Friends,

Dr. Harley has not yet made any distinction between soluble uranium compounds and insoluble inhaled uranium compounds.  According to a manual "Handbook of Health Physics & Radiological Health Edited by B. Slaback Jr. and B.K. Birky,   3rd. Edition, Publishers Williams & Wilkins, inhalation of one gram of insoluble compounds of U-238 deposits 300 REMS or 3 SIEVERTS (Sv.) to the lungs as organ dose. The biological half life was considered to be 500 days for the oxide.

Depleted uranium contains U-234, U-235 and U-236 besides U-238. The amounts of U-234, U-235 and U-236 are small ( `0.2 per cent) but they have very high specific activity (rate of emission of radiation from a gram of the radioisotope) compared to that from U-238. The radiation insult from one gram of depleted uranium according to the manual will amount to 400 REMS or 4 Sv.

It has been reported that the Gulf War veterans exposed to depleted uranium dioxide in 1991 are still excreting depleted uranium in urine. This fact implies that the biological half life of depleted-uranium dioxide dust must be longer than 500 days. It appears that the ceramic form of the oxide resists solubilization in body fluids and hence it resides in the lungs much longer. In a Canadian Uranium Fuel Fabrication Study: I. Intake, Retention and Excretion Monitoruing Results & II. Comparison of Results with Metabolic Models by Avadhanula et. al of Atomic Energy Control Board, Pomroy of Radiation Protection Bureau and J.R. Johnson et. al of Chalk River Nuclear Laboratories, the clearance rate of uranium dioxide was determined for workers who had chronic exposure to uranium dioxide over a number of years. The clearance rate can be estimated by a short-term clearance component (half life of about three days) and three long-term components (half lifves of approximately 280, 800 and 3500 days).

For intake of ceramic form of uranium dioxide and excretion rate measured eight years after the exposure the biological half lives of 800 and 3500 days will be applicable. It is believed that biological half life may be even longer. Measurements of excretion rate and the amount of uranium in the lungs can provide the important parameter, namely, the biological half life of the ceramic dioxide. In any event, if the biological half life is longer, the radiation insult to the lungs will be much higher. If the dose is high enough, radiation-induced leukemia can result. I don't understand why Dr. Harley is so positive that radiation dose from uranium-dioxide to humans cannot be the cause of leukemia.

Hari Sharma.