Savannah River Site Health Effects Subcommittee (SRSHES) Meeting

Meeting Minutes
September 5, 2002

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Dr. Mark Evans of ATSDR conveyed that the purpose of this activity was to respond to a request by the Lawrence Livermore National Laboratory (LLNL) community to evaluate tritium exposures in general and OBT in particular. Concerns were raised about existing tritium monitoring activities excluding OBT in regular environmental measurements and neglecting a particular component of the radiological dose. Before addressing these concerns, however, ATSDR first noted three major problems in conducting a tritium dose assessment. First, tritium is present in several different forms in the environment, i.e., gas, water and organic forms. Second, past and current monitoring programs do not directly measure OBT. Third, the effective dose of OBT may be greater than water since most dosimetry models assume all tritium is present as water.

ATSDR also noted uncertainties in an OBT dose assessment, such as lack of data on specific activity ratios between OBT and water; the possibility of a longer biological half-life creating a larger tritium dose; and the potential for OBT decay to have a relatively greater impact than water decay. To effectively evaluate tritium in the environment, ATSDR needed to first identify appropriate questions for the data to produce useful answers. ATSDR will only recommend or conduct environmental sampling if warranted by public health. To respond to the LLNL community, ATSDR would need to know the expected levels of OBT in the environment, appropriate analytical methods for anticipated levels and biological media to test. ATSDR formed an expert panel to assist in formulating questions for the assessment, identifying significant public health issues with respect to OBT, and conducting environmental monitoring or undertaking other public health actions.

ATSDR solicited input from the LLNL community during all phases of the expert panel process, including formulating tasks, identifying areas of expertise for the panel, nominating members, participating in meetings, and reviewing and commenting on both the report and health consultation. The California State Health Department and residents from the LLNL and SRS sites assisted ATSDR in obtaining community input. Agreement was reached for the expert panel to address two major questions: (1) Are potential total tritium exposures at SRS and LLNL at levels of public concern that warrant the collection and analysis of OBT data? (2) If warranted, what media should be sampled and what would be appropriate analytical procedures?

ATSDR then released a request for proposal with six criteria for the nomination and selection process. The five expert panel members who were selected presented with the most expertise in radiochemistry of tritium; sampling and analysis of tritium or hydrogen in environmental media; human uptake of environmental radiological contaminants; partitioning of organic hydrogen compounds in humans and animal models; dosimetry of OBT, tritium health effects or other low-energy beta emitters; and uncertainty analyses. For purposes of the evaluation, "OBT" was defined as non-interchangeable tritium bound to carbon. OBT production is biochemical in bacteria or food, has no direct transformation in air from gas or water, and is a chemical process similar to methylation. The effective half-life of OBT varies by type of organic molecule.

Several components are incorporated into models to calculate tritium dose, including energy of decay, effective biological half-life, whole body mass, radiation weighting factor, dose and dose rate effectiveness factor (DDREF), environmental concentrations and intake rates. Due to the absence of documented morbidity and mortality cases from tritium exposure, ATSDR acknowledges the complexity in calculating dose from low environmental levels of radionuclides and identifying associated adverse health effects. Because of these data gaps, ATSDR’s model included radiation exposures from Hiroshima, Nagasaki and other releases with extremely high doses. Adjustments were then made to convert extremely high radiation doses to low-level environmental radionuclides.

The expert panel developed ranges for parameters underlying the dosimetry process to make probable dose estimates. The panel concluded that the weighting factor for tritium was between 1 and 3 with the most likely value being 1.3 to 1.5. The DDREF was found to vary between 0.10 and 1 with the most likely value being 0.40. To calculate SRS doses for environmental tritium activities, the expert panel made the following conservative assumptions based on data in the SEMP 1999 Annual Report. Tritium activity in water was calculated at ~40 Bq/L with a mean drinking water value of 37 Bq/L and a maximum value of 60 Bq/L. Water from the Savannah River at the site boundary was ingested at the rate of 2 L/day. Tritium activity in food was calculated at ~10 Bq/L with a range from non-detectable limits to 90 Bq/L; the mean value was calculated at a range of 4-12 Bq/L. Tritium activity in food was measured as water in vegetation-free water; 100% of food was assumed to be from locations near SRS.

The expert panel incorporated corrections for OBT in the dose calculations and also reviewed historical tritium activities. Data showed that past concentrations were on the order of 100 times current tritium activities. Annual tritium doses from SRS releases to the offsite community were evaluated in drinking water, food, water in food, OBT in food, food decay as water, food decay as OBT, and food plus water for a total dose. Values established for the public and ATSDR’s minimal risk level for tritium of 100 mrem per year were compared to the expert panel’s dose calculations.

ATSDR used the expert panel conclusions to make decisions about conducting the OBT health consultation from a public health perspective. First, total annual doses of tritium and OBT were found to be below levels of public health concern. Second, protective assumptions were found to be adequate to account for uncertainties in monitoring and estimating dose. Third, OBT will increase a tritium dose by a multiplier of 1.3 to 1.5, but estimated tritium doses were not found to warrant the collection of additional OBT data. Fourth, OBT is produced in the environment, but actual releases from SRS were found to be negligible.

Discussion.

Dr. Bustos pointed out that the most damaging action by tritium would be genetic effects in the cell nucleus rather than the whole body. Dr. Evans noted that although whole body is the standard process to evaluate tritium health effects, the expert panel report contains a section on reproductive impacts, risks and decays. Mr. Lockridge requested details on OBT production at SRS. Dr. Evans explained that OBT is produced in the environment or the body and is not released from SRS to any significant degree. However, he reiterated that OBT has been excluded from routine environmental monitoring activities.

Dr. Bustos added that OBT would be generated in any instance where tritium attaches to a molecule in the body, hydrogen is present and can be exchanged. Ms. Kato questioned whether the role of VOC releases at SRS in OBT uptake was considered during the expert panel evaluation. She acknowledged that SRS annual releases are low, but cumulative effects from air, water and other sources increase the potential for adverse health effects. Ms. Kato noted that the expert panel report stated the increase in OBT dose was 100% in all normal diets and 16%-35% in fresh plants. She mentioned that these data were based on doses to an average sized male rather than a smaller female, infant or fetus.

Dr. Evans confirmed that the release of tritium as OBT may have been caused by VOCs from SRS. Although this source is probably insignificant, the expert panel did consider VOCs during the assessment. Tritium accounts for more than 50% of the total dose around SRS, but he agreed that cumulative doses from all sources must be considered. With respect to OBT consumption, Dr. Evans clarified that a large proportion of ingested OBT may be excreted or enter the human body as water rather than OBT. Data show that retention and ingestion of OBT in the human body affect dose and vary by individual.

He also agreed that the evaluation was flawed in some areas. For example, the expert panel questioned the reliability of one study and the assessment did not include acute tritium doses. As a result, ATSDR followed up the LLNL evaluation by developing an acute tritium release scenario for the site. Nevertheless, several laboratory studies and solid research conducted around SRS on OBT in deer served as strong data sources to support tritium and OBT processing by mammalian bodies. These references are listed in the expert panel report.

Ms. Guess inquired about research that has been conducted to determine if OBT binds with molecules in the human body more than in water or other environmental media. Dr. Evans explained that data have been collected demonstrating some doses are greater from OBT than water due to the time of residence in the body. While water remains in the human body no more than 30 days, OBT can reside in the body up to 300 days.