TAB M - Characterizing DU Aerosols

The actual level of aerosols generated during the various impact tests has varied widely. One of the first hard impact tests conducted on DU ammunition was reported in Characterization of Airborne Uranium from Test Firings of XM-744(sic) Ammunition, 1979.^[288] This report concluded that as much as 70% of the 105mm penetrator would turn into aerosol upon impact. Although this 70% has been frequently cited, it is flawed and misleading—mainly because it was "back-calculated" from cloud data and represented a worst-case scenario (i.e., an impact against a hard target, which was not penetrated). The 1982 report from the Ballistic Research Laboratory entitled Aerosolization Characteristics of Hard Impact Testing of Depleted Uranium Penetrators contradicted the results of the 1979 test. In this test, about 3% was aerosolized 2-3 minutes after impact. Allowing for error, it is highly unlikely that more than 10% of the penetrator was aerosolized in the 1982 test. The 1982 test found that 70% of the aerosolized particles were less than 7 microns—i.e., respirable particles.^[289]

Hard impact testing in 1990 of the M829A1 120mm cartridge and the XM900E1 105-mm cartridge produced somewhat contradictory numbers. This study characterized particulate levels after hard impact with both complete and partial penetration of the armor. The tests were performed with both the M829A1 and XM900E1 rounds, as well as two non-DU rounds, the M865 and DM13. (The purpose of the non-DU round firings was to evaluate DU resuspension during hard impact tests.) The sample results were questioned when only about 0.2% to 0.5% of the DU was aerosolized for the M829A1 and 0.02% to 0.04% for the XM900E1. (These values were approximately two orders of magnitude below expected values.) After comparing Real-Time Aerosol Monitor (RAM) data with RAM data from a previous test, researchers eventually estimated that the percent aerosolized was closer to 18%—substantially less than the 70% previously cited by Battelle in the 1979 test. The respirable aerosol fraction [less than 10 m m AED (Aerodynamic Equivalent Diameter)] was 91% to 96% for the M829A1 and 61% to 89% for the XM900E1. Evaluation of the respirable dust fraction indicated that 57% to 76% was class "Y" material and 24% to 43% was class "D" material, in keeping with other studies which indicated that a high percentage of the respirable dust from hard impact testing was soluble in the lungs. (Note: Class "D" materials have dissolution half-times less that 10 days, class "W" materials have dissolution half-times of 10 to 100 days and class "Y" materials have dissolution half-times greater than 100 days.)^[290] The resuspension tests indicated that most of the resuspended dust was non-respirable, which is consistent with the theory that most of the respirable dust was removed by the filtering system in the enclosure. The aforementioned tests are but a few of the tests performed on DU munitions in an attempt to characterize aerosol formation and assess potential exposures. As a result of recommendations made in the 1995 Health and Environmental Consequences of Depleted Uranium Use in the US Army: Technical Report, Battelle’s Pacific Northwest Laboratory conducted an evaluation of existing test data for predicting aerosol exposures. Their report (entitled Evaluation of DU Aerosol Data: Its Adequacy for Inhalation Modeling) identified some of the technical problems with estimating exposure under various combat scenarios. The following is a brief discussion of DU aerosol generation scenarios present in the report:^[291]

• Fires. During a munitions "cookoff," the burning propellant does not consume oxygen since the propellant supplies its own oxygen. Little if any oxidation of the DU metal occurs because combustion is so rapid. Studies have shown that few of the particles generated during a fire are small enough to be caught up in the thermal currents unless there are violent explosions. The solubility of the oxides formed during a fire are low. Most of the particles produced in a tank fire end up deposited on the interior walls of the tanks, but openings (hatches, holes created by explosions, etc.) could let particles out into the surrounding atmosphere.
• Vehicles Punctured by Projectiles. As noted in other studies, the level of oxides formed during impact is largely a function of the "hardness" of the target. The heavier the armor, the more oxides that will be formed as the DU penetrator expends its kinetic energy "burning" through the armor. During the Gulf War, there were numerous DU hits on lightly armored vehicles, which typically left round, golf-ball-sized entrance and exit holes. Because lightly armored vehicles offered little resistance, unless the round struck the engine or similar obstructions, DU aerosolization was limited in these cases. Conversely, harder targets (like Abrams M1A1 Heavy Armor tanks involved in friendly fire incidents) tend to produce higher levels of DU aerosolization. Aerosolization is enhanced if the penetrator splits into fragments and those fragments remain inside the vehicle. Aerosol levels inside the vehicle also depend on such factors as the number of open hatches and other ruptures or openings. Eventually, particles from inside the tank are either deposited on the inside surfaces of the tank or released to the atmosphere through any opening. As particles are deposited on the interior surfaces, the particle size, distribution, and mass change.
• Entry of Contaminated Vehicles. For Battle Damage Assessment Team (BDAT) personnel, recovery personnel, or souvenir hunters entering the damaged vehicles, the primary concern is resuspension of DU dust. Resuspension depends on the air turbulence inside the vehicle and other conditions (e.g., oily surface walls minimize resuspension). Physical activity inside the vehicle (like lifting or moving equipment or personnel) would obviously increase the level of resuspension. For emergency rescue personnel who enter the tank shortly after impact, the aerosols generated at impact would be the primary concern. These impact aerosol levels should be higher than the resuspension levels generated after the aerosols in the tank have had time to settle or to be vented through open hatches, etc.
• Inspection and Repair Activities on Contaminated Vehicles. Entry into contaminated vehicles for inspection and repair activities can cause significant DU resuspension. And some of the actual repair activities—like cutting and welding—have the potential to raise resuspension levels even higher. Cleaning operations can also cause resuspension.
• Routine Combat Activities. The report, Evaluation of DU Aerosol Data: Its Adequacy for Inhalation Modeling, also indicated potential exposures from DU penetrators that did not penetrate the target or were deflected. The penetrator would be hot enough to generate aerosols, so oxides would continue to be formed for a while once the penetrator was buried in the soil. The report also cited potential exposure to troops near the target at impact, or troops exposed to resuspension from subsequent activities on, in, or near the target.

Two recent tests conducted after the Battelle Summary report raise some questions concerning the nature and extent of respirable particulates generated during fires and hard impact testing. In June 1995, the Army fired 120 mm and 25 mm DU munitions against Soviet armored equipment. Although technical and procedural difficulties seriously affected the data and limited the conclusions that could be drawn from the test, several key findings were cited in the Draft report. They were:

• DU aerosols, containing particles of respirable sizes, are generated inside armored vehicles by DU penetrator impact. The concentration of airborne DU aerosol decreases with time, but measurable concentrations of respirable particles remain suspended hours later.
• Measurable quantities of DU oxide particles that settle on surfaces can be resuspended during routine personnel re-entry activities, and that the resuspended aerosols contain particles of respirable sizes.^[292]

The second test was the 1994 burn test of a Bradley Fighting Vehicle (BFV) equipped with TOW anti-tank missiles and 1,125 M919 25mm cartridges. This was the first time that a vehicle with a full combat load of DU munitions was actually used in a burn test. Most of the previous data for fires were generated from stack testing wooden or metal shipping crates. The BFV was completely engulfed by the fire and burned vigorously for about an hour. The fire subsided after an hour, but continued to emit a plume over the next five hours with smoldering hot spots into the next day.^[293] Of the 1,125 DU penetrators, 625 were accounted for, including nine live rounds found within a few meters of the test pad. Although 500 rounds were unaccounted for, the report indicated that a large percentage was trapped within the melted remains and a significant amount of the DU oxide was mixed within the ash and settled inside and around the hull of the vehicle. Six piles of DU oxide were detected on the vehicle surface after the fire. Analysis of the DU oxide indicated that approximately 33% of the oxide particulates were respirable. However, only trace amounts of DU oxide were detected on the air monitoring filters at various distances during the 29 hours of air sampling.^[294] Although the higher percentage of respirable particulates (33%) measured in the piles of DU oxide after the fire is an important consideration for assessing resuspension potential during recovery, further research is needed to determine whether the higher values of respirable particulates were unique to this test or if results are truly valid for vehicle fires involving DU munitions.

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