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8
Evaluation of the Army’s Capstone Report
In this chapter, the committee evaluates the U.S. Army’s Capstone Report (Guilmette et al. 2005; Parkhurst et al. 2005, 2004a,b). First, the committee evaluates the Army’s estimates of exposure to depleted uranium (DU) in various scenarios. It then reviews potential health risks to exposed personnel in the context of the exposure estimates and the toxicologic and health information presented in the preceding chapters.
EXPOSURE ASSESSMENT
The Capstone Report estimated the exposure of military personnel to DU from “friendly-fire” incidents in the first Gulf War. The assessment used the results of the Capstone DU-aerosols study (Parkhurst et al. 2004a) and a series of exposure scenarios based on interviews with veterans of Operation Desert Storm, after-action reports, and discussions with other military experts (OSAGWI 1998, 2000; USACHPPM 2000). Three exposure groups were defined as follows:
Level I includes military personnel in, on, or near combat vehicles at the time of impact and perforation by DU munitions and personnel who entered vehicles immediately after they were struck (and perforated) by DU munitions. Those personnel could have been exposed to DU from fragments resulting from impact, inhalation of DU aerosols, ingestion of DU residues, or any combination thereof.
Level II includes military personnel and a small number of U.S. Department of Defense (DOD) civilian employees whose jobs required them to work in and around vehicles containing DU fragments and particles. They were not in vehicles at the time of impact and did not immediately enter a vehicle after it was struck. They performed a variety of tasks, such as battle-damage assessment, repairs, explosive-ordnance disposal, and intelligence-gathering.
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They typically entered vehicles well after the initial suspended aerosol had dissipated or settled on interior surfaces. They may have inhaled DU residues that were resuspended by their activities, ingested DU through hand-to-mouth transfer, or spread contamination on their clothing.
Level III is an “all others” group whose exposures were brief or incidental.
The Capstone program performed a series of experiments to provide information on the amounts and characteristics of aerosols generated in or near vehicles hit by DU munitions. The experiments (Parkhurst et al. 2004a,b) involved 12 firings of large-caliber (LC) DU cartridges into Abrams tanks and a Bradley fighting vehicle. Specifically, the scenarios involved firing into three types of stripped-down vehicles (with no operating ventilation systems): an Abrams tank with conventional armor, a Bradley fighting vehicle with conventional armor, and an Abrams tank with DU armor. In addition, one shot was fired into an operational Abrams tank with DU armor that had an operating ventilation system. Aerosols were sampled in the vehicles by using filter cassettes, eight-stage cascade impactors, a five-stage cyclone separator, and a moving filter sampler. Sampling outside the vehicles was accomplished with high-volume air samplers or cascade impactors, and wipe samples were collected to evaluate potential DU ingestion.
Aerosol samples were collected in the target vehicles as a function of elapsed time after the shot, and the samples were analyzed for uranium content, particle size distribution, and other chemical and physical characteristics. The resulting dataset formed the basis of estimates of the amount and characteristics of aerosols that might be inhaled by soldiers in vehicles struck by DU munitions. The total quantity of DU aerosol generated by impact with armor cannot be measured directly, because of losses of absorption into or spallation of the DU onto the target. Using aerosol data, the Capstone study estimated that a maximum of 7% of the LC-DU penetrator was aerosolized inside the heavily armored Abrams tank and a maximum of 1% in the lighter-armored Bradley vehicle.
DU intakes, chemical concentrations, and radiation doses to selected organs were calculated for each phase (vehicle type), shot, and sampling position for each scenario. The intakes were based on scenarios of human exposure (described below) that included exposure duration and breathing rates. The time histories of uranium concentration in key organs (including maximal concentrations in kidneys) and the resulting radiation doses were estimated with human biokinetic models developed by the International Commission on Radiological Protection (ICRP). Specifically, three models were integrated in the computer programming to mathematically describe the toxicokinetics of uranium: the human respiratory tract model, the gastrointestinal tract model, and the uranium systemic biokinetic model. The respiratory and gastrointestinal tract models are described below, and the uranium systemic biokinetic model is described in Chapter 2.
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The human respiratory tract model (ICRP 1994a, 1997) divides the respiratory tract into five distinct anatomic compartments: the anterior portion of the nose (anterior nasal passage); the back of the nose and mouth and the throat (posterior nasal and oral passages, pharynx, and larynx); the trachea, the split of the air pathway into the two lobes of the lung, and the first seven later divisions of the two pathways into the lung (trachea, main bronchi, and generations 2-8 bronchi); the next seven divisions (bronchioles and terminal bronchioles); and (5) the final divisions of the airway and the alveolar sacs where gas exchange with blood occurs (alveolar-interstitial region). Separate lymphatic tissues are included in the model. Physiologic data include breathing rate and the amount of air space in the lung that is typically used during breathing. The deposition portion of the model describes deposition of particles as a fraction of the intake in each of the five regions of the respiratory tract in terms of particle size, from 0.6 nm to 100 μm. Each region is modeled as a filter, and all five regions act as a series of filters for both the inhalation and exhalation phases of breathing. Deposition is not considered to depend on the element, radionuclide, or chemical form. Particulate material is removed from the respiratory tract by particle transport (mechanical clearance) and by absorption into blood, which act independently and competitively on the material in each deposition region. In the anterior nose, only mechanical clearance applies, and material is removed quickly out of the body through the front of the nose. In the other regions of the respiratory tract, particle transport includes clearance to the gastrointestinal tract and the lymphatic system. Concurrent with the mechanical clearance of particles is dissolution of the particles and absorption into blood. This mechanism depends on the physical and chemical forms of the deposited material, and the rates of dissolution are modeled to occur at the same rates in all respiratory tract regions. Dissolution is modeled by assuming that a fraction of the deposited material dissolves relatively rapidly and the rest more slowly. The Capstone Report used dissolution rates obtained from in vitro dissolution experiments to define specific dissolution rate constants and associated fractions for the mixtures of uranium forms encountered in the aerosols measured in the vehicle interior.
The gastrointestinal tract model (ICRP 1979) consists of four compartments that represent the stomach, small intestine, upper large intestine, and lower large intestine. Material enters the stomach and clears to the small intestine, from which a fraction is absorbed by blood and the remainder clears to the upper large intestine. The material in the upper large intestine clears to the lower large intestine, and material in the lower large intestine is excreted in feces. There is no feedback between the compartments. The absorbed fraction depends on the solubility of the material in gastrointestinal tract fluids and is generally related to the absorption type used in the respiratory tract model.
The biokinetic models used in the Capstone Report reflect scientific consensus based on years of studies of animals and on human data where possible. Capstone scientists programmed special applications of the models for the human respiratory tract, the gastrointestinal tract, and uranium systemic biokinetics. Best estimates of intakes, radiologic doses, and peak chemical concentra-
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tions were calculated. Uncertainty analyses were also performed. The committee found that the models used in the Capstone analyses provide adequate information to support the risk analysis.
Two principal types of uncertainty are associated with the calculation of organ doses and committed effective doses (lifetime radiation doses) from inhaled DU aerosols: uncertainty due to variability in the measurement data and uncertainty in the biokinetic and dosimetric models used to calculate doses as central estimators for the population. The uranium concentration values were derived from beta-radioactivity counting of cascade-impactor collection substrates. Uncertainty calculations in the Capstone Report considered counting statistics, uncertainty in regression-model parameter values, and uncertainty in the ingrowth correction applied to account for the state of disequilibrium of the beta-emitting progeny. The measurement uncertainties were evaluated in terms of the likelihood function by using Bayesian statistics. Posterior distributions were calculated, and then all the distributions for a particular vehicle type were summed. The summed distribution represents the probability distribution of dose when all the shots and positions making up the dataset for a particular phase are considered equally likely; in other words, it is the probability distribution when the type of vehicle and the intake scenario are known but the geometry of the shot or subject placement is not known. Uncertainty analyses showed that the resulting distributions could not be described by any standard distribution, so the results were reported as medians with 10th and 90th percentiles. The committee found that the uncertainty analyses were appropriately performed and well done.
Level I Exposures
The primary focus of modeling level I exposures was to provide estimates of DU inhalation exposure to personnel in an Abrams tank or a Bradley fighting vehicle. Modeling of the level I inhalation exposures in a vehicle is a matter of determining the aerosol source characteristics in the vehicle (DU concentration, particle size, and solubility), the timing, the duration of exposure, and the breathing rates associated with the physical activities being performed. Those factors influence the magnitude of the intake and the consequent doses. Five level I exposure scenarios were modeled in the Capstone Human Health Risk Assessment (Guilmette et al. 2005): four for crew members present in the vehicle at the time of perforation and one for first responders who entered the vehicle after perforation.
Four exposure times were selected: 1 min, 5 min, 1 h, and 2 h. The first two exposure times assumed that the crew members would be able to leave the vehicle readily. The second two assumed continued exposure in a still-functioning vehicle. Longer stay times would increase exposure, but the DU aerosol concentrations after 2 h are orders of magnitude smaller and therefore add little to the intake. The assumptions used in creating the four scenarios for personnel in a vehicle during vehicle perforation are presented in Table 8-1.
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TABLE 8-1 Capstone Summary of Level I Exposure Scenario Conditions
Scenario
Time of Exposure
Exposure Duration
Breathing Rate
Crew Inside Vehicle
A
From impact to exit 1 min after shot
1 min
3 m3/h
B
From impact to exit 5 min after shot
5 min
3 m3/h
C
From impact to exit up to 1 h after shot
1 h
3 m3/h for first 15 min, 1.5 m3/h thereafter
D
From impact to exit up to 2 h after shot
2 h
3 m3/h for first 15 min, 1.5 m3/h thereafter
First Responder
E
Entry 5 min after shot, exit 10 min later
10 min
3 m3/h
Source: Parkhurst et al. 2005. Reprinted with permission; copyright 2005, Battelle Press.
The key input for each of the scenarios is a description of the amount and characteristics of DU in the air, including total mass, particle size distribution, and solubility. For the present review, the committee performed an independent assessment of those characteristics.
Before the Capstone experiments, a number of experiments were performed with DU munitions to determine the aerosol characteristics of the dusts and fumes produced when a DU penetrator strikes a hard target (e.g., Glissmeyer and Mishima 1979; Jette et al. 1990; Parkhurst et al. 1995; Gilchrist et al. 1999). The committee used the earlier datasets to make its own estimates of exposure.
Finely divided uranium metal is reactive (pyrophoric), oxidizing to triuranium octaoxide in air. The chemical form of the pure uranium oxide is uranium trioxide when formed at 1 atm oxygen pressure and below 500°C; triuranium octaoxide is the stable phase when formed above 500°C. In low-oxygen environments, or as an intermediate, uranium dioxide is formed (Parkhurst et al. 1995). In outdoor tests, Glissmeyer and Mishima (1979) found that 105-mm penetrators striking metal targets produced uranium oxides as 75% triuranium octaoxide and 25% uranium dioxide in particles that had an aerodynamic equivalent diameter (AED) of about 2.5-3 μm and of which about 50% were in the respirable size range. Particles created with 105-mm rounds as measured by Gilchrist et al. (1999) had an AED of 2.3-5.8 μm. Jette et al. (1990) found that aerosols from 120-mm rounds had particles of which 91-96% were less than 1 μm in AED and from 105-mm rounds particles of which 61-89% were less than 10 μm in AED. Through statistical sampling of walls, floors, equipment, ducting, and filters, Sutter et al. (1985) were able to recover up to 97% of the uranium from projectiles fired in an indoor testing range as nonaerosol particles (for example, pieces) and particulate oxides.
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From those data and evaluation of the Capstone information, the committee developed a simplified description of the particulate material in the air immediately after a DU munition penetrates a target vehicle. It is assumed that about 50% of the dust generated by an impact is larger than respirable size. The remaining 50% is evenly distributed between “large” respirable particles (mean, 5 μm) and “small” respirable particles (mean, 1 μm). The material is assumed to be in a moderately soluble form (corresponding to ICRP solubility classification M [for moderate]; see Chapter 2).
The initial concentration of dust and fumes generated in the vehicle depends on the event. Approximations made after outdoor tests indicated peak concentrations greater than 106 μg/m3 (Glissmeyer and Mishima 1979); these tests are not directly applicable to concentrations inside vehicles. In the Capstone firing tests (Parkhurst et al. 2004a), the peak concentrations in the Abrams tank was around 107 μg/m3, which was used in the committee’s analysis as the starting point.
The Capstone studies evaluated air concentrations as functions of time after impact in two vehicle types: with and without functional ventilation systems. That provided a complex set of curves that the Capstone staff used to analyze potential impacts. A simplified theoretical approach was taken for the committee’s independent assessment. The reduction in particle concentration in the air will be a function of both the ventilation and the settling of the particles and can be described mathematically as
where C is the DU concentration in air as a function of time, V is the ventilation rate in air exchanges per hour, and s is the deposition rate constant. The deposition rate constant will be a function of the particle size; larger particles deposit more rapidly than smaller ones. The deposition rate may be approximated by using a particle-deposition velocity vd as
where h is the distance from the floor to the ceiling, approximated as 2 m. The three particle-size classes described above were assigned the following approximate values, which are commonly used in environmental assessments (Sehmel 1984):
Particle-Size Category
Deposition Velocity (m/s)
Very large (>10 μm)
0.1
Large (~5 μm)
0.01
Small (~1 μm)
0.001
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The ventilation rates for Abrams tanks and Bradley fighting vehicles are described in Parkhurst et al. (2004a). Even an “unventilated” vehicle will have some leakage, which results in loss of particles from the vehicle. The assumed rates of ventilation and leakage are as follows:
Vehicle
Unventilated (h−1)
Ventilated (h−1)
Abrams
4
47.2
Bradley
8
40
The removal of particles by ventilation and deposition will be countered to some extent by resuspension of deposited material in the air by the activities of the personnel in the vehicle. A lower limit of concentration is assumed to be 10 μg/m3 for unventilated vehicles and 1 μg/m3 for ventilated vehicles (see discussion below on resuspension).
With those assumptions, the time history of the air concentration in the vehicle after penetration by a DU munition can be estimated. The results are presented in Table 8-2 for the two vehicles and ventilation states. The results, shown graphically in Figure 8-1, compare favorably with both the early experiments (Gilchrist et al. 1999) and the Capstone measurements (Parkhurst et al. 2004a).
The curves were used to estimate the time-integrated air concentrations of DU particles in each size category (very large and nonrespirable, large respirable, small respirable) for the five exposure scenarios for the two types of vehicles. Those are presented in Table 8-3. The values are somewhat larger than those estimated on the basis of outdoor shot tests by Gilchrist et al. (1999), as would be expected.
The time-integrated air concentrations may be used with the breathing rates defined for the five Capstone level I exposure scenarios to estimate intakes. Those are shown in Table 8-4. The intakes estimated for the unventilated Abrams tank are very close to the Capstone estimates. The intakes for the ventilated Abrams tank are 2-10 times larger than the corresponding Capstone estimate. For the unventilated Bradley fighting vehicle, the independent estimates are about 3 times larger than the Capstone estimates. (However, the initial quantity of DU measured in the Capstone Report for Bradley vehicles was only about one-third the initial value assumed in these independent re-estimations.) A Bradley vehicle with an operational ventilation system was not available for the Capstone study. Intakes were also independently estimated by the Royal Society (2001); the upper-bound estimates were about 3 times higher than any of these. However, the approximations to the time-integrated air concentrations were grounded on less information.
To estimate radiation-dose equivalents, the dose-conversion factors for DU presented in Chapter 6 (Table 6-2) were used to calculate the effective doses. The committee’s estimated doses are generally within a factor of about 2 of the more sophisticated Capstone estimates, of which the results for vehicles
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with conventional armor are compared in Table 8-5. The only exception is the dose to the first responder, for which the committee’s estimates are somewhat lower than the Capstone values. Overall, there is excellent agreement between the estimates, and the committee’s independent assessment supports the validity of the Capstone results.
TABLE 8-2 Committee-Predicted Concentrations of DU in Air in Vehicles after Impact (mg/m3)
Time (min)
Abrams
Bradley
Unventilated
Ventilated
Unventilated
Ventilated
0
10,010
10,001
10,010
10,001
1
4,245
2,062
3,972
2,325
5
1,952
54
1,401
98
10
1,025
2
531
4
15
607
1
229
1
30
148
1
29
1
45
42
1
12
1
60
18
1
10
1
120
10
1
10
1
FIGURE 8-1 Committee-predicted mass concentrations of DU in air in vehicles after impact.
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TABLE 8-3 Committee’s Estimated Time-Integrated Concentrations of DU in Air for Various Conditions after Impact (mg-h/m3)
Start Time
Stop Time
Abrams
Bradley
Unventilated
Ventilated
Unventilated
Ventilated
Large Respirable Particles
A: Exit 1 min
0
1
39.6
25.9
38.5
27.2
B: Exit 5 min
0
5
99.1
38.6
88.9
43.2
C: Exit 60 min
0
60
115.7
38.8
98.9
43.5
D: Exit 120 min
0
120
115.7
38.8
98.9
43.5
E: First responder
5
15
18.7
0.3
11.9
0.5
Small Respirable Particles
A: Exit 1 min
0
1
45.0
29.0
43.8
30.5
Small Respirable Particles
B: Exit 5 min
0
5
171.5
50.7
148.6
58.5
C: Exit 60 min
0
60
437.6
51.6
262.4
60.4
D: Exit 120 min
0
120
438.9
51.6
262.4
60.4
E: First responder
5
15
173.8
1.7
99.6
2.7
TABLE 8-4 DU Intakes Independently Estimated by Committee for Five Capstone Level I Exposure Scenarios
Scenario
Abrams
Bradley
Unventilated
Ventilated
Unventilated
Ventilated
Total Intake (mg)
A: Exit 1 min
254
165
247
173
B: Exit 5 min
812
268
713
305
C: Exit 60 min
1 660
271
1,084
312
D: Exit 120 min
1 664
271
1,084
312
E: First responder
577
6
335
10
Delivered Dose (mg/kg)
A: Exit 1 min
3.6
2.4
3.5
2.5
B: Exit 5 min
11.6
3.8
10.2
4.4
C: Exit 60 min
23.7
3.9
15.5
4.5
D: Exit 120 min
23.8
3.9
15.5
4.5
E: First responder
8.2
0.1
4.8
0.1
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TABLE 8-5 Committee’s Estimates of Effective Lifetime Committed Radiation Dose Equivalents from DU in Air for Level I Exposure Scenarios [rem (Sv)] and Selected Capstone Results for Comparison
Scenario
ABRAMS
Unventilated
Ventilated
Committee
Capstone
Committee
Capstone
A: Exit 1 min
0.94
2.0
0.61
0.09
(0.0094)
(0.020)
(0.0061)
(0.0009)
B: Exit 5 min
3.1
3.7
1.0
0.44
(0.031)
(0.037)
(0.010)
(0.0044)
C: Exit 60 min
6.5
4.8
1.0
1.02
(0.065)
(0.048)
(0.010)
(0.0102)
D: Exit 120 min
6.5
5.0
1.0
1.20
(0.065)
(0.050)
(0.010)
(0.0120)
E: First responder
2.3
0.92
0.02
0.41
(0.023)
(0.0092)
(0.0002)
(0.0041)
BRADLEY
Unventilated
Ventilated
Scenario
Committee
Capstone
Committee
A: Exit 1 min
0.91
0.59
0.64
(0.0091)
(0.0059)
(0.0064)
B: Exit 5 min
2.7
1.7
1.1
(0.027)
(0.017)
(0.011)
C: Exit 60 min
4.2
2.1
1.2
(0.042)
(0.021)
(0.012)
D: Exit 120 min
4.2
2.4
1.2
(0.042)
(0.024)
(0.012)
E: First responder
1.3
0.89
0.04
(0.013)
(0.0089
(0.0004)
Assessment of Level II and Level III Exposures
Exposures to DU via inhalation were estimated from breathing-zone monitors of Capstone personnel (which measured both internal vehicle exposures and external exposures) and from area monitors that measure only exposures in vehicles. Exposure estimates in the Capstone Report are for unprotected personnel not using such equipment as respirators or gloves. The Capstone Report provides estimates of DU exposures via inhalation and hand-to-mouth ingestion for levels II and III unprotected personnel. Exposures are highest for levels II and III personnel involved in activities in perforated vehicles. Hence, durations of exposure in perforated vehicles particularly need to be monitored for these personnel.
The primary exposure pathways are the same for level II and level III personnel; however, the time spent by personnel in the vehicles is different. The physical activities performed in and around the vehicles may also be different for level II and level III personnel. Level II personnel spend more time in vehi-
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cles because their jobs require them to work in and around damaged vehicles to repair them, gather intelligence, assess battle damage, or dispose of explosive ordnance. Level III personnel enter damaged vehicles because of curiosity rather than mission requirements. Therefore, because the exposures are not better defined, Capstone and this independent evaluation looked at the rates of exposure.
Evaluation of Resuspension
The Capstone measurements included samplers running in the vehicles for 2-3 h after initiation of the test. As described in Szrom et al. (2004), during these periods personnel were actively taking pictures and retrieving samples in the vehicles. Resuspension arrays (PI-6-L and PI-7-L) were running during this period. With the methods described in Szrom et al., it was determined for this analysis that resuspended concentrations of DU in air ranged from about 4 to 25 mg/m3 during periods of personnel activity in the vehicles. The resuspended material had a distribution of particle sizes somewhat different from that of the original material: there was a much lower concentration of the very large (non-respirable) particles. Reanalysis of the data indicated that about an hour after impact, the resuspended particles were primarily in the respirable range. On the basis of the results of resuspension array PI-7-L, it is estimated that perhaps 16% of the resuspended DU was in the roughly 5-μm fraction and about 83% in the 1-μm fraction. Therefore, it is assumed that the concentration of resuspended respirable particles averaged about 10 mg/m3: 1.6 mg/m3 at 5 μm and 8.4 mg/m3 at 1 μm. This value was also used as a minimum for the level I inhalation exposures described above.
For military personnel involved with postbattlefield conditions, a breathing rate associated with moderate levels of activity is assumed to be 1.2 m3/h. As a result, DU would be inhaled at about 12 mg/h. That corresponds with a radiation effective dose of about 0.052 rem/h of exposure, which corresponds with values estimated by Capstone (Szrom et al. 2004) of about 14.5 mg/h and 0.078 rem/h. The agreement between the Capstone and committee estimates is reasonable.
It should be noted that both these and the Capstone estimates are based on entry into a contaminated vehicle that has not been cleaned. Neither assumes any sort of respiratory protection. Although the dose rates are not extreme, they indicate that training in ALARA (as low as reasonably achievable) techniques and some degree of respiratory protection would be appropriate for personnel working in the vehicles.
Evaluation of Incidental Ingestion
For comparability with the above inhalation estimates, it is assumed that the initial dust loading in the vehicles is about 10,000 μg/m3. The vehicle interior dimensions are about 3 m × 2 m × 2 m (Royal Society 2001), for a volume
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Dermal (burn)
Nitrate
Man
10
3
++
Albuminuria
Butterworth 1955
Level I: Bradley vehicle; conventional armor, no ventilation; crew exits in 5 min
220
2.9
REG 1
Not likely to become illc
Guilmette et al. 2005
Level I: Abrams tank; DU armor, no ventilation; crew exits in 5 min
710
2.6
REG 1
Not likely to become illc
Guilmette et al. 2005
Inhalation
Hexafluoride
Man
24
2.5
+
Transient proteinuria and glucosuria
Fisher et al. 1990
Injection
Nitrate
Two men
5
1.8
—
No abnormalities
Luessenhop et al. 1958
1.4
—
Inhalation
Hexafluoride
Three men
40-50
4
+
Albumin and casts in urine
Kathren and Moore 1986
4
+
1.2
+
Inhalation
Hexafluoride
11 men
6-18
0.05-1.9
+
Transient proteinuria
Fisher et al. 1990
Inhalation
Hexafluoride
19 men
6-18
0.05-1.9
—
No abnormalities
Fisher et al. 1990
Level I: Abrams tank; DU armor, no ventilation; first responders
160
1.5
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Bradley vehicle; conventional armor, no ventilation; first responders
99
1.4
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Abrams tank; DU armor, no ventilation; crew exits in 1 min
250
1.1
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Bradley vehicle; conventional armor, no ventilation; crew exits in 1 min
83
1.0
REG 0
No detectable effectsd
Guilmette et al. 2005
Ingestion
Nitrate
Man
470
1
+
Transient albuminuria
Butterworth 1955
Inhalation
Hexa-fluoride
Man
20
1
—
No abnormalities
Boback 1975
Occupational (<10 to >20 y)
Yellowcake
39 workers
ND
1
++
Mild increase in aminoaciduria, ß2m
Thun et al. 1985
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Route of Exposure
Chemical Form
Subjects
Uranium Intake (mg)
Peak Renal Uranium (μg/g)
Renal Effectsa
Outcome
Reference
Embedded, inhalation (6-10 y)
Metal and oxides
16 soldiers
ND
1
++
Increased retinol binding protein excretion
Squibb et al. 2005
Level I: Abrams tank; DU armor, no ventilation; first responders
200
0.67
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Abrams tank; DU armor, ventilation operating; crew exits in 2 h
110
0.56
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Abrams tank; DU armor, ventilation operating; crew exits in 1 h
91
0.46
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Abrams tank; DU armor, ventilation operating; crew exits in 5 min
43
0.23
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Abrams tank; DU armor, no ventilation; first responders
27
0.14
REG 0
No detectable effectsd
Guilmette et al. 2005
Level I: Abrams tank; DU armor, ventilation operating; crew exits in 1 min
10
0.05
REG 0
No detectable effectsd
Guilmette et al. 2005
aBiochemical indicators of renal dysfunction: +++ = severe with clinical symptoms; ++ = protracted; + = transient; — =, no detectable effects.
bCommittee interprets “may become ill” to mean may experience clinical symptoms of renal dysfunction and require medical attention.
cCommittee interprets “not likely to become ill” to mean may exhibit low-level transient renal effects.
dPredicted outcome from Guilmette et al. 2005. Committee interprets “no detectable effects” to mean no low-level transient renal effects and no clinical symptoms.
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TABLE 8-9 Capstone Predicted Renal Uranium Concentrations in Level II and Level III Personnel
Exposure
DU Intake (mg/h)
Peak Renal Uranium (μg/g-h)
Renal Effects
Predicted Outcome
Levels II and III: inhalation, breathing zone (mean)
0.447
2.8E-03
REG 0
No detectable effectsa
Levels II and III: inhalation, area monitor (mean)
14.5
1.43E-01
REG 0
No detectable effectsa
Level II: ingestion, hand to mouth (mean)
10.6
7.67E-02
REG 0
No detectable effectsa
Level III: ingestion, hand to mouth (mean)
1.78
1.30E-02
REG 0
No detectable effectsa
aCommittee interprets “no detectable effects” to mean no low-level transient renal effects and no clinical symptoms.
Source: Adapted from Szrom et al. 2004.
ventional armor and no ventilation, level I crew exiting in 1 min from the Abrams tank with DU armor and no ventilation, level I first responders in the Bradley fighting vehicle with conventional armor and no ventilation, and level I first responders in the Abrams tank with conventional armor and no ventilation. With a lower REG-0 range, people in those exposure categories may exhibit transient renal effects, including excretion of albumin and low-molecular-weight proteins; time of recovery from these effects depends on excretion of the uranium. The committee agrees that for all other level I personnel exposures and for all level II and level III exposures modeled, detectable renal effects are not likely to occur.
The committee found REG 2 and REG 3 to be appropriately defined in the Capstone Report as over 6.4 to 8 μg/g and over 18 μg/g, respectively.
Cancer
Comparison of Radiation-Dose Estimates
The Capstone Report’s cancer risk assessment for DU exposure is based on the radioactive properties of DU. In the absence of cancer risk factors specifically related to DU, the estimate of the risk of developing cancer in the Capstone Report is based on the radiation risks posed by alpha-emitters. The concern that DU may increase the risk of cancer is based on knowledge that radiation doses can be delivered to various organs by inhaled DU and that radiation is a known carcinogen.
The radiation dose estimates in the Capstone Report (presented earlier in Table 8-5) are within U.S. radiation standards for occupational exposure. The
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U.S. annual limit for routine occupational exposure is 5 rem (10 CFR 20). The committed effective doses listed in Table 8-5 accrue over 50 y instead of a single year and do not directly correspond to annual doses. The true annual dose is much less than the 50-y committed effective dose. Thus, this is a conservative comparison. Furthermore, the Capstone-estimated median exposures are below the U.S. Nuclear Regulatory Commission annual dose limit of 10 rem for occupational workers with a planned special exposure (10 CFR 20), for example, protecting critical property during an emergency.
Estimates of radiation exposure in the Capstone Report compare well with previously reported exposure estimates. For a crew exiting in 1 min from a perforated vehicle without ventilation, the Capstone Report estimates median 50-y lung exposure as 5.2-17.5 rem; the Royal Society (2001, 2002) obtained a central estimate of 17.8 rem, and the U.S. Army Center for Health Promotion and Preventive Medicine (2000) estimate ranged from 1.5 to 13.2 rem.
Lifetime Cancer-Mortality Risk Estimate
Increased mortality based on organ-specific cancer risk coefficients of alpha-emitting radionuclides was used in the Capstone Report to estimate the risk of fatal cancer in selected organs. Biokinetic-model calculations of organ doses multiplied by organ-specific cancer risk factors estimate that the cancer risk posed by internally deposited DU is primarily to the lungs, which are relatively sensitive organs.
One of the strengths of the Capstone Report is that the calculated risks of cancer mortality were based on the sum of individual organ risks rather than on the whole-body effective dose. That provides a more refined assessment in that use of organ risk factors allows for the nonuniformity of dose distribution among organs. The approach could be used because of the availability of risk factors for lung-cancer mortality and mortality from cancer of other major organs as a function of alpha-emitter dose (ICRP 1991). The risk-factor coefficients for various organs are listed in Table 8-10. Summed organ risks resulted in total cancer-mortality risks that were about 35% higher than the estimated risks based on whole-body effective doses.
Lifetime cancer mortality risks were calculated with the conservative linear (no-threshold) dose-response model. That is, the estimated cancer mortality risk for an organ is proportional to the organ dose. The model might overestimate risks at the low doses predicted in the Capstone Report. The following is an example of the risk-assessment calculations included in the Capstone Report. For crew members who left an Abrams tank with DU armor and no ventilation 5 min after perforation, the median 50-y radiation dose to the lungs is estimated to be 44 rem. The cancer risk-factor coefficient for the lungs is 0.68 × 10−4 per rem. Using the linear model, the committee estimated the median lifetime risk of fatal lung cancer to be 3.0 × 10−3, or three in 1,000 [(0.68 × 10−4 per rem) × 44 rem]. Similarly, the fatal lifetime risks for the other organs
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TABLE 8-10 Risk-Factor Coefficients for Fatal Cancers in Worker Population
Organ or Tissue
Probability of Fatal Cancer per rem
Bladder
0.24 × 10−4
Bone marrow
0.40 × 10−4
Bone surface
0.04 × 10−4
Breast
0.16 × 10−4
Colon
0.68 × 10−4
Extrathoracic tissues
0.10 × 10−4
Kidney
0.11 × 10−4
Liver
0.12 × 10−4
Lung
0.68 × 10−4
Esophagus
0.24 × 10−4
Ovary
0.08 × 10−4
Skin
0.02 × 10−4
Stomach
0.88 × 10−4
Thyroid
0.06 × 10−4
Remainder
0.19 × 10−4
Whole body
4.00 × 10−4
Source: Guilmette et al. 2005. Reprinted with permission; copyright 2005, Battelle Press.
were calculated by multiplying their cancer risk factors by their estimated 50-y alpha-radiation doses and summed with the lung-cancer risk to find the estimated median total fatal-cancer risk of 3.2 × 10−3. The total estimated fatal-cancer risk is due primarily to the alpha-radiation exposure from DU retained in the lungs.
As noted earlier in this chapter, the committee found the Capstone Report’s radiation-dose estimates to be reasonable predictions. The committee’s estimates of level I exposures for the unventilated Abrams tank and Bradley fighting vehicle with conventional armor and the Capstone Report’s estimates agree to within a factor of about 2 (see Table 8-5). The cancer risk estimate is proportional to exposure, so estimates of cancer risks also would agree to within a factor of about 2 for the level I exposure scenarios.
For unprotected level II personnel working in and around vehicles with a single perforation by a DU munition, the committee estimates a DU inhalation rate of up to about 12 mg/h compared with the Capstone Report’s estimate of up to 14.5 mg/h; both considered ingestion of DU to be negligible. Because the estimated cancer risk is proportional to exposure, level II cancer risk estimates would be similar on the basis of the committee’s estimate or the Capstone Report’s estimates.
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Limitations and Interpretation of Level I Risk Estimates
Exposures and cancer mortality risks were estimated for level I personnel. For the most likely exposure scenarios, median lifetime cancer mortality risk estimates presented in the Capstone Report ranged from 2.3 × 10−5 to 3.2 × 10−3 (one in 43,500 to one in 312; see Table 8-11). For the upper-bound exposure scenarios of 1 and 2 h, estimated median lifetime cancer mortality risks ranged scenarios of 1 and 2 h, estimated median lifetime cancer mortality risks ranged from 5.7 × 10−4 (one in 1,750) to 4.5 × 10−3 (one in 222) for the crew members confined in an Abrams tank with DU armor and no ventilation for 2 h after perforation.
A limitation of the median estimates in the Capstone Report is that they do not consider the inherent variability in the exposure estimates. At the 10th and 90th percentile estimates of exposure, lifetime cancer mortality estimates for some exposure scenarios are lower by as much as a factor of about 6 and higher by as much as a factor of about 3, respectively. For the 90th percentile of the exposure scenarios evaluated, the estimated lifetime cancer mortality risks approach 6 × 10−3 (0.6%). If a vehicle is penetrated twice, the lifetime cancer mortality would be expected roughly to double. That would result in median and 90th percentile estimated lifetime cancer risks of 9 × 10−3 (0.9%) and less than 12 × 10−3 (1.2%), respectively.
Given those levels of risk, it would be difficult to distinguish increased cancer mortality rates in exposed Gulf War personnel from background lung-cancer rates because 7.35% of U.S. males smoke and the overall lifetime risk of fatal cancers in males is 23.6% (Ries et al. 2003). In the small group of about 100 level I personnel, most of whom had lower exposure to DU, it is not likely that whatever fatal tumors develop could be attributed to DU exposure. Consistently with that conclusion, the Health Physics Society (HPS 1995) has issued a position statement that recommends against calculating risk estimates for exposure of less than about 10 rem (lifetime), because the risks would be either too small to be observed or possibly zero. It should be remembered, however, that multiple perforations can occur on the battlefield. As noted in the Capstone Report, cancer risks would roughly double if a vehicle suffered two perforations. for soldiers in tanks penetrated by two DU munitions, estimated radiation exposure in the Abrams tank without ventilation ranges from 1.8 to 17.4 rem. Hence, for the worst-case level I exposure scenario of 2 h in a twice-perforated Abrams tank with DU armor and no ventilation, the estimated median increased risk of fatal lung cancer would be 0.9% (one in 111), which is not an insignificant cancer risk.
The cancer risk estimates in the Capstone Report are also limited by their lack of consideration of additional risks to personnel who sustained DU fragment wounds and thus have higher exposure to DU over their lifetime than those calculated in the Capstone Report. Risks from embedded fragments were explicitly excluded from the Capstone Report. Thus, the cancer risks to people with embedded DU fragments are probably underestimated.
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TABLE 8-11 Capstone Summary of Median (10th-, 90th-Percentile) Estimates of Increased Lifetime Risk of Fatal Lung Cancer (Expressed as %) from Inhalation Exposures of DU for Level I Personnel from Single Perforation of Vehicle
Exposure
Abrams Tank: Regular Armor, No Ventilation
Abrams Tank: DU Armor, No Ventilation
Abrams Tank: DU Armor, Ventilation
Bradley Vehicle: Regular Armor, No Ventilation
Exit in 1 min
0.11
0.12
0.0049
0.034
(0.07, 0.14)
(0.08, 0.24)
(NA)
(0.009, 0.059)
Exit in 5 min
0.20
0.32
0.025
0.099
(0.17, 0.40)
(0.24, 0.52)
(NA)
(0.019, 0.180)
First responder
0.05
0.10
0.023
0.052
(0.03, 0.11)
(0.06, 0.16)
(NA)
(0.016, 0.077)
Exit in 60 min
0.27
0.44
0.057
0.12
(0.17, 0.44)
(0.32, 0.64)
(NA)
(0.06, 0.40)
Exit in 120 min
0.28
0.45
0.065
0.14
(0.16, 0.44)
(0.33, 0.65)
(NA)
(0.07, 0.41)
NA = not available.
Source: Parkhurst et al. 2005. Reprinted with permission; copyright 2005, Battelle Press.
Level II and Level III Risk Estimates
The Capstone Report is limited in not providing estimates of fatal cancer risks for potential exposure scenarios for level II or level III personnel. Such risks are difficult to predict because they depend on the duration and level of exposure. Because levels II and III exposures are not in the battlefield setting, some mitigating factors need to be considered, such as the use of personal protective equipment and decontamination of the vehicles. On the basis of exposure estimates in the Capstone Report (see Table 8-12), the potential for cumulative exposure suggests that fatal-cancer risks might be substantial in unprotected level II personnel working for several hours in perforated vehicles. The committee recommends that the number of hours that level II personnel work in perforated vehicles be limited or that protective equipment, particularly respirators, be used. The committee also recommends that if level II Gulf War personnel who had several hours of unprotected exposure in perforated vehicles can be identified, they should be included in the Department of Veterans Affairs health-surveillance program for DU-exposed soldiers.
Exposure of level III unprotected personnel in vehicles with a single DU-munition perforation is expected to be the same as that of level II personnel. Hence, exposure estimates for unprotected level III personnel in perforated vehicles are the same as the upper estimates reported for level II personnel, and similar risk estimates would apply.
For other exposure scenarios, estimates of level III exposure were presented in the Capstone Report. Upper estimates of level III exposure are listed
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TABLE 8-12 Various Methods of Estimating Level II Mean Exposures of DU per Hour of Work by Unprotected Personnel Around and in Vehicles with Single Perforation by DU Munition
Dose Metric
Lower Estimate
Upper Estimate
Cumulative 50-y lung dose via inhalation
0.012 rem/h
0.56 rem/h
Intake via inhalation (ingestion risk negligible)
0.45 mg/h
14.5 mg/h
Cumulative 50-y whole-body dose (inhalation + ingestion)
2.7 × 10−3 rem/h
7.9 × 10−2 rem/h
Source: Adapted from Parkhurst et al. 2005.
here in Table 8-13. The estimates are very low, so it is reasonable not to calculate fatal cancer risks for these exposure scenarios.
Uncertainty of Estimates of Cancer Risk
The risk of fatal cancer is estimated by multiplying exposure to DU (expressed as rem) by the risk coefficient (expressed as risk per rem). That assumes a linear nonthreshold relationship of risk to exposure, which may overestimate risk at low doses by an unknown amount. Conversely, the risk coefficient is based only on the radiologic (alpha-emitter) effects of DU and does not include any potential risk due to chemical carcinogenesis of DU, so it might underestimate risk by an unknown amount. The uncertainty of risk estimates depends primarily on the uncertainty of exposure estimates and the uncertainty associated with the risk coefficient.
Potential cancer mortality from exposure to DU appears to be due almost entirely to lung cancer. Capstone lung-cancer risk estimates used a risk coefficient of 0.68 × 10−4 per rem (ICRP 1991), which is consistent with the data presented in Chapter 6 (Tables 6-2 and 6-3). The NRC (1988) estimated a lung-cancer risk coefficient of 0.35 × 10−4 per rem, which is about half the ICRP value. Koshurnikova et al. (1998) estimated a lung-cancer risk coefficient of 1.2 × 10−4 per rem, which is roughly twice the ICRP value.
Uncertainty of Chemical Carcinogenicity of Uranium
The cancer risk estimates in the Capstone Report were calculated on the basis of radiation doses associated with DU exposure and did not take into account chemical genotoxic effects of DU. That is consistent with historical approaches and recent reports, such as that of the Royal Society (2001), but it does not consider carcinogenic risks that could be posed by the chemical properties of uranium. Recent research has indicated that the mechanism of uranium’s carcinogenicity might involve chemical reactions of the uranium ion with DNA (see Chapter 7) and DNA damage due to uranium’s radioactive properties (re-
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TABLE 8-13 Capstone Upper Estimates of Dose per Hour of Exposure via Inhalation by Unprotected Level III Personnel
Exposure Scenario
Intake (mg/h)
50-y Dose (rem/h)
Downwind of burning uploaded Abrams tank
2.8 × 10−3
4.0 × 10−5
Entry of burned uploaded Abrams tank
2.5 × 10−2
4.0 × 10−4
Downwind of vehicle perforated by DU munition
4.4 × 10−2
7.0 × 10−5
Source: Adapted from Parkhurst et al. 2005.
viewed in Chapter 6) and suggests that cancer risk from exposure to DU might be higher than estimated in the Capstone Report.
The extent to which the chemical carcinogenicity of DU affects cancer risk estimates is not clear and should be studied in greater detail. The committee recommends that studies be conducted to determine the relative contribution of chemical and radiologic mechanisms of uranium carcinogenesis. If the chemical contribution is found to be substantial, studies should be undertaken to calculate cancer risks resulting from DU’s combined chemical and radiologic effects.
SUMMARY
The committee independently evaluated the Capstone exposure assessment. It used data developed largely outside the Capstone program to estimate the time-integrated concentrations of DU in the air in Abrams tanks and Bradley vehicles struck by DU munitions. The results compare favorably with the Capstone measurements. The estimated time-integrated air concentrations were used to estimate inhalation intakes for the five level I exposure scenarios defined in the Capstone Report. The committee’s intake estimates are within a factor of about 2 of the Capstone results. Using those intake estimates, the committee also independently assessed the Capstone dose and risk estimates; its estimates are within a factor of about 2 of the Capstone estimates. The committee’s results for level II and level III exposure resulting from surface contamination resuspended in the air and from incidental ingestion are similar to the Capstone results.
The committee concurs with the Capstone Report that the kidneys are the critical organs for acute chemical effects of uranium. Toxicity is due primarily to damage to renal tubular cells that leads to nephritis. Human occupational and accidental exposure to uranium consistently results in renal effects, and renal effects are also consistently noted in animal studies that report effects on targets other than the kidneys.
The committee had difficulty in verifying the REG-0 classification range for renal effects presented in the Capstone Report; it had questions about the interpretation of some studies and the relevance of the exposure in the studies to that encountered in military settings. Human exposure data suggest that transient proteinuria and albuminuria have occurred in humans with renal uranium concentrations as low as 1 μg/g. Thus, the REG-0 value may have to be redefined;
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any revision to the upper-bound REG-0 value would also require that the REG-1 range be redefined. REG 2 and 3 should remain as defined in the Capstone Report.
Although epidemiologic studies of uranium workers indicate that the risk of cancer from exposure to uranium is low, the possibility of radiation-induced cancer from inhalation of insoluble DU particles cannot be ruled out, especially given that DU emits alpha particles. However, the latent period associated with radiation-induced lung cancer is at least 10 y and might be much longer.
The committee’s estimates of level I, II, and III exposure are similar to those in the Capstone Report. The radiologic-cancer risk estimates are proportional to exposure, so cancer risk estimates based on radiation doses would be similar on the basis of Capstone Report exposure estimates or committee exposure estimates. On the basis of the exposures provided in the Capstone Report, the committee agrees with the radiologic-cancer risk estimates calculated in the Capstone study for the level I inhalation-exposure scenarios.
The Capstone Report does not provide estimates of radiologic-cancer risks for levels II and III personnel. The committee believes that that constitutes a deficiency in the report. On the basis of estimated exposure of levels II and III unprotected personnel working in and around vehicles 2 h or more after a single DU munition perforation, the 50-y whole-body dose (inhalation plus ingestion) is up to 0.079 rem/h of exposure, and the 50-y lung dose via inhalation is up to 0.56 rem/h of exposure. The estimated exposure would be higher and not insignificant for extended exposure in vehicles with multiple perforations.
The Capstone Report does not include cancer risk estimates for soldiers who have embedded DU fragments. That intentional omission is perhaps being addressed separately. Its exclusion from the Capstone Report leads to an underestimation of risk due to increased, prolonged systemic exposure to DU in this cohort of soldiers and of the risk of developing sarcomas in the vicinity of the embedded fragments.
RECOMMENDATIONS
The committee recommends that the Army review the accuracy of the data presented in the Capstone Report on acute human exposures by verifying that uranium intakes were estimated appropriately from the original data, verifying that peak renal uranium concentrations were estimated appropriately with the same model, re-evaluating its interpretation of the Fisher et al. (1990) study, and re-evaluating the dataset by considering the relevance of route of exposure and chemical form to the military exposure scenarios. Depending on the outcome of that review and later calculations, the upper bound of the REG-0 range might need to be revised and the lower bound of the REG-1 range modified. Because of the uncertainties associated with any estimate, the Army should avoid setting REG values that suggest a great deal of precision, particularly in renal concentrations below 3 μg/g.
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Cancer risk estimates should be calculated for levels II and III exposure to determine whether vehicles perforated by DU munitions should be decontaminated to reduce the fatal-cancer risk from later exposure of unprotected people.
For level II personnel working in vehicles perforated by DU munitions, the number of hours should be limited, or protective equipment, particularly respirators, should be used to reduce otherwise potentially important cumulative exposure to DU.
If Gulf War level II personnel who had several hours of unprotected exposure to DU in perforated vehicles can be identified, they should receive additional health monitoring.