Previous chapter Table of contents Next chapter

7. RADIOLOGICAL EFFECTS

7.1 Radiation Dose Equivalent

Effects of radiation on matter are characterized by radiation dose, or absorbed energy per unit mass. Dose is measured in units Gray (1 Gy = 1 J/kg) or rad (1 Gy = 100 rad). In order to evaluate biological effects of different types of radiation (a, b, g, or neutrons), the absorbed dose is multiplied by a Relative Biological Effectiveness (RBE) factor. The resulting quantity is called dose equivalent and it is measured in units Sievert (1 Sv = RBE 1 Gy) or rem (1 Sv = 100 rem). The RBE factor is 1 rem/rad (1 Sv/Gy) for b- and g-radiation, 20 rem/rad for a-radiation with energy below 2 MeV, and it gradually drops to about 2.5 rem/rad for a-particles with energy 100 MeV [6]. Since the energy of a-particles from radioactive decay is always between 2 - 9 MeV, the Nuclear Regulatory Commission (NRC) simply uses RBE factor of 20 rem/rad for all a-emitters [20].

Dose equivalents for individual uranium isotopes and their decay products uniformly distributed in individual organs or in the whole body are assembled in Table 7. Calculation of the annual dose equivalent DE from the specific activity 1 pCi/kg of a pure a-emitter such as 235U is particulary simple:

DE = RBE Ea R Y = 1.32 10 - 5 Sv/year = 1.32 mrem/year

where RBE = 15 rem/rad = 15 Sv/Gy is the Relative Biological Effectivness, Ea = 4.678 MeV = 7.527 10 - 13 J is the energy of a-particles, R = 1 pCi/kg = 3.7 10 - 2 Bq/kg is the specific a-activity under consideration, and Y = 3.156 107 sec/year is the number of seconds per year. In calculating the dose equivalents from b-radiation, it is assumed that about 2/3 of the maximum energy is carried away by neutrinos and not absorbed in matter. Dose equivalents from a-emitters far outweigh those from b-emitters when the source of radiation is inside the body. Dose equivalents from both natural and depleted uranium uniformly distributed in the whole body or in individual organs are listed in Table 8. The DU exposure rate is about 43% less compared to that from natural uranium at the same mass concentration.

Table 7: Dose equivalent rates from uranium isotopes and decay products
Isotope
Radiation
Energy*
[keV]
Yield
[%]
RBE
[rem/rad]
Dose Equivalent/Activity
[(mrem/year)/(pCi/kg)]
238U
a
a
g
4,267
4,219
48
77
23
23
16
16
1
1.28
234Th
b-
b-
g
193
100
93
67
33
33
1
1
1
0.0028
234mPa
b-
b-
b-
g
g
2,290
1,480
1,250
810
230
98
1
1
2
1
1
1
1
1
1
0.0146
234U
a
a
g
4,856
4,803
53
72.5
27.5
27.5
15
15
1
1.30
235U
a
4,678
100
15
1.32
231Th
b-
387
100
1
0.0024
* For b-radiation, maximum energy of b-spectrum.

Table 8: Dose equivalent rates from uranium
Uranium
Dose Equivalent/a-activity
[(mrem/year)/(pCi/kg)]
Dose Equivalent/Mass
[(mrem/year)/(g/kg)]
Natural uranium
with daughter products
1.30
0.885
Depleted uranium
with daughter products
1.30
0.505

7.2 Regulatory Limits on Radiation Exposure

The International Commission on Radiological Protection (ICRP) recommends and the Nuclear Regulatory Commission (NRC) mandates an occupational annual dose equivalent limit for the whole body no more than 5 rem/year and no more than 10 rem in 5 years [4], [20]. No short-term health effects (i.e., no blood changes) are detectable at this dose equivalent. The occupational annual dose equivalent limit for the eye lens is 15 rem/year and for other individual organs and body extremities (hands or feet) 50 rem/year. The whole-body limit (5 rem/year) is further internally reduced by various US federal agencies, for example to an administrative control level 2 rem/year for the Department of Energy (DOE) radiation workers [22], [38].

For comparison, the onset of acute radiation illness (vomiting, diarrhea, hair loss, anemia, hemophilia, eye lens clouding, temporary sterility in males, etc.) occurs at short-term dose equivalents between 50 - 100 rem. Without medical care, 50% mortality was reported at 250 - 300 rem, primarily because of immunodeficiency due to bone marrow failure. With modern medical care, 100% survivability is possible for short-term dose equivalents up to 1,000 rem [87].

Since the dose equivalent does not exhibit any low threshold for the risk of long-term health effects such as cancer [11], [72], the non-occupational annual dose equivalent limit for general public is selected as 100 mrem/year, which comparable to the average background of 363 mrem/year (200 mrem/year from radon, 100 mrem/year from cosmic radiation, terrestrial and internal radioactivity, and 63 mrem/year from artificial sources such as medical exams and consumer products) [85], [93].

7.3 External and Internal Exposure to Uranium

The aluminum jacket of a DU projectile provides shielding from a-particles. The a-particles cannot penetrate the dead skin layer even from a bare DU penetrator. The skin dose equivalent rate from the b- and g-emissions of uranium daughter products in a bare DU projectile in direct contact with skin is approximately 200 mrem/hour [72], [85], two orders of magnitude more than the maximum permissible radiation dose equivalent rate for general public (2 mrem/hour) [20]. This dose equivalent rate was confirmed by our own estimates based on the specific a-activity of 238U (see Table 1), equal to the specific b-activity of each of its equilibrium daughter products 234Th and 234mPa, and on the dose equivalent rates caused by these b-g radionuclides (see Table 7). Between unpacked DU cartridges, the exposure rate is up to 2 mrem/hour. However, only hands could be exposed to this rate and the annual dose equivalent limit for body extremities is 10 higher than the whole body limit.

The accumulated dose equivalent becomes significant when spent but unexploded DU penetrators are worn by army personnel as war souvenirs in direct contact with skin (1,800 rem/year) or when used by children as toys [55]. The annual skin dose equivalent limit of 50 rem/year for radiation workers would be reached in about 10 days. However, neither army personnel in general and certainly not minors are classified as radiation workers. Applying the usual factor of 50, we obtain the annual skin dose equivalent limit 1 rem/year for general public. This limit would be reached in approximately 5 hours (!).

The whole body dose equivalent rates for the crew of a tank armored with DU vary between 0.04 mrem/hour for the commander to 0.18 mrem/hour for the driver [85]. The accumulated dose equivalent for the driver would reach the annual limit for general public (100 mrem/year) after 70 days of 8-hour shifts.

Depleted uranium poses its greatest danger to human health when inhaled or ingested. DU can be internalized as a result of breathing smoke containing DU particles, hand-to-mouth transfer as a result of contact with contaminated vehicles, inhalation or ingestion of resuspended particles, ingestion of food or water contaminated by DU, contamination of wounds by DU dust, or from wounds caused by DU shrapnel [54].

Table 9:
NRC occupational annual limits on uranium intake
Intake
Solubility
a-activity
[Ci/year]
NU Mass
[g/year]
DU Mass
[g/year]
Inhalation
Insoluble
0.05
0.074
0.13
Inhalation
Soluble
1
1.5
2.6
Ingestion
?
10
15
26

7.4 Radiological Limits on Uranium Intake

The US Nuclear Regulatory Commission (NRC) occupational annual limits on intake of uranium [20] (see Table 9) are based on the annual dose equivalent of 5 rem/year and on the uranium excretion rate in urine (see paragraph 8.2). The limit for inhaled insoluble uranium is significantly lower than that for inhaled soluble uranium, because inhaled soluble uranium enters the blood stream and is excreted in urine, while inhaled insoluble uranium particles remain embedded in lungs for years. Since the insoluble uranium cannot enter the blood stream when ingested, only one limit is given for ingested uranium.

Limits on a-activity concentration of natural and depleted uranium (NU and DU) are identical. Radiological limits on mass concentration may be calculated by simply dividing the limits on a-activity concentration by the specific a-activity of natural or depleted uranium from Table 4. Due to 43% lower specific a-activity of depleted uranium, the radiological limits on mass concentration of depleted uranium are 1/(1 - 0.43) = 1.75 higher compared to those for natural uranium.

Table 10:
NRC occupational derived air concentrations
Solubility
a-activity
[pCi/m3]
NU Mass
[g/m3]
DU Mass
[g/m3]
Insoluble
20
30
52
Soluble
500
740
1300
Table 11:
DOE occupational limits for surface contamination
Contamination
a-activity
[nCi/m2]
NU Mass
[g/m2]
DU Mass
[g/m2]
Removable
45
66
116
Fixed
225
330
580
Table 12:
NRC limit for drinking water
Water
a-activity
[pCi/L]
NU Mass
[g/L]
DU Mass
[g/L]
Drinking water
300
440
770

The NRC occupational derived air concentrations (DAC) of uranium [20] (see Table 10) are based on the NRC occupational annual limits on intake by inhalation, on the average volume of respired air (6,500 - 8,400 m3/year) and on the maximum time a radiation worker is exposed (2000 hours/year, or 40 hours/week for 50 weeks). Annual limits on intake by inhalation for general public can be obtained simply by dividing the occupational limits by a factor of 50, in order to receive an annual dose equivalent less than 100 mrem/year. Derived air concentrations of uranium for general public can be obtained by dividing the occupational concentrations by a factor of 50 and by another factor of 4.4 in order to account for the maximum time (2000 hours/year) a radiation worker can be exposed to the occupational concentrations, for a total factor of 220 (NRC uses a factor of 220 - 330).

Department of Energy (DOE) occupational limits for removable and fixed surface contamination level from a-activity of uranium [22], [38] are given in Table 11. Please note that in the above references these limits are given in practical units dpm/100 cm2 (where dpm stands for decays per minute), so that the values must be multiplied by a factor 100/60 = 16.7 in order to obtain the limits in Bq/m2 and divided by another factor of 3.71010 to express the limits in Ci/m2.

Current NRC standard for drinking water [20] (see Table 12) is based on the annual dose equivalent less than 100 mrem/year permitted for general public, on the average daily water intake (2.5 L/day, half from drinking), and on the uranium excretion rate in urine (see paragraph 8.2). No occupational limit is given, because drinking contaminated water is not a permissible occupational hazard.

7.5 Contamination by DU Penetrator Impact

Let us consider the impact of one 120 mm tank round with the 11.8 lb. (5.35 kg) DU penetrator on an armored target, with 18 - 70% of the penetrator rod oxidizing into aerosol dust. The surface contamination level will approximately follow a 2-dimensional gaussian (bell-shaped) distribution. The maximum surface contamination level CS at the center is given by:

Cs = y (M R) /(sigma sqrt(2 pi))^2 = 810 nCi/m2

where y = 44% is the average dust yield (see paragraph 6.6), R = 0.39 mCi/kg = 0.39 106 nCi/kg the specific activity of DU from Table 4, M = 5.35 kg mass of the DU round from Table 5, p = 3.142, and s = 13.5 m the standard deviation of the distribution. The standard deviation is based on the 50 m perimeter value, inside which the army personnel should wear protective clothing and respiratory masks [42]. The maximum surface contamination level at the center is 18 higher than the DOE limit for radiation workers and 900 higher than the allowed contamination level for general public. The contamination level drops to the DOE limit for radiation workers the perimeter 2.4s = 32 m and to the allowed contamination level for general public at the perimeter 3.7s = 50 m.

The initially contaminated area from the impact of one DU tank round inaccessible to general public (50 m radius circle) is about 0.8 hectares (2 acres). If no decontamination is performed and the contamination spreads with weather elements, the amount of released radioactive DU dust is sufficient to make up to 38 hectares (94 acres) inaccessible to general public. The maximum contaminated area S is calculated as

S = y (M R) / (e Cs) = 38 x 10^4 m2 = 38 hectares

where e = 2.718 is the base of natural logarithm and CS = 0.9 nCi/m2 the allowed surface contamination for general public (based on the occupational limit in Table 11 divided by a factor of 50).

The air concentration after the impact and before the DU dust settles can be estimated in a similar manner, by assuming a 3-dimensional gaussian distribution with the same standard deviation. The maximum air concentration CV in the center is given by:

Cv = y (M R) /(sigma sqrt(2 pi))^3 = 24 nCi/m3

Since on the average only 33% of the created DU dust is water soluble (see paragraph 6.6), the maximum air concentration of soluble uranium is 16 higher than the NRC limit for radiation workers and 3,500 higher than the allowed air concentration for general public. The air concentration of soluble uranium drops to the NRC limit for radiation workers at the perimeter 2.4s = 32 m and to the allowed air concentration for general public at the perimeter 4s = 54 m. The maximum air concentration of insoluble uranium is 800 higher than the NRC limit for radiation workers and 180,000 higher than the allowed air concentration for general public. The air concentration of insoluble uranium drops to the NRC limit for radiation workers at the perimeter 3.7s = 50 m and to the allowed air concentration for general public at the perimeter 5s = 67 m. The contaminated air from the impact of one DU tank round initially covers a circular area with 67 m radius, or about 1.4 hectares (3.5 acres). The respirable DU aerosol remains airborne for hours [72].

7.6 Residual DU Contamination in Iraq

In April 1999, 8 years after the Gulf War, the Rumeila oil fields just north of Kuwaiti border (see detailed maps) were visited by Scott Peterson, a reporter for The Christian Science Monitor. Accompanied by an official from Iraq's Atomic Energy Commission carrying a radiation detector, they measured radiation levels 35 above the background over parts of the battlefields and 50 above the background over the rusting tanks hit by DU ammunition. The area was not parched and deserted. They noticed people searching for mushrooms, unaware of any DU contamination, and fresh imprints of bare human feet in the wet soil [88]. The average annual dose equivalent from background radiation is 363 mrem/year (see paragraph 7.2). Excluding the dose equivalent from radon (present mostly in buildings) and artificial sources (medical exams and consumer products), the average dose equivalent from background cosmic radiation, terrestrial, and internal radioactivity is 100 mrem/year [85], [93]. Incidentally, the excess non-occupational dose equivalent allowed for general public is also 100 mrem/year [4], [20]. Radiation level 50 over the background must therefore result in an excess dose equivalent approximately 50 over the limit for general public, or 5 rem/year.

7.7 Animal and Radiation Worker Studies

In response to health concerns of the Gulf War veterans, an extensive review of scientific literature on radiological and toxic effects of uranium [85] was prepared by RAND, a non-profit organization formed by the US Air Force after the Word War II and funded largely by the US Government agencies such as the Department of Defense (DoD).

There is an extensive evidence of excess lung cancers in underground uranium miners. The carcinogenic agent in the mines is the radon isotope 222Rn, a member of the uranium decay series. As a noble gas, it cannot accumulate in lungs and it is quite harmless by itself. However, decay products of radon are solids - they are formed as chemically reactive ions, rapidly attach to ambient dust particles resuspended by human activity in the mines, and become embedded in lungs when inhaled. Although not mentioned in the review [85], excess cancers in former watch makers using radium paint, who sharpened miniature paint brushes using their tongues, are also well documented. Radium isotope 226Ra is also a member of the uranium decay series.

Uranium mill workers have not shown increased mortality or excess lung cancers despite their increased exposure to uranium dust and radon decay products at uranium concentrations 0.5-2.5 mg/m3 for about 5 years. No explanation is given for this discrepancy, yet it is likely that uranium mill workers are exposed to much less dust than uranium miners. Two studies link lymphatic malignancies to uranium exposure in uranium millers, but according to [85] the authors suggest that not uranium, but the thorium isotope 230Th (present in uranium ore with slightly less than half the a-activity of natural uranium and 88% of the a-activity of depleted uranium) was the causative agent. Another study observed an excess in leukemia deaths but according to [85] was unable to determine if there was an association with radiation.

Workers in metal processing plants, including those who make DU penetrators do not exhibit increased mortality or excess lung cancers either. This is not surprising: These workers are not exposed to radon or to respirable uranium dust, they have to undergo a refresher radiation training, and the regulatory limits for uranium intake are enforced. Radiation workers properly informed of the dangers tend to cooperate in limiting their exposures.

Because of high chemical toxicity, radiological effects of natural or depleted uranium are difficult or impossible to study in laboratory animals. Indeed, according to [20], chemical toxicity may be the limiting factor for soluble natural uranium. While various laboratory animals tolerated 10 mg/m3 of (insoluble) UO2 for 5 years, increased mortality was observed in a variety of animals exposed to 19 mg/m3 of (soluble) UO3 for 33 hours/week for 4 weeks. 50% mortality was observed when rats were exposed to about 8 g/m3 of soluble uranium for 10 min. The cause of death, in most cases, was chemically induced kidney failure. Based on these animal studies, the average lethal dose for humans (with 50% expected mortality) can be estimated to less than 1 g of inhalled soluble uranium dust.

A variety of cancers, including leukemia and cancer of the lungs and kidney developed when rats were exposed to enriched uranium (93% of 235U). The review [85] claims that, because weapon grade enriched uranium has 2 orders of magnitude higher a-activity compared to DU (approximately 47 mCi/kg, primarily because of accidental enrichment by 234U), these data are of little relevance to possible radiation-related health effects of DU exposure. Yet increasing the radioactivity while keeping the toxicity constant can separate and highlight the radiological effects, which can be observed in small groups of (healthy) laboratory animals and then extrapolated to lower dose equivalents in large human populations including vulnerable individuals. With this insight, we maintain that the enriched uranium animal studies are even more valuable than natural or depleted uranium studies in evaluating the radiological effects. Similar methodology is normally used to investigate effects of a variety of other carcinogens.

The RAND review concludes that no negative health effects resulting from radioactivity of depleted or natural uranium have been observed in humans. The listed studies investigate effects of natural or enriched uranium, implying that no cancer study of the US Gulf War veterans exposed to depleted uranium has ever been undertaken.


Previous chapter Table of contents Next chapter



back to DU | radiation | rat haus | Index | Search | tree

.

.

.

.

.

.

.

.