 1983-1989 (CBDMP 1996). The CBDMP study found no evidence of increased rates of birth defects among people living in the Livermore zip code areas. The years 1983 to 1989 are the only years that the CBDMP operated in Alameda County.

In addition to the above community-based health studies, several studies of LLNL worker disease incidence have also been conducted. These studies include:

  • A 1980 CDHS study of the LLNL worker populationfound increased rates of malignant melanoma relative to the regional population (Austin, et al. 1981). For the 19 cases diagnosed (from 1972 to 1977) work involving ionizing radiation was not associated with the melanoma diagnoses, however, work as a chemist was associated with the diagnoses.


  • A follow-up case control study, which examined worker diseases for the years 1969 to 1980, found higher rates for salivary gland and rectal cancers for female workers and other central nervous system tumors (excluding brain tumors) for male workers (Austin and Reynolds 1984). The case control study of melanoma incidence among LLNL workers was conducted to assess potential occupational factors that might be related to the melanoma diagnoses. Several occupational factors were identified that were more common than expected including: (1) exposure to radioactive materials, (2) work at Site 300, (3) exposure to photographic chemicals, (4) participation at the Pacific Test Site, and (5) duty as a chemist. The Austin and Reynolds (1984) study found that melanoma incidence was consistent with personal factors, genetics, and outdoor recreation. Out of the 39 potential industrial exposures, only occupational exposure to alcohols was more common among persons with diagnosed melanoma.

Neither the community nor the LLNL worker health studies have identified excess disease incidences that appear to be related to the relatively low tritium doses from exposure to the accidental releases. There is no indication that rates of leukemia and non-Hodgkins' lymphomas, which are thought to be potentially radiogenic cancers (ATSDR 2001), were elevated in any of the community or worker health studies. Although several of the studies have documented significant increases in the rate of malignant melanomas, such skin cancers have no observed or expected cause and effect relationship with tritium exposure (ATSDR 2001). The study of birth defects, which found no increased birth defect rates, includes birth data from 19 to 24 years after the accidental releases. While this study would not specifically include any babies born shortly after the releases, it would be potentially applicable to babies whose mothers might have had in utero or early neonatal exposures (if they still lived within the Livermore area).

In addition to the above completed health studies, the CDHS Environmental Health Investigations Branch is currently conducting a review of health studies and the underlying health outcome data related to the Livermore community and the LLNL facility.The public comment version of this health consultation has been released (CDHS 2003) and does not provide any new information or conclusions relating off-site tritium exposures and disease incidence in the Livermore community.


SECTION 5: CONCLUSIONS AND RECOMMENDATIONS

On the basis of current peer-reviewed scientific literature, the one-time exposure to tritium resulting in a committed effective adult dose of 42 mrem (0.42 mSv) or a child dose of 149 mrem (1.49 mSv) from the LLNL accidental HT releases are no apparent public health hazard. No apparent health hazard means that while some public exposure to tritium probably did occur as a result of the accidental releases of tritium gas (HT), estimated maximum exposures are below levels of public health concern and no adverse health effects would be expected. This conclusion, based on tritium doses developed from analytical models, is supported by measured results of tritium concentrations in human urine samples following the 1970 release. After that accidental release, tritium was not detectable in the urine of either LLNL workers or affected community members. The above estimated doses represent the 95th percentile doses based on health protective exposure and dosimetry (radiation dose assessment) assumptions. It is unlikely that actual doses approached these estimated levels.

The estimated tritium doses from the 1965 release are lower than those from the 1970 release. Meteorological conditions during the 1970 release indicate that the maximally exposed population consisted of fewer than 52 people living 1 to 2 miles north-northeast of the LLNL. Due to meteorological conditions during the January 1965 release, estimated concentrations are less than those from the 1970 release. Because the 1965 accidental release involved a similar quantity and form of tritium (~350 kCi HT) and had lower estimated off-site tritium concentrations, the 1965 release is also considered to be no apparent health hazard.

All of the adverse health effects from exposures to tritium (or low-energy external gamma radiation or x-rays) that we found in the medical literature occurred at much higher levels than the exposure levels we estimated for people living near the LLNL facility at the time of the accidental releases. Therefore, we conclude that inhalation and ingestion of tritium from those releases plus any chronic or long-term exposures were not public health hazards. Specifically, these releases are considered to be no apparent health hazards, which means that while some exposure probably did occur, those exposures are not likely to produce adverse health effects and are below levels of public health concern.

Although these conclusions are specific to the accidental releases of tritium from LLNL that occurred in 1965 and 1970 and the short-term exposures resulting from those accidental releases, the total tritium doses (Table 3) do include potential contributions from long-term or chronic LLNL tritium releases and potential exposures. As these historic accidental releases are below levels of public health concern, no specific recommendations are warranted. Conclusions and recommendations regarding chronic or long-term LLNL tritium releases and ongoing tritium monitoring are addressed in a separate health consultation.


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PREPARERS OF REPORT

Mark W. Evans, PhD
Environmental Geologist
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation

Paul Charp, PhD
Health Physicist
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation


Reviewers of Report

Burt J. Cooper, MS
Supervisory Environmental Health Scientist
Energy Section Chief,
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation

Sandra G. Isaacs
Chief,
Federal Facilities Assessment Branch
Division of Health Assessment and Consultation


Regional Representative

William Nelson
Senior Regional Representative
Regional Services, Region IX
Office of Regional Operations


13 For the purposes of this assessment, 1 rad (absorbed dose) is equal to 1 rem (dose equivalent, or 1,000 mrem); 100 mrem is equal to 1 mSv; and 1 gray (absorbed dose) is equal to 1 sievert (dose equivalent).
14 Fetal doses are assumed to be 1.7 times the dose to the mother (Sikov et al. 1993). Estimated in utero doses for 1970 at the maximum exposure location average about 19 mrem with a 95th percentile value of 71 mrem.
15 Although the ATSDR MRLs for ionizing radiation are specific to external exposure, the value of 100 mrem/year is consistent with those for either external or internal exposures promulgated by the US Nuclear Regulatory Commission, the National Commission on Radiation Protection, and the International Commission on Radiation Protection (as referenced in ATSDR 1999).
16 Although theoretical exposures occurred to residents living beyond the 2-mile distance evaluated in this assessment, the potential doses are such a small fraction of normal background radiation that the additional radiation dose from the tritium releases cannot be differentiated from the range of background values.
17 This reference section also includes references for following appendices.

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