TAB O -- DU Dose and Risk Estimates for the Gulf War Theater, 1990-1991

The Gulf War exposed personnel (Table 1 on page 8) to DU in several ways. US tanks firing DU sabot rounds mistakenly destroyed or damaged some US combat vehicles. Personnel worked inside US vehicles contaminated with DU fragments and particles. Several accidental tank fires and an ammunition explosion and fire at Camp Doha, Kuwait in July 1991 burned, oxidized, or fragmented DU rounds, which created potential exposures to personnel operating in the vicinity. Other US personnel entered Iraqi armor disabled by DU rounds or passed by enemy vehicles set on fire by DU rounds. Determining the medical consequences of these exposures requires a systematic, scientifically sound evaluation using recognized principles and methods.

The Office of the Special Assistant for Gulf War Illnesses (OSAGWI) recognized that veterans would want to know more about their possible exposures, doses, and their related health effects. OSAGWI tasked the US Army Center for Health Promotion and Preventive Medicine (USACHPPM) to perform exposure, dose, and risk estimates for the 13 exposure categories in the 3 exposure levels identified in Section IV of this report. This appendix summarizes OSAGWI and USACHPPM efforts to estimate and report intakes and radiation doses from DU and to characterize their risks to health.

A.  General Considerations on Assessment Methods

Chemical intakes and radiation doses from materials that enter the body usually cannot be measured directly. Bioassay methods (e.g., measuring uranium in urine or in the air breathed in) provide a reasonable estimate of the amount of material taken into the body. Except for seven soldiers involved in the April 13, 1991 tank fire incident, medical personnel did not evaluate veterans for DU until 1993. So, the chemical intake, radiation dose, and risk assessments for most Gulf War participants relied on estimates produced with established models using data measured under conditions similar to those in the Gulf War. Once the quantity of material taken into the body (called intake) is estimated, concentrations in body organs and radiation doses can be combined with existing knowledge about the toxicity of the material to form some reasonable conclusions about possible health effects.

The accepted method for estimating dose and risk for Gulf War participants involves:

1.  Exposure Scenarios

As the first step in assessing the health risks from DU, OSAGWI identified the potential exposures that took place for people in Levels I, II and III. This required a thorough, focused investigation that relied on hundreds of eyewitness interviews and thousands of pages of official and unofficial documents, records, reports, memos, personal diaries, and personal photographs to determine the essential facts of each event. As information on the exposure scenarios developed, OSAGWI provided specific details to USACHPPM. The specific discussions on Level I, II, and III assessments contain additional details of the exposure circumstances, activities performed, identity of the groups exposed, and details of events.

2.  DU Exposure Source Terms

For the Gulf War veterans, the most common sources of DU exposure included:

During the Gulf War, measurements of the DU levels in the air, inside vehicles, or on the ground were not performed, were lost, or had limited utility for exposure assessment. Consequently, USACHPPM evaluated data on air concentrations and residual DU contamination levels obtained during DU testing and development to determine whether the data were relevant and useful for performing intake and dose estimates.[555,556] In its August 1998 interim report, USACHPPM concluded that the available data, although limited, were sufficient for estimating dose and risk and assessing whether further medical evaluations were needed. However, USACHPPM also identified data gaps that should be filled if better estimates are needed for future operations involving DU munitions. In a September 2000 revised report, USACHPPM reported further evaluations of the data that were applied to intake and radiation dose estimates for Levels I, II and III. Additional details about the data used for estimating the intakes and doses are discussed in the individual Level I, II, and III sections below.

3.  Routes of Entry

The common routes of entry for DU into the body of participants in the various Gulf War scenarios were inhalation, ingestion, and wound contamination. Wounding by DU fragments represents a fourth route of entry that is not evaluated in this report because soldiers in that category are participating in the Baltimore Veterans Affairs (VA) DU program. Inhalation exposure produced the most concern in each of the Gulf War scenarios because of the expected high DU concentrations in air and the higher possible uptake from the respiratory tract. By comparison, the expected lower DU absorption from ingestion or wound contamination would cause less concern. Inhalation was of particular concern for Level I participants because they could be exposed to the higher air concentrations from the burning DU inside vehicles penetrated by DU rounds.

4.  DU Behavior in the Body

Inhaled DU enters the body through the nose and mouth. Parts of the initial amount are exhaled, remain in the lungs for later absorption into the blood, or transfer to the intestines. Intake is the amount of DU that gets into the body. For inhalation, intake is the volume of air inhaled during a period of time, combined with the concentration of DU in the air. Depending on the particle sizes, DU particles will remain in the lung, will be exhaled, or will be trapped in the mucus of the upper airway, swallowed, and cleared through the gastrointestinal tract. Ingested DU enters the body through the mouth, is swallowed, and is either absorbed into the blood or excreted through the gastrointestinal tract. DU absorbed in the blood is distributed to other organs or excreted through the kidneys in urine. Figure O-1 provides a schematic view of intake, uptake, and excretion.

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Figure O-1.  Schematic representation of intake, uptake and excretion[557]

Uptake is the amount of DU absorbed into the blood. This amount depends on two main factors -- the solubility of the DU compound in bodily fluids, particularly lung and intestinal fluids; and on the permeability of the pulmonary and gastrointestinal membranes for uranium ions and molecules. Once absorbed into the blood, DU's behavior depends on complex chemical and physiological factors. Understanding these behaviors provides a methodology for assessing intakes, organ concentrations, radiation doses, and urine and fecal excretion.

The International Commission on Radiological Protection (ICRP) developed models for the physiological behavior of materials in humans for use in radiation safety. The ICRP developed and over time improved models for the respiratory tract, gastrointestinal tract, biokinetic behavior of absorbed material, and retention and excretion. These models are currently available:

USACHPPM evaluated software for performing these calculations and selected the Lung Dose Evaluation Program (LUDEP) Version 2.06, discussed below. Although LUDEP 2.06 uses the previous ICRP-30 biokinetic and excretion models, USACHPPM concluded that the program could produce reasonable estimates of intake and radiation dose. This section summarizes the models' features, and the parameters required for obtaining reasonable estimates. These models include:

USACHPPM's report[558] contains detailed discussions about these models' features and their application to estimating intake, radiation dose, kidney concentration, and urine concentration. The sections below summarize the major assumptions and data used for inhalation exposures, ingestion exposures, and wound contamination.

a.  Models for Evaluating Inhalation Exposures

1.  Deposition in Respiratory Tract and CEDE

ICRP-66, published in 1994, updates the ICRP-30 human respiratory tract model and identifies the anatomical regions of the respiratory tract for dosimetric purposes. This respiratory tract model better details the anatomy and physiology of the respiratory tract than the ICRP-30 model it replaces. The ICRP-66 also uses absorption rate Types F (fast), M (medium), and S (slow), which are related to the ICRP-30 inhalation clearance Classes D (days), W (weeks), and Y (years), respectively. In contrast to the ICRP-30 respiratory tract clearance Classes D, W, or Y, which are respiratory tract parameters defining the overall respiratory clearance, the ICRP-66 absorption Types F, M, and S parameters relate only to absorption into body fluids. Lung retention calculated using the Type M and Type S parameters is greater than for Class W and Class Y, respectively.

USACHPPM calculated Dose Conversion Factors (DCFs), which convert intake into committed effective dose equivalent (CEDE) for different absorption types based on the fractional mass amount of DU, that is U234, U235, U236, and U238, for a 1 mg intake of a 5 m activity median aerodynamic diameter (AMAD) aerosol inhaled by a mouth breather having a breathing rate of 3 m3/hr (or 50 L/min), simulating "heavy activity," which produces a conservative intake estimate.[559] USACHPPM compared those DCFs with others calculated using a 1 m AMAD aerosol, persons breathing through the nose, and with breathing rates of 1.2 m3/hr, and 1.688 m3/hr in addition to 3 m3/hr. Those comparisons showed that the selected DCFs produced the most conservative estimates for Gulf War conditions.[560] The inhalation DCFs used for the dose calculations are 0.000852 rem/mg (Type F), 0.0125 rem/mg (Type M), and 0.0218 rem/mg (Type S).[561]

The DCFs apply to the inhalation of forms of DU that are entirely one absorption type. The DU oxides encountered during the Gulf War were generally mixtures of relatively soluble (Type F or M) and insoluble (Type S) materials. USACHPPM calculated the estimated intakes and doses by combining the corresponding fraction of each type of material. For example, the DU oxides from the impact of DU munitions were assumed to consist of 17% Type M and 83% Type S material as determined from test data. Therefore, the dose was calculated using 17% of the Type M DCF and 83% of the Type S DCF.

2.  Transfer to the Kidney

The quantity of uranium or DU present in the kidney is of interest because of chemical toxicity considerations. USACHPPM coupled the ICRP-30 respiratory tract model with the ICRP-30 kidney retention function to derive kidney intake retention functions.[562] The parameters that control the ICRP-30 kidney retention model for uranium are two components representing uptake from the blood: one component has a fractional uptake of about 0.12 with a biological half-life of 6 days, and the other has a fractional uptake of about 0.00052 with a biological half-life of 1,500 days. Uranium transfers from the kidney (renal tubules) to the urinary bladder with a half-life of 7 days.

The amount of DU reaching the kidney is related to the AMAD of the aerosol inhaled and DU's solubility. The fractions that reach the kidney will do so over varying time intervals. Smaller, more transportable materials will enter the blood faster, while larger, less transportable materials will enter the blood more slowly. USACHPPM assumed all the DU reaches the kidney at the same time. Consequently, this approach estimates a higher peak concentration in the kidney than would be expected and thus produces a more conservative estimate of possible health effect. The fraction of DU reaching the kidney for a particle size of 5 m AMAD for each respiratory clearance class is 0.0642 (Class D), 0.0174 (Class W), and 0.0034 (Class Y).[563] The uranium concentration in the kidney is calculated by combining the contributions from the DU form inhaled (e.g. 17% Class W, 83% Class Y).

b.  Models for Evaluating Ingestion (Hand-to-Mouth) Exposures

Gulf War Level I, II and III participants could have ingested DU while inside US or enemy tanks penetrated by DU munitions. Once the DU reaches the mouth, a gastrointestinal (GI) transfer coefficient for uranium or DU is employed to calculate the uptake to blood. The GI transfer coefficient for soluble uranium or DU of Class D and W (or Type F and M) is 2 percent. For insoluble Class Y (or Type S), the GI transfer coefficient is 0.20 percent.[564] The kidney concentration is calculated using the assumption that all the absorbed DU passes through the kidney at once. According to the International Atomic Energy Agency (IAEA) Safety Series 115, the ingestion dose conversion factors (DCFs) for uranium are 1.61 x 10-5 rem/mg (insoluble) and 1.24 x 10-4 rem/mg (soluble).[565]

c.  Models for Evaluating Wound Contamination

Some Level I Gulf War Veterans involved in DU friendly fire incidents had wounds contaminated with DU and retained embedded fragments.[566] The dose assessment models required to estimate the radiation doses and chemical intakes from these exposures are not available at this time. The National Council on Radiation Protection and Measurements is in the process of developing the models required for this assessment. Therefore, USACHPPM did not estimate intakes and doses from possible wound contamination.

VA monitoring of a subset of those veterans with embedded fragments provides information that reported constant, elevated urine DU concentrations throughout the monitoring period with the highest on the order of 31 g of uranium per gram of creatinine.[567] These studies also estimated the radiation dose from the uranium measurements at 0.01 rem to 0.1 rem per year from this constant excretion. If those doses were entirely due to wound contamination, which they are not, the increased dose would equal the 0.1 rem in a year guideline for members of the public.

5.  Methods for Estimating Dose

Estimates of dose begin with an estimate of intake. Once the intake is established, then the distribution of DU throughout the body and the concentrations of DU in various organs must be calculated using models of the bio-kinetic behavior of DU in the body. The process and methods used to estimate both chemical and radiological doses are essentially the same. Radiation dose estimates require additional calculations to convert organ concentrations into the 50-year whole body committed effective dose equivalent (CEDE) -- the standard quantity for reporting results in this report. Committed dose equivalents (CDE) to individual organs may also be calculated, but the CEDE provides the primary value for assessing the risk of radiation effects from an exposure.

This office recognizes that reporting radiation quantities in SI units (grays, sieverts, and becquerels) is becoming standard practice. However, it also understands that veterans of the Gulf War, who form a major part of the audience for this document, may be more familiar with the traditional units (rads, rem, and curies). Therefore, the traditional units will be used in presenting the results of these evaluations. Health and safety professionals can easily make the conversions to SI units if desired.

USACHPPM used accepted internal dosimetry models to convert intakes into radiation dose -- the primary one being the Lung Dose Evaluation Program (LUDEP 2.06), developed by the National Radiological Protection Board in the United Kingdom. LUDEP uses the most current model of the human respiratory tract from the International Commission on Radiological Protection (ICRP).[568] Although it uses the former ICRP-30 gastrointestinal tract model and systemic model, USACHPPM's analysis concluded that the doses calculated with LUDEP agree to within one percent with the doses calculated using the current models. In addition to testing by the ICRP, LUDEP has also been tested by several national laboratories and consensus groups on radiation safety. USACHPPM compared the results from LUDEP with the results from other models and manual calculations to verify that it obtained credible results.

Input parameter selection for the calculations affects the results. Particularly critical are the breathing rate, the particle size distribution of DU aerosols, and the solubility class of the DU forms for inhalation and ingestion. Details of those parameters are provided in the sections that discuss the specific exposure levels.

6.  Risk Estimates

Risk is a measure of the chance that an undesirable event or effect may actually happen. For example, a risk of 1x10-7 means that there is one chance in ten million of the event happening. As applied to radiation risk, this means that if 10,000,000 persons received a certain radiation dose, then we would expect that one person may show the harmful effect. The estimated risk from low doses of radiation, where no harmful effects have actually been seen, is extrapolated from high radiation doses, where the effects have been seen. In making this extrapolation, it is assumed that the chance of injury from the low dose is proportional to the chance of injury from the high radiation dose.

Once the intakes, radiation doses, and kidney concentrations are estimated, risk estimates can be produced that communicate the likely outcomes from the doses. Discussing the likely outcomes generally involves some comparison with consensus or regulatory guidelines (i.e., the standardized limits), a consideration of current scientific and medical knowledge, and/or observations of the health of the group being evaluated.

Section III covered a number of regulatory limits and guidelines for radiation including a limit for the public (0.1 rem in a year), a limit for workers (5 rem in a year), and guidelines for a lifetime occupational dose (50 rem). Guidelines for chemical toxicity include the Occupational Safety and Health Administration's (OSHA) permissible exposure levels and the American Conference on Governmental Industrial Hygienists (ACGIH) threshold limit values and short-term exposure limits for limiting inhaled DU. Guidelines for ingested DU at any level and inhaled DU above OSHA and ACGIH limits are sparse. Some scientists have suggested that acute intakes should not exceed 8 milligrams of inhaled soluble uranium to avoid temporary kidney effects, and 40 milligrams of inhaled soluble uranium to avoid permanent kidney effects.[569] The Nuclear Regulatory Commission (NRC) regulations limit inhaled soluble uranium intakes to 10 milligrams in a week for routine operations in an industrial or laboratory setting.[570] The NRC also recommends a limit of 30 milligrams of inhaled soluble uranium in single emergency events at gaseous diffusion plants (where uranium is enriched).[571] A maximum permissible organ concentration (MPOC) of 3 micrograms of uranium per gram of kidney tissue was selected as the basis for assessing kidney effects for this study.

These limits and guidelines are, of course, not absolutes: the 0.1 rem limit for the public doesn't mean that 0.09 rem is good and 0.101 rem is bad. The limits and guidelines are only meant to provide practical, scientifically acceptable limits for risk management and regulation. It should also be remembered that if a certain dose has never before been observed to cause effects in humans, then it is unlikely that this dose will cause observable effects in humans in the future. In addition, the reliability of predicted dose and risk estimates can be tested by comparing them with the dose estimates, risk estimates, and medical conclusions from studies like the Baltimore VA medical follow-up program.

USACHPPM's chemical and radiological dose and risk estimates and the consequent assessments are summarized in Sections IV.B through IV.D below.

7.  Uncertainty and Sensitivity Analysis

In any complex evaluation based on scientific principles and test data, the parameters used to estimate quantities such as intake, DU concentration in the kidneys, and radiation dose may have variations from measurement, from generalizations in developing models based on many, diverse cases, or from missing data that require assumptions to complete the analysis. For the three Gulf War scenarios, all these sources of variation are present. The most prevalent source of these variations arises from any attempt to use the estimates to represent the exposure of any individual. Exposure times, exposure conditions, and individual characteristics were simply not available. Therefore, the estimates provided in this report should only be used as indicators of the range of possible exposures that Gulf War participants experienced.

In addition to the variations mentioned, USACHPPM's use of test data introduced possible variations because the data may have been measured under conditions that were not exactly similar to those in the Gulf. Furthermore, the measured data, as with any measurements, have some amount of possible error. Also, the various models used to estimate intake to the body, transfer to the kidney, excretion in urine, and conversion to radiation dose all contain variations.

For Level I, USACHPPM applied statistical techniques to assess the variations caused by the various parameters used. These techniques provide a scientifically sound assessment of the possible variation in the estimates, as well as information on the parameters that influence results the most. For Level II and III, the uncertainty and sensitivity are based on qualitative assessments of the various assumptions used to prepare the estimates. These methods and results are discussed in the sections below.

B.  Level I Exposures (Friendly Fire)

Friendly-fire incidents in which US tanks fired DU rounds at occupied US vehicles during the Gulf War resulted in the damage or destruction of 6 Abrams tanks and 14 Bradley Fighting Vehicles (BFVs). These incidents were distinct from non-DU friendly-fire incidents or cases where friendly vehicles were evacuated and then struck by DU rounds (sometimes deliberately, to prevent their capture). The featureless desert terrain; large, fast-moving formations; and low visibility (from darkness, heavy rains, sandstorms, etc.) were contributing factors in all these incidents.[572]

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Figure O-2.  M1A1 lost to friendly fire

Level I soldiers were in or around combat vehicles at the time they were struck by DU rounds, or immediately afterward. These soldiers were exposed to DU by inhaling DU aerosols generated by fires or generated by the impact of the DU projectile penetrating the target. Some of them entered vehicles immediately after the attacks to render aid or to rescue fellow soldiers.

1.  Exposure Scenario Details

The Abrams tank crewmen and the Bradley "dismount" infantry manning the vehicles struck by DU reacted to the friendly fire in a variety of ways. These veterans usually attempted to immediately exit the vehicle. Sometimes doors jammed, or key crewmen assigned to open doors were incapacitated, delaying escape. Some veterans delayed their own escape to assist fellow crewmen injured in the assault. Usually, soldiers in the same platoon or company witnessed the friendly fire on their comrades. Typically these soldiers would rush to the aid of the stricken vehicle's occupants to perform emergency first aid and rescue operations -- often entering the damaged or destroyed vehicles moments after they had been hit. In the process, these personnel may have exposed themselves to DU residues or oxides still airborne from the impacts, or they may have inadvertently stirred up DU residues during rescue activities.

The occupants of the vehicles were exposed to flying DU fragments, airborne soluble and insoluble forms of DU, and DU residues, which could be inhaled, ingested, or could contaminate wounds. Personnel outside the vehicle could be exposed through inhalation, ingestion, or by wounds from contacting the outer surfaces of the vehicle. Soldiers entering the struck vehicles could inhale DU aerosols from the initial impact or from resuspended DU residues, ingest DU (through hand-to-mouth transfer), or have DU settle in breaks in their skin (burns, wounds, or scratches).

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Figure O-3.  Bradley Fighting Vehicle

A depleted uranium strike on the exterior of an Abrams differs from one on a Bradley. The thicker armor of the Abrams Heavy Armor Model -- reinforced at the front of the turret by DU panels inserted between regular steel armor -- offers much greater resistance to the impacting DU round than does the thinner, lighter- weight aluminum-alloy skin of the Bradley. Consequently, DU aerosolization and fragmentation at the point of penetration (and at the exit point) and in the interior of the Abrams are greater. By contrast, the Bradley is less vulnerable to interior contamination because DU penetrators typically punch right through the Bradley's relatively thin armor, producing little DU aerosol. During one incident, two DU rounds penetrated and flew through one Bradley and struck a second Bradley standing twenty feet away.[573] The likely exposures from a DU strike, therefore, can span a broad range of values.

2.  Source Terms

The primary routes of entry for Level I participants evaluated in this report are inhalation, ingestion, and wound contamination. Estimates of DU intake for each of these routes of entry are needed for these personnel. These estimates require the following information:

Air Concentrations

Since DU air concentrations were not measured during the Gulf War, USACPPM identified and reviewed technical data obtained during development and testing of DU munitions for possible use in performing these intake and dose estimates. For the Level I exposure scenarios, USACHPPM's review indicated no specific data on impact studies involving DU munitions and the BFV. They also concluded that the best data available to estimate the bounds of DU that may have been internalized during the Gulf War were contained in Technical Report BRL-TR-3068, Radiological Contamination from Impacted Abrams Heavy Armor (summarized in Tab L, Report #27).[574] That report describes a series of tests against a heavy armor Abrams tank, including tests involving impact of the DU armor plate by a 120-mm round and a tungsten round under conditions that would cause penetration of the armor plate and entry to the crew compartment. Personal air samplers, which sampled an average flow rate of 5 L/min, were placed inside the tank at locations, representing likely crew stations, during all tests to attempt measurements of the air concentration following impact.

The air concentration measurements during the DU penetration of the armor plate (Test 5A) suffered from several problems, including apparent air sampler shutdown almost immediately after impact, inconsistencies and errors in transcribing data from field notes to the report, and uncertainties in calculating the actual run-time of the samplers. USACHPPM and the report's authors collaborated to resolve those issues and to attempt to extract data that would support calculating a range of possible air concentrations. They based their approach on two assumptions.

USACHPPM recognized that the approach would not produce definitive estimates of the air concentrations because of the number of assumptions involved and their associated uncertainties. Therefore, they investigated a means of systematically dealing with these uncertainties. Two techniques were considered for dealing with the uncertainties in the data and the uncertainties for the key parameters required to estimate intake. The first technique considered, the best-estimate technique, was to develop a best estimate for each parameter and data point. This technique was used in the August 1998 Interim report. Estimating uncertainty is relatively simple as long as the data are robust and the numbers of parameters are small. When uncertainties exist in the data and a large number of parameters exist, estimating total uncertainty is difficult. Justification of the values selected becomes impossible if a key parameter may have two discrete, very different, and equally justifiable values. This technique becomes particularly problematic when attempting to establish upper and lower bounds for the estimated parameters, because there is no method for excluding estimates based upon an unrealistic set of parameters. Given this dilemma, USACHPPM did not use the best-estimate technique in their September 2000 report.

The second technique considered was to use a Monte Carlo simulation program that will generate a probabilistic distribution of air concentrations and intakes based upon a defined probability distribution of potential parameter values. This technique provides a scientifically defensible method for dealing with the uncertainty generated by each parameter. It also provides a justifiable method for establishing reasonable upper and lower bounds for intake.

For Assumption 1, sampler run-times were estimated based on the assumption that the removable contamination on the interior of the tank came primarily from the settling of the DU aerosols in the tank. Contamination levels determined from wipe tests performed during the Fliszar tests were used to estimate the total DU in the interior of tank and used to estimate the range of times the air samplers ran.[575] For Assumption 2, the distribution of possible sampler run-times were generated using the same process to estimate an air concentration from the amount of DU on the filter and the sampling flow rate. Those analyses produced estimates of the air concentration ranging from 468 mg/m3 to 8,690 mg/ m3 (median value of 2,410 mg/m3) for Assumption 1, and from 147 mg/ m3 to 530 mg/ m3 (median value of 267 mg/ m3) for Assumption 2.

Since the DU was produced by impact and penetration, the material was assumed to contain 17% of soluble DU oxide (Class W, Type M) and 83% of insoluble DU oxide (Class Y or Type S).[576]

Surface Contamination Levels

Removable surface-contamination data from Test 5A were used to estimate the hand-to-mouth transfer for secondary ingestion. The greatest removable radioactivity (4,815 dpm/100 cm2) found on the vehicle surface after Test 5A was converted to mass per unit area (mg/100 cm2). Since the DU was produced by impact and penetration, the material was assumed to contain 17% of soluble DU oxide (Class W, Type M) and 83% of insoluble DU oxide (Class Y or Type S).[577]

Resolution of Government Accounting Office Concerns

During the review of the 1998 OSAGWI DU Environmental Exposure Report, the GAO raised questions about the reliability of the data used in the USACHPPM evaluation for Level I. USACHPPM continued their reassessments in an attempt to obtain reasonable estimates. Those reassessments, reported in their September 2000 report, revealed additional inadequacies with the air sampling data. They applied various innovative approaches, including probabilistic (statistical modeling) techniques (discussed above) to estimate the range of possible air concentrations inside the tank.[578]

Although those efforts produced technically sound estimates, they also further emphasized the need for additional testing of the behavior of DU aerosols during impact of DU munitions with combat vehicles. Consequently, in October 1999, the Special Assistant directed that additional testing be conducted to bolster the validity of the health effects assessments. That program, funded in part by this office, conducted the first test firing in November 2000, and is expected to be completed in Fiscal Year 2002.

3.  Intake and Dose Estimates

USACHPPM used the source data discussed above to estimate the DU intakes, radiation doses and kidney concentrations for a DU sabot round penetrating the DU-protected section of an Abrams. Such DU-on-DU penetrations did not, however, occur during the Gulf War.[579] Nevertheless, the test data were judged to provide the basis for a reasonable upper limit for the exposure from one penetration. Since two Abrams tanks were hit more than once by DU rounds that penetrated non-DU sections of their armor, USACHPPM needed an approach for applying the estimates for one penetration to these actual events involving two penetrations. USACHPPM expected that a second shot would increase the air concentration and resulting DU intakes. However, they also realized that the increases were probably not a simple factor of 2. Therefore, USACHPPM reasoned that a second hit could possibly increase the air concentration by factors ranging from 1.5 to 3 times a single hit. They multiplied their estimates for a single DU round by 1.5 to 3.0 based on professional judgement. Those factors compensated for events that could lower (NBC system operating, hatches blown off, etc.) or increase (rounds did not exit, possible internal fires, etc.) the air concentration.[580]

a.  Inhalation Exposures

USACHPPM combined the air concentrations with the Dose Conversion Factors for inhalation and an estimated exposure time of two minutes inside the tank to calculate estimated DU intake, radiation dose (CEDE), and kidney concentration for the Level I participants. As mentioned above, the airborne DU was assumed to have a particle size distribution of 5 m AMAD and participants breathed through their mouths at a rate of 3 m3/hour, representing "heavy activity" and producing a maximum intake estimate. The DU material was assumed to consist of 17% moderately soluble (Type M) and 83% insoluble (Type S) forms. Their results represented by the median values obtained from the simulation are summarized in Table O-1.

Table O-1. Estimated intake, kidney concentration, and radiation dose for Level I

spacer

Intake (mg)

Radiation Dose
(CEDE) (rem)

Kidney Concentration
(g U/g kidney)

One Penetration

Assumption 1

79

1.6

1.46

Assumption 2

9

0.18

0.17

1Two Penetrations

Assumption 1

237

4.8

4.38

Assumption 2

27

0.54

0.51

Notes: 1. Value obtained by a multiplying one penetration result by 3

b.  Ingestion Exposures (Hand to Mouth)

In estimating intake from ingesting contamination on the interior surface of the vehicle, USACHPPM assumed:

The assumptions produced intake values of 12 to 24 milligrams, radiation dose (CEDE) of 0.0003 to 0.0006 rem, and kidney concentrations of 0.2 to 0.4 g of uranium per gram of kidney tissue from a single penetration. For two penetrations, the corresponding upper limit values would be 36 to 72 milligrams intake, radiation dose (CEDE) of 0.0009 to 0.0018 rem, and kidney concentrations of 0.6 to 1.2 g of uranium per gram of kidney tissue.[581]

4.  Risk Characterization

Comparing the kidney concentrations and radiation doses from inhalation and ingestion to the guidelines discussed in Section III of the main report reveals that only the value of 4.38 g of uranium per gram of kidney tissue obtained for two penetrations exceeds 3 g of uranium per gram of kidney tissue MPOC guideline. Furthermore, none of the radiation doses (CEDE) exceed the 5 rem guideline. Readers are reminded that these assessments for the two penetrations overestimate exposures that could be expected for Level I participants because:

USACHPPM's 1998 and 2000 Level I assessments provide tentative estimates of exposures to and intakes of DU since they are based on numerous assumptions to compensate for the limited test data. A better measure of potential Level I medical effects comes from a direct evaluation of the exposed Level I veterans, which is being done through the Baltimore VA Medical Follow-up Program discussed below.

Despite the concerns about possible temporary and permanent kidney damage resulting from the Level I potential exposures, to date the Baltimore VA medical monitoring indicates Gulf War veterans do not show adverse kidney effects such as would be expected from excessive exposures by inhalation, ingestion, embedded fragments, or wound contamination. In the Baltimore VA program, the observations of the veterans with no embedded DU fragments show no unusual uranium concentrations in urine. In soldiers with embedded fragments, the urine uranium concentrations are well above normal -- implying that the embedded DU fragments are a constant source of DU to the blood and other bodily organs.[582] The 1995 clinical evaluations found no adverse clinical effects from uranium exposure.[583] Additional studies through 1997 found that veterans with fragments continue to excrete elevated concentrations of uranium.[584] The VA reported committed effective dose equivalents to the whole body of about 0.01 to 0.1 rem per year in those veterans with embedded fragments.[585] If these doses remain constant over 50 years, the individual with the highest urine concentration would receive a cumulative 50-year dose of 5 rem, which is the annual limit for workers and is about equal to USACHPPM's 2000 estimated CEDE for Level I veterans who inhaled or ingested DU but retain no fragments in their bodies. While the VA noted some subtle variations in the latest medical examinations (which the VA will continue to monitor), the 1997 examinations did not attribute any adverse medical effects to the veterans' DU exposures, other than those arising from their traumatic injuries.

5.  Uncertainty and Sensitivity Analysis

The most significant source of uncertainty associated with the Level I estimates of DU intake, kidney concentration and radiation dose (CEDE) results from the collection time for air samplers in Test 5A of the 1989 test report. USACHPPM and the Army Research, Development, and Engineering Center (ARDEC) estimated lower and upper bound values for these parameters using Monte Carlo estimation techniques that produced a distribution of possible results based on two assumptions about the air sample data. Assumption 1 derived estimates of the air concentration from the DU contamination measurements in the Test 5A tank. Assumption 2 derived the air concentrations using the results of one air sample collected at the driver's location during the same test. The two simulations produced a range of values, rather than a single value.

Interpretation of the simulation results required use of the most meaningful values based on an understanding of statistical techniques related to central tendency. Central tendency can be measured using several techniques. One of the most useful measures is the arithmetic mean. The median or midpoint, another measure of central tendency, is the value where exactly half of the points in a probability distribution lie below the median value and half lie above the median value. The median value is generally a more robust measure of central tendency, because it is not sensitive to the shape of the "tail" of a probability distribution or to the occurrence of outliers in a data set, as is the arithmetic mean. The mode often referred to as the most likely value (MLV) is the value associated with the maximum of the probability density function. USACHPPM selected the median as the most meaningful value for the lower and upper bound estimates for Level I exposures.[586] The September 2000 USACHPPM Report contains additional discussions about these estimates.

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Figure O-4. Estimated intake by a Level I crew member

The range of results from each Assumption is represented by a distribution of possible values. As shown in Figure O-4 for the DU intake by a crew member, the distribution is not a "normal" distribution, but rather a skewed or unimodal distribution. More values occur at lower intakes than at higher intakes. This distribution is characterized by intakes of 11.7 milligrams (2.5 percentile), 79 mg (median or 50 percentile), 106 mg (mean), and 366 mg (97.5 percentile). In a sense, the distribution represents the risk, or probability, that an intake will occur. Kidney concentration and radiation dose (CEDE) results show similar distributions.

In addition to the uncertainties from the sampling time, other factors contribute additional uncertainty. These factors include:

These uncertainties reinforce the main point of this analysis -- that the estimates represent very rough order of magnitude values for the possible intakes and radiation doses that can only be improved by further testing. That testing is underway, with the first test firings conducted in November 2000. Furthermore, the best assessment of Level I participants' health comes from the VA's DU Follow-up Program, which to date has demonstrated no detectable adverse medical effects among Level I veterans attributable to DU's chemical or radiation toxicity.

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