Biomedical subjects

D M Hamby

Publications and source records attributed to D M Hamby.

At least 19 recordsLinked to original sources

PubMed2006

Uncertainty of the thyroid dose conversion factor for inhalation intakes of 131I and its parametric uncertainty.

Inhalation exposures of 131I may occur in the physical form of a gas as well as a particulate. The physical characteristics pertaining to these different types of releases influence the intake and subsequent dose to an exposed individual. The thyroid dose received is influenced by the route through which 131I enters the body and its subsequent clearance, absorption and movement throughout the body. The radioactive iodine taken up in the gas-exchange tissues is cleared to other tissues or absorbed into the bloodstream of the individual and transferred to other organs. Iodine in the circulatory system is then taken up by the thyroid gland with resulting dose to that tissue. The magnitude of and uncertainty in the thyroid dose is important to the assessment of individuals exposed to airborne releases of radioiodine. Age- and gender-specific modelling parameters have resulted in significant differences between gas uptake, particulate deposition and inhalation dose conversion factors for each age and gender group. Inhalation dose conversion factors and their inherent uncertainty are markedly affected by the type of iodine intake. These differences are expected due to the modelling of particulate deposition versus uptake of gas in the respiratory tract. Inhalation dose estimates via iodine gases are very similar and separate classifications may not be necessarily based on this assessment.

Administration, Inhalation↗

PubMed2004

A modified ICRP 66 iodine gas uptake model and its parametric uncertainty.

Intakes via inhalation may occur from radionuclides released in the form of a gas. The chemical characteristics pertaining to the release influence the intake and subsequent dose to an exposed individual. Gases are taken up or absorbed in the entire respiratory tract and the associated uptake mechanisms are quite different from deposition of particulates. Gaseous iodine can exist in various chemical forms, e.g., elemental iodine, inorganic, and organic iodine compounds. These different chemical species play an integral role in the gaseous uptake o f iodine in t he respiratory tract. Gas uptake in the various regions of the respiratory tract results in the intake of iodinated material into the body. The radioactive iodine taken up in the gas-exchange tissues is absorbed into the bloodstream of an individual and subsequently transferred to other organs. Iodine in the circulatory system can then be taken up by the thyroid gland, with resulting dose to the thyroid. The magnitude and uncertainty in regional gas uptake is important in the assessment of individuals exposed to airborne releases of radioiodine. The current ICRP 66 model is rudimentary and estimates regional gas uptake based on solubility and reactivity of the different radionuclides entering the respiratory tract. The modified model proposed here employs methodology and a mathematical structure to determine estimates of fractional gas uptake rather than defaulting to literature values, as in the current ICRP model. Model parameters have been assigned input distributions and estimates of uncertainty have been determined. A sensitivity analysis of these parameters has been performed to demonstrate the importance of each of these parameters. The sensitivity analysis ranks the model-input parameters by their importance to estimates of regional gas uptake. The model developed herein may be used for improved estimation of gas uptake in the respiratory tract and subsequent dose estimates from the different chemical forms of radioiodine.

Adult↗

PubMed2003

Age-specific uncertainty of the 131I ingestion dose conversion factor.

The production of weapons-grade nuclear materials and their by-products has resulted in a number of releases from United States Department of Energy facilities. 131I, a fission by-product, is one of the most common radionuclides generated and released to the environment. It is known that there are differences in various physiological parameters over all age groups when considering biokinetic modeling of iodine. The establishment of age-specific dose conversion factor uncertainty is necessary for accurate internal dose assessment. The 131I dose conversion factor determined herein is log-normally distributed with varying age-specific distribution characteristics. The two most important parameters for determination of the dose conversion factor, in all age groups, are thyroid mass and iodine uptake fraction. These parameters are assumed to be highly correlated with a relationship that is quite important to dose conversion factor uncertainty. Dose estimates to individuals exposed to radioiodine can be determined more accurately with an increased understanding of the correlation between thyroid mass and uptake fraction. Improved dose estimates following oral intakes of 131I can be made from the consideration of age-specific dose conversion factors and their input parameters.

Administration, Oral↗

PubMed2003

Uncertainty in transport factors used to calculate historical dose from 131I releases at the Savannah River Site.

During the 1950's, atmospheric release of 131I was one of the largest contributors to offsite dose at the Savannah River Site. Computer models used to estimate offsite dose involve the use of many parameters with wide ranges of uncertainty. The overall uncertainty in dose can be estimated by propagating the uncertainty of each parameter through the model. A major component of the calculational model can be solved for a given release scenario and condensed into a transport factor, which, when multiplied by the air concentration (or deposition) and the appropriate dose conversion factor, can be used to estimate a specific pathway dose. Uncertainties are estimated for the period of 1955-1961 for all parameters contributing to the 131I transport factor for each pathway. The overall transport factor including all pathways has ranges characterized by maximum-to-minimum ratios (95% to 5%) of about 40. The parameter shown to have the greatest impact on the transport factor calculation was the fraction of elemental iodine released.

Air Pollutants, Radioactive↗

PubMed2002

Uranium, thorium, and potassium in soils along the shore of Lake Issyk-Kyol in the Kyrghyz Republic.

The Kyrghyz Republic, located in the southeastern region of the former Soviet Union, maintains a population of more than one-half-million persons and is heavily dependent on Lake Issyk-Kyol, both to draw tourists to the area and for its utilization by some as a food and recreation source. Historical surveys, conducted primarily for geological exploration, have indicated that localized areas of shoreline on Lake Issyk-Kyol have relative radiation levels in excess of ambient background by as much as a factor often. Uranium mining operations in the mountains bordering the lake to the south may have resulted in the contamination of a number of areas on the lake's southern shore. Concentrations of naturally occurring uranium, thorium, and potassium are present in these soils in elevated quantities. This paper presents the results of an investigation of soil concentrations along the shoreline of Lake Issyk-Kyol relative to previously discovered areas of high exposure rate.

Environmental Monitoring↗

PubMed2002

The Gaussian atmospheric transport model and its sensitivity to the joint frequency distribution and parametric variability.

Reconstructed meteorological data are often used in some form of long-term wind trajectory models for estimating the historical impacts of atmospheric emissions. Meteorological data for the straight-line Gaussian plume model are put into a joint frequency distribution, a three-dimensional array describing atmospheric wind direction, speed, and stability. Methods using the Gaussian model and joint frequency distribution inputs provide reasonable estimates of downwind concentration and have been shown to be accurate to within a factor of four. We have used multiple joint frequency distributions and probabilistic techniques to assess the Gaussian plume model and determine concentration-estimate uncertainty and model sensitivity. We examine the straight-line Gaussian model while calculating both sector-averaged and annual-averaged relative concentrations at various downwind distances. The sector-average concentration model was found to be most sensitive to wind speed, followed by horizontal dispersion (sigmaZ), the importance of which increases as stability increases. The Gaussian model is not sensitive to stack height uncertainty. Precision of the frequency data appears to be most important to meteorological inputs when calculations are made for near-field receptors, increasing as stack height increases.

Air Pollutants, Radioactive↗

PubMed2002

Age-specific uncertainty in particulate deposition for 1 microm AMAD particles using the ICRP 66 lung model.

The ICRP 66 lung model may be used to determine age-specific dose estimates for members of the public via the inhalation pathway. A significant source of uncertainty in internal dosimetric modeling is due to particulate deposition in regions of the respiratory tract. Uncertainties in estimates of particulate deposition are present because of the inherent variability in the deposition model and its various input parameters. An improved understanding of the uncertainty in particulate deposition will further guide research efforts and improve our ability to quantify internal dose estimates. The ICRP 66 lung deposition model is most sensitive to breathing rate when 1 microm AMAD particles are inhaled by members of the public. Uncertainties in deposition fractions are shown to span an order of magnitude with their distributions varying by age and sex for a particular lung region. Age-specific uncertainties of deposition fraction demonstrate increased estimates of extrathoracic deposition in younger age groups due to the decreased size of the respiratory tract airways. This age-specific trend becomes more pronounced for very young children and infants. The largest fractional deposition occurs in the alveolar and extrathoracic regions, regardless of age and sex.

Adolescent↗

PubMed2001

Uncertainty in particulate deposition for 1 microm AMAD particles in an adult lung model.

The ICRP 66 lung model may be used to determine dose estimates for members of the public via the inhalation pathway. A significant source of uncertainty in internal dosimetric modelling is due to particulate deposition in regions of the respiratory tract. Uncertainties in estimates of particulate deposition are present because model input parameters have their own inherent variability. These sources of uncertainty need to be examined in an effort to understand better model processes and to estimate better the doses received by individuals exposed through the inhalation pathway. An improved understanding of the uncertainty in particulate deposition will further guide research efforts and improve our ability to quantify internal dose estimates. The ICRP 66 lung deposition model is most sensitive to breathing rate when 1 microm AMAD particles are inhaled by members of the public. Uncertainties in deposition fractions are shown to span an order of magnitude with their distributions varying by gender for a particular lung region. The largest fractional deposition occurs in the deep lung alveolar and extrathoracic regions.

Administration, Inhalation↗

PubMed2001

Analysis of an internal kinetic model for free and bound tritium.

Internal dosimetry models that currently drive regulatory compliance decisions assume that tritium retention kinetic behavior can be modeled by a single exponential function. This is contrary to the results of a number of modeling techniques, which indicate that while elemental tritium (HT) and tritiated water (HTO) are the most commonly released forms of tritium, organically-bound tritium (OBT) doses can be quite significant. In this paper, a unified two-compartment model of the retention kinetics of HTO and OBT is examined for the purpose of investigating the importance of metabolic routes not considered in the ICRP one- and two-exponent models; namely the transfer of tritium from the HTO compartment to the OBT compartment and vice versa. In particular, the effect of intake ratio is investigated, and a detailed analysis of dosimetric implications is performed. For typical combined intakes of HTO and OBT, the number of disintegrations from the two tritium forms can be roughly equal. This result, when combined with the suggested greater biological effectiveness of OBT, indicates effective doses will be greater than those obtained from a single exponential model. The results of this study corroborate previous findings using the two-compartment model for the cases of HTO-only and or OBT-only intakes and compare well with data taken from studies on animals and human subjects.

Adult↗

PubMed2000

Radiocesium discharges and subsequent environmental transport at the major US weapons production facilities.

Radiocesium is one of the more prevalent radionuclides in the environment as a result of weapons production-related atomic projects in the USA and the former Soviet Union. Radiocesium discharges during the 1950s account for a large fraction of the historical releases from US weapons production facilities. Releases of radiocesium to terrestrial and aquatic ecosystems during the early years of nuclear weapons production provided the opportunity to conduct multidisciplinary studies on the transport mechanisms of this potentially hazardous radionuclide. The major US Department of Energy facilities (Oak Ridge Reservation in Tennessee, Hanford Site near Richland, Washington, and Savannah River Site near Aiken, South Carolina, USA) are located in regions of the country that have different geographical characteristics. The facility siting provided diverse backgrounds for the development of an understanding of environmental factors contributing to the fate and transport of radiocesium. In this paper, we summarize the significant environmental releases of radiocesium in the early years of weapons production and then discuss the historically significant transport mechanisms for 137Cs at the three facilities that were part of the US nuclear weapons complex.

Animals↗

PubMed2000

Ingestion pathway model developed for use with an acute atmospheric dose model at the Savannah River Site.

AXAIRQ is a straight-line Gaussian plume dose model used for prospective accident assessment at the Savannah River Site and currently includes the following dose pathways: inhalation, ground shine, and plume shine. In the event of an accident, another possible pathway for dose would be through ingestion of locally produced contaminated foodstuffs. A model, AXINGST, has been developed that would incorporate this pathway. Currently available ingestion models were referenced as a basis for AXINGST. The model calculates an ingestion dose following an atmospheric release of radionuclides and includes site-specific variables where applicable. AXINGST estimates tritium vegetation concentrations that are within a factor of 20-40 of measured data during previous accidental releases at SRS.

Air Pollutants, Radioactive↗

PubMed2000

Environmental releases of radioactivity and the incidence of thyroid disease at the Ignalina Nuclear Power Plant.

The Ignalina Nuclear Power Plant consists of two Russian-made RBMK-1500 reactors. The plant uses Lake Druksiai as a natural reservoir for cooling water. Within the framework of the revised radiation dose limitation system, site-specific routine release conversion factors and maximum annual effective doses for the dominant radionuclides and pathways were evaluated for both atmospheric and aquatic releases. Using calculated release conversion factors, the locations of the highest predicted activity concentrations were determined for air and for the dilution zone of heated effluent water during the period 1984-1998. Committed effective doses for critical group members were less than 0.001 mSv for Ignalina Nuclear Power Plant airborne releases and less than 0.05 mSv for aquatic releases. These dose estimates are lower than the 1 mSv dose limit for the adjacent population. In the case of Ignalina Nuclear Power Plant, taking into account the uncertainties, a recommendation for the administrative dose constraint is 0.25 mSv y(-1). This dose level may scarcely affect human health. Interestingly, during screening for thyroid disorders, endocrinologists and pediatric-endocrinologists determined a dominance of abnormal thyroids (up to 60%) among school children in the vicinity of Ignalina NPP. The data on neonatal screening for congenital hypothyroidism and transient hyperthyrotropinemia, however, suggested a possibility that the majority of abnormal thyroid cases were related to stable iodine deficiency. Thus, the influence of Ignalina Nuclear Power Plant on thyroid disorders is highly conjectural and unlikely to be associated with the observed levels of childhood thyroid disease.

Adult↗

PubMed2000

In vivo biodistribution of 125IPIP and internal dosimetry of 123IPIP radioiodinated agents selective to the muscarinic acetylcholinergic receptor complex.

The development of new radioiodinated ligands for imaging the muscarinic acetylcholinergic complex (mAChR) using single photon emission computed tomography (SPECT) requires the evaluation of human organ doses prior to approval for human use. Animal biodistribution and excretion data were obtained and evaluated for IPIP, a new mAChR agent. Preliminary biodistribution studies were performed on four different stereoisomers of IPIP. A biokinetic model of the Z-(S)-IPIP stereoisomer was constructed for the rat and used to estimate the internal absorbed dose in humans based on an extrapolation of the rat model. The thyroid is the critical organ for this radiopharmaceutical, with an absorbed dose estimate of 2.4 mGy/MBq for both males and females, when labeled with 123I. Even when blocked, the thyroid is still the critical organ, yet with a 90% dose reduction. The heart and brain receive the next highest doses in both males and females. Effective dose estimates for the use of pure 123I-PIP in humans are 0.16 mSv/MBq for males and 0.14 mSv/MBq for females. The biodistribution studies of the Z-(S)-IPIP stereoisomer showed the most promise as a successful agent for imaging muscarinic receptor sites in the heart and brain. IPIP also demonstrated potential as a therapeutic radiopharmaceutical for some colon carcinomas where muscarinic receptor sites are expressed in the tumor cells. These results provide preliminary data for use of IPIP in clinical studies on humans.

Animals↗

PubMed1999

Uncertainty of the tritium dose conversion factor.

Environmental releases of tritium oxide at a number of Department of Energy nuclear weapons facilities contribute to a significant portion of environmental dose. Several conversion factors are utilized in the estimation of human impact from these releases, e.g., dispersion coefficients, consumption rates, uptake factors, transport factors, dose conversion factors, and risk coefficients. A probabilistic determination of the tritium dose conversion factor was generated in this work to assess the uncertainty of the internal dosimetry required to estimate dose equivalent given an intake of tritium oxide. The tritium dose conversion factor was found to vary by a factor of about 15 with a median value of 2.2 x 10(-11) Sv Bq(-1) when considering orthovoltage x rays as the standard for estimating the relative biological effectiveness of tritium. The median dose conversion factor increases by about 50%; however, when gamma radiation is considered as the standard. The current deterministic estimate of the tritium dose conversion factor published by the DOE and the EPA is 1.7 x 10(-11) Sv Bq(-1), 25-50% lower than the median probabilistic values. The tritium oxide dose conversion factor model was found to be most sensitive to biological half-life and quality factor and is highly dependent on the standardizing radiation for RBE assessments.

Adult↗

PubMed1999

A screening assessment of external radiation levels on the shore of Lake Issyk-Kyol in the Kyrghyz Republic.

The Kyrghyz Republic, located in the southeastern region of the former Soviet Union, holds a long history of atomic weapons development activities. Historical surveys, conducted primarily for geological exploration, have indicated that areas of shoreline on Lake Issyk-Kyol in the Kyrghyz Republic have relative radiation levels in excess of background by as much as a factor of ten. Nuclear testing in China and uranium mining operations in the mountains surrounding the lake may have resulted in the contamination of a number of areas on the lake's southern shore. The valley region maintains a population of more than one-half-million persons and is heavily dependent on the lake to draw tourists to the area and its utilization by some as a food and recreation source. In this note, we show the results of a screening assessment of relative radiation levels along the shoreline of Lake Issyk-Kyol to pin-point areas of relatively high exposure rates.

Developing Countries↗

PubMed1998

Three-parameter model for estimating atmospheric tritium dose at the Savannah River Site.

The models used in the NRC approach to assess chronic atmospheric releases of radioactivity generate deterministic dose estimates by using standard assumptions about exposure conditions and environmental transport mechanisms. This approach has been used at the Savannah River Site since 1983. Total dose to off-site maximally exposed individuals at the SRS from atmospheric releases has been on the order of 1 microSv y(-1), three orders of magnitude lower than the applicable dose limit. When estimating atmospheric dose many parameters remain unchanged each time calculations are performed. These parameters, therefore, are essentially unimportant with regard to routine modeling. It is proposed, therefore, that transport and dosimetry models can be reduced to simple functions of a few parameters that essentially determine dose at all locations across the site. The three-parameter transport and dosimetry model developed in this work is useful for quick and easy estimates of chronic atmospheric tritium dose that are within a factor of 2 of estimates by more sophisticated models. The three parameters critical to estimating annual average concentration at the Savannah River Site are wind-direction frequency, downwind distance, and physical stack height. The model is bounded by physical stack heights between 10 and 61 m and downwind distances between 800 m (0.5 mi.) and 32 km (20 mi.) and should not be used outside its intended domain. It requires knowledge of wind-direction frequency, downwind distance, and physical stack height to estimate an Atmospheric Dose Factor (ADF; in units of microSv GBq(-1)) for the conversion of long-term release activity to maximum individual effective dose equivalent. This concept is being carried forward to the development of a reduced model for particulate emissions from SRS stacks.

Air Pollutants, Radioactive↗

PubMed1997

Predicted versus measured tritium oxide concentrations at the Savannah River Site.

Measured tritium oxide concentrations in air at various offsite locations are compared with concentrations predicted by three computer codes that are utilized at the Savannah River Site to estimate doses to maximally exposed offsite individuals. Annual average concentrations calculated by the computer models were compared with measured average concentrations taken from monitoring data collected over the last 10 y. The computer programs used for the comparison are AXAIRQ, MAXIGASP, and CAP88. The 10-y averaged ratios of predicted-to-measured tritium oxide air concentrations using AXAIRQ, MAXIGASP, and CAP88 are 1.89+/-0.56, 1.70+/-0.48, and 1.40+/-0.39, respectively. The difference in ratios is primarily due to different wind speed averages used within each of the models. These results show exceptional agreement, considering Gaussian plume models typically over predict annual average air concentrations by a factor of two to four.

Air Pollutants, Radioactive↗

PubMed1997

Fundamentals for establishing a risk communication program.

This paper provides a suggested outline for developing a risk communication organizational plan that could be used by a variety of federal, state, or private agencies. Drawing on various techniques presented in the literature and on the authors' insights, suggestions are provided as to how to formulate and convey risk messages. First, the paper provides a few risk communication fundamentals including definitions, the goal of informing vs. influencing, the importance of public participation in risk management, building trust and credibility, the consideration of outrage, and the importance of oral and visual communications. Second, a stepwise approach synthesized by the authors is presented that can be applied in developing a risk communication program. The approach is a 13-step method based on the premise that the risk communication program should be dynamic, flexible, and involve interaction with the public at every possible step.

Communication↗