That's a quote from my Lancet paper. If we start off with the fact that depleted uranium, chemically speaking, has identical toxicity to natural uranium and that radiologically it's less toxic, because most of the more radioactive isotopes are reduced in the uranium, then we can go back and look at the experience within industry where people have been working with uranium for years.
There were two large studies, one conducted in the United States with about 20,000 people and one conducted in the U.K. with about 20,000 people. The one in the United States looked at cancer and the one in the U.K. looked at all causes of death. The conclusion of both was that they could find no evidence of any adverse effects in those populations, even though in the early days of the nuclear industry when most of the exposures occurred people just threw uranium yellow cake around without taking very many precautions. People sometimes wore dust masks, sometimes they didn't wear a dust mask. So there were considerable exposures in those days, but there's no indication of any toxicity within those groups.
One or two studies suggest there might be a link with lung cancer, but the suggestion is very weak, and lung cancer studies are notoriously difficult to interpret because you have to control for smoking, and unless you know exactly what the smoking habits were of every one of the subjects, it's difficult to interpret.
If it were a more radioactive form of uranium, the more toxic one, and you asked me what I would expect, I would tell you that when that uranium is breathed in, it is breathed in as particles and goes into the lungs. About one-third of it is exhaled with the next breath. Most of the remainder is deposited on the airways of the lungs, and about 6%, depending on the size, is deposited in the deep lung, meaning the respiratory sacs, the alveoli, of the lung.
That which is on the airways rapidly comes up and is swallowed. So there is a flow of mucous up the respiratory tract, but you swallow it. So within one or two days that would have cleared. Then you have 6% left behind in the lung and that gradually dissolves. It can take a long time to dissolve. More insoluble forms of uranium can take months or years to dissolve in the bottom lung. While it's there it irradiates the lung. So one risk that we would associate with a highly radioactive form of uranium, like uranium-233, which is an isotope, is lung cancer.
When it gets into the bloodstream, the vast majority of it is excreted. While it's being excreted, some of it gets hung up in the kidney for a while. So you get a radiation dose to the kidney and might expect to get kidney tumours.
The other thing is that because uranium behaves in the body like calcium, it goes into the skeleton and some of it is in the skeleton. There you might expect to find bone tumours.
So the three tumours that you would be looking for within a population exposed to uranium would be lung tumours, bone tumours, and renal tumours. And those are not found, as I said—with the possible exception of lung cancer, but with a very big uncertainty associated with that. Some studies suggest there's an excess, some suggest there's no excess, and it depends critically on whether there's any smoking.