The Human Plutonium Injection Experiments

before the radium was fixed in tissue.

Without data to support another conclu-

sion, Hamilton probably assumed that

the behavior of plutonium would be

similar—much of it would be eliminat-

ed quickly.  

Hamilton also suggested that “integra-

tion of 24-hour urine samples, checked

every 2 weeks will give a fairly good

indication of intake of Pu by an indi-

vidual, and so a gauge of Pu deposition

in body.”  This statement is consistent

with the assumption that, like radium,

plutonium would take time to become

fixed in tissue.  Thus, an accurate deter-

mination of a body burden would re-

quire that the measurements be made

after the plutonium circulating in the

blood was either excreted or fixed.  At

that later time, only plutonium re-enter-

ing the blood from fixed tissue sites

would be circulating, and measurements

of the fraction excreted would more ac-

curately reflect the level of retained

plutonium.

Eleven milligrams of plutonium were

diverted to Hamilton at the beginning

of February 1944 (about 2 per cent of

the total Clinton output of plutonium at

that point) to enable him to begin bio-

medical experiments with animals.  The

research involved administering soluble

15-microgram portions of plutonium-

239 compounds to rats, using different

plutonium valence states (+3, +4, and

+6) and different methods of introduc-

ing the plutonium (oral, intramuscular,

intravenous, subcutaneous, and intrapul-

monary procedures).  

A Met Lab progress report for February

containing Hamilton’s input stated:

Product studies: - Oral absorption

of all valence states is less than

0.05%; lung retention high; ad-

sorbed material predominately in

skeleton; excretion very small in

urine and feces.

And the report for March noted:

Product behaves differently in the

three valence states.  The plus 4

state is retained to considerable ex-

tent at 16 days, the plus 3 is re-

tained to a less degree and the plus

6 to a still less degree.

By April, Hempelmann was discussing

Hamilton’s results at Los Alamos, say-

ing that “plutonium in all three valence

states is very poorly absorbed when

taken by mouth—less than .005%” and

“the organ which took up most of the

absorbed plutonium was the bone, with

more than half of the element going to

the skeletal system in each case.”

Additional quantities of plutonium were

made available to Hamilton, and he was

authorized to extend his research to the

uptake of plutonium dust from the

lungs of rats.  He soon learned that

only about 20 per cent of the plutonium

originally inhaled was eventually de-

posited in the skeleton.  Almost half

was trapped in the upper air passages

and eliminated; about 25 per cent re-

mained in the lung, although some of

that was slowly eliminated.  The actual

percentages depended on whether or

not the plutonium compound was solu-

ble—plutonium nitrate was quite readi-

ly absorbed, whereas the oxide was not

absorbed at all.  

In the spring of 1944, plutonium was

made available for animal studies at the

Chicago Met Lab, and research was ini-

tiated there on the acute toxicity of plu-

tonium.  Those studies involved the in-

jection of microgram and milligram

quantities of plutonium-239 into mice,

rats, rabbits,.and dogs.

The results of the studies at Berkeley

and Chicago showed that plutonium’s

physiological behavior differed signifi-

cantly from that of radium.  Two facts

were particularly alarming: there was

significant deposition of plutonium in

the liver, and the overall excretion rates

were very low (see Table 1).  Neither

of these facts were anticipated when the

tentative 5-microgram tolerance limit

for plutonium was adopted early in

1944.  Furthermore, the rate of plutoni-

um elimination in excreta differed be-

tween species of animals by as much as

a factor of five.  Such variation made it

difficult to estimate what the rate would

be for man.  

The studies also showed that plutonium

was similar to radium in being a bone

seeker, but only a little more than half

of what was retained went to the bone,

compared to 99 per cent for radium.

Also, the two metals deposited at dif-

ferent locations.  Radium (similar,

chemically, to calcium) deposited in

mineralized bone, whereas plutonium

remained on the surface in the “actively

metabolizing” portion of the bone, an

area intimately associated with bone

marrow and the production of blood

cells.  (However, because plutonium

deposits on the endosteal surfaces of

the red marrow and the alpha particles

have a limited range, the blood-forming

tissue is not irradiated uniformly.)

The initial animal excretion rate for

plutonium was low (less than 10 per

cent of what had been introduced ap-

peared in the urine and about 6 per cent

in the feces over the first four days),

The Human Plutonium Injection Experiments

188

Los Alamos Science Number 23  1995

Joseph Hamilton carried out the ini-

tial metabolic studies of plutonium in

animals.

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