We have now obtained the sequel to the paper¹ cited by Dr Guthrie. This second paper² looked for Uranium concentrations and histological changes in animals up to five years after the end of their exposure to inhaling Uranium Oxide (UO2) dust.

It must be borne in mind that compared with the most of the likely exposures from military use of DU, the Uranium burdens in lung and TBLNs were monstrously high in this study; radiation doses to TBLNs were of the order of 1,000,000 times natural background at the end of the 5 year exposure, and doubled over the ensuing 6½ years.

Unsurprisingly lymph nodes suffered extensive destruction:

The complete loss of lymphoid tissue with replacement by pigment and fibrous tissue was characteristic of all nodes in the tracheobronchial group from monkeys with post-exposure periods over 12 months.

Lungs received such high doses that

pulmonary fibrosis seen in late post- exposure monkeys had progressed to the point where severe impairment of respiratory function, while not measured, must have occurred.

Tumours were found in none of the monkeys, but in dogs, whose accumulated dose was roughly half that of the monkeys, pulmonary neoplasms were 50 - 100 times the spontaneous (natural) rate. The discussion abstains from drawing conclusions. So do we, with three exceptions.

1     The study confirms that uranium persists in TBLNs, with a biological half-life of 50 - 65 months in monkey TBLNs. An interesting observation, possibly relevant to DU, is that in the lower dose monkeys (i.e. those exposed for one year rather than the full 5) the TBLN burden continued to increase (by a factor of >3) throughout the 6 years of the post exposure study.
2     Dr Guthrie's assertion that Part I of the inhalation study had shown no ill effects on the subjects is not supported by Part II.
3     The paper notes that tumours occurred in the UO2 exposed dogs at estimated alpha radiation doses one-fourth to one-fifth those producing the same response with Plutonium 239. [i.e. in other studies] We suggest that this observation, which Leach et al make in the context of speculating about the relationships between carcinogenic qualities of different isotopes, undermines the conventional model of low dose radiation protection and supports our contention that lower doses are disproportionately dangerous.

Leach et al conclude:

Pulmonary neoplasia developing after exposure to natural uranium dust, even in a single species, is a sobering thought. The possibility of this occurring in man under occupational conditions, however remote, should be fully investigated. We cannot rely on animal experiments alone to yield data that will answer this question, therefore we believe that future investigations should be focused on human exposures and post mortem data.

The Gulf War veterans are a potential "at risk" population big enough to give statistically significant results. We note that the Ministry of Defence has now offered to commission slightly more sensitive testing for Uranium in those veterans who have tested positive in monitoring conducted at their own expense in Canada and the USA. This is completely inadequate. MAP should be extended to all veterans.

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References

1 Leach LJ Elliott AM et el A five year inhalation study with natural uranium dioxide dust. 1: retention and biological effect in the monkey dog and rat. Health Physics 1970 18 599-612

2 Leach J L, Yuile C L, et al A five year inhalation study with natural uranium dioxide dust. II Postexposure retention and biologic effects in the monkey, dog and rat. Health Physics 25 239-258 (Sept 1973)