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2003 June 09 Monday.
Today's naive, ignorant question is this: how much uranium dust can a person get stuck in their lungs?

Depleted uranium burns. When used on the battlefield, it burns fiercely, forming an aerosol which can be breathed in.

A couple of days ago, I wrote an e-mail enlarging on this subject, and my intelligent antagonist came back and asked me "Just how many milligrams of this stuff do you think a person can breathe in?"

I didn't have a clue. Well, can I find out? What happens if I go to www.google.com and punch in the search "uranium dust lungs"?

.... and there seems to be an answer in the form of an article by Steve Fetter and Frank von Hippel published, apparently, in "The Bulletin of the Atomic Scientists" November/December 1999, Vol. 55, No. 6, pp. 42-45. The article has the title "After the Dust Settles".

After the Dust Settles

In summary:-

(1) The article is sceptical about the possibility of any significant damage caused by external exposure to DU. The argument (which seems reasonable) is that if DU is outside the body then it is unlikely to be much of a problem.

(ii) Having indicated that the radiation burden from external battlefield DU is insignificant, the article then says that "Internal body doses could by higher". If 20 percent of a depleted uranium projectile burns, "a reasonable estimate based on army tests," then "a heavy DU penetrator might generate a kilogram of uranium oxide aerosol."

(iii) The article says that "For a heavy penetrator, the released energy would be equivalent to the explosion of as much as a kilogram of TNT, lifting the aerosol upward on a column of hot air. Because of this vertical dilution, the amount of depleted uranium inhaled by a nearby person would probably not exceed 0.1 milligrams. The dose to a person a mile away directly downwind would be about ten times less."

(iv) The article cites a model indicating that "15 percent of an insoluble inhaled uranium oxide aerosol could be retained in the lungs for more than a year."

(v) The article says that "For someone close to the battle who inhaled one milligram of depleted uranium - an unlikely scenario because he would have to be exposed to several close hits - the equivalent whole-body dose would be up to 0.1 rem. That is roughly half the annual average dose from inhaled radon and its decay products in a typical single-family home in the United States. An individual's estimated added risk of dying from cancer from such a dose would be about one in 20,000. (To put that figure in perspective, we in the United States have a one-in-five risk of dying of cancer.)"

(vi) The article does some math and figures that depleted uranium used in Desert Storm could have resulted in ten "excess lung cancer deaths in the lifetimes of the exposed populations".

(vii) Apart from the radiation hazard, there is a chemical hazard, in that uranium is chemically toxic. The article says that "The greatest hazard from depleted uranium's chemical effects would come from its soluble oxides. Army tests indicate that 17 to 43 percent of the uranium oxides produced initially as a result of hard impacts of DU penetrators are relatively soluble. For very small aerosol particles, about half of the inhaled soluble oxides would dissolve into the blood after being inhaled and five percent after being ingested."

(viii) Regarding the chemical toxicity, the article says that "The U.S. Occupational Safety and Health and Administration sets a limit on the eight-hour average exposure for unprotected workers to soluble forms of uranium at 0.05 milligrams per cubic meter, the same as for lead."

(ix) The article says that "The International Commission on Radiological Protection model assumes that about one-eighth of the uranium that finds its way into the bloodstream will deposit in the kidneys. The rest will either be rapidly eliminated in the urine or attach itself temporarily to bone surfaces. For one milligram of uranium to be deposited in the kidneys, the blood would have to absorb about eight milligrams of soluble uranium compounds. If the fraction of soluble uranium oxides were one-third of the total uranium oxides released in the DU attack, then about 50 milligrams would have to be inhaled."

(x) The article suggests that some soldiers who were in vehicles hit by DU rounds (and their rescuers, and any unprotected cleanup workers who spent "prolonged periods" in the vehicles) "may have inhaled enough DU dust to suffer heavy-metal effects". Because of the residual toxic risk (and the risk of unexploded munitions) such wrecked vehicles "should be made inaccessible, perhaps by being buried and then pumped full of concrete."

Dilution Assumptions

Having read the article, I now have one big question, which is this: are the dilution assumptions reasonable?

A DU projectile strikes. It burns. Twenty percent of the projectile is converted into a breathable aerosol - a kilogram of uranium oxide.

A person nearby breathes in no more than 0.1 milligrams of depleted uranium.

Now, when a person takes in a breath, they take in, on average, half a liter of air - 500 cubic centimeters of air. If a person breathes in twice, then they breathe in a liter of air - a thousand cubic centimeters.

A cubic meter of air is 100 x 100 x 100 cubic centimeters, which is a million cubic centimeters, or, to put it another way, a thousand liters.

To contaminate a cubic meter of air to the tune of 0.1 milligrams per liter (0.1 milligrams per two breaths) would take 0.1 x 1,000 milligrams, or 100 milligrams, or 100/1000 of a gram, which is a tenth of a gram.

A kilogram is a thousand grams, so, assuming I haven't somehow misplaced a decimal point, a kilogram of uranium oxide could contaminate 1,000 x 10 cubic meters to the 0.1 milligram per liter level - that is, 10,000 cubic meters.

I went to the cube and cube root calculator, punched in 10,000, and got a figure of just over 21.54 as the cube root. I multiplied 21.54 x 21.54 x 21.54 on my calculator (which is the ordinary "let's go shopping for vegetables" variety, and does not do fancy things like cube roots) and got 9,993 plus some small change.

(I really though I'd left the cube root behind once and for all when I escaped from high school.)

So, anyway, to figure that "A person nearby breathes in no more than 0.1 milligrams of depleted uranium" you really have to believe that the one kilogram aerosol from a heavy DU projectile is dispersed over an area equal to or greater than a cube which is 21 meters or so along one side.

The article assumes that the aerosol would be lifted "upward on a column of hot air". But is this a reasonable assumption?

This is a question about the behavior of explosions, and I don't have a clue how to answer it. However, there must be, somewhere, an accessible database dealing with how things behave when they explode.

Presumably uranium oxide has a known weight, and presumably there's some formula somewhere for calculating how particles of a given weight rise and fall in fires and explosions ... as I feared right from the start, this whole adventure into the world of online research is looking painfully mathematical.

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