shallowdeep
Posts: 343
Joined: 9/1/2006
From: California
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This seems to have gone downhill, but one last post that I'd already started on:
quote:
ORIGINAL: Owner59
What if the dust is spread around evenly on a farm field or grazing pasture where farm animals feed?
This sort of dispersal would actually be a good thing. Assume the worst case - that we could evenly distribute the roughly 200 metric tons of DU used over just the permanent cropland in Iraq (0.61% of the 432,162 km^2 total, according to the CIA World Factbook). Let's say the dust gets mixed in with the top six inches of topsoil during tilling. Those top six inches already have roughly 1800 tons of uranium present naturally - or more if phosphates have been used as fertilizers. So you have roughly an 11% increase in the amount of uranium present, which falls well within the natural variations in uranium concentration and poses virtually no threat from a toxicological standpoint. If the only thing we're considering is chemical toxicity, dilution like this eliminates concentration levels that could be dangerous.
Cancer risk is a bit different. Unlike non-carcinogenic toxins, and barring evidence of a specific mechanism, the conservative operating assumption is that cancer risks have a direct linear relationship to an individual's exposure, with no minimum threshold. While dilution would lower the risk to any one individual to negligible levels, theoretically you would expect a few additional cancer cases in the aggregate. So, if you could actually get all the uranium into cropland somehow, you might see something like a 7% increase over cancers caused by natural uranium (reduced from 11% to account for the lower radioactivity of DU). Of course, the number of cancers caused by natural uranium exposure is minute to the point I've never heard of anyone worrying about it.*
The problem though is that DU is not readily dispersed. You rarely get detectable levels beyond 400 m or so from the impact [source, section 3.2]. As a result, there are localized areas that might be dangerous. If the impact occurred in the middle of an uninhabited desert, this localization is probably a good thing. If it occurred near a village well, not so much.
There are legitimate grounds for concern over localized contamination like this, although the studies to date have not clearly established a substantial risk. Chances are a few grams of uranium aerosol, or a mostly intact 30 mm penetrator, ending up in a well aren't going to affect the health of anyone in the village (the oral LD50 is thought to be around 5 g, and the cancer risk for any single individual is small) but exposing others to such a risk is something most people wouldn't want on their conscience, hence the WHO recommendation for clean up and monitoring.
*Well, until I looked into it. The CDC's toxicological profile for uranium, amid a plethora of good information on the risks of of uranium and the studies that support them, referenced the BEIR IV committee's calculations of risk from natural uranium exposure. For the sake of quantifying the risk of DU use in Iraq, I played with the committee's calculations a bit. Assuming 200 metric tons of DU were used and that you could get 27.5 million Iraqis (roughly the country's current population) to ingest all of it over the next 70 years, you could achieve 10.2% of the BEIR IV dosage, which would translate into 3.94 expected additional cases of bone cancer (probably uranium's highest risk due to its affinity for phosphates) in those 27.5 million people over those 70 years.
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"I believe imagination is stronger than knowledge." R. Fulghum ... For both better and worse.
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RE: depleted Uranium - 
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