A human approach to the study of plutonium

29 October 1999

In a human experiment designed to study the metabolic pathways of plutonium, two scientists inhaled nitrates of plutonium; its movement through the men is closely monitored. by DICK KOVAN

The two men involved in the plutonium inhalation testing, which took place in the UK nearly two years ago, were also subjects in an earlier experiment in which they were injected with plutonium. The importance of the recent experiment is that inhalation is much more representative of how people would take in plutonium following an accidental release.

The tests involved only “trace” amounts of plutonium that could have no harmful health effects. However, last summer, the popular press got hold of the story and generally misreported it as a study of the biological effects of internal plutonium. Had this been true, the ethics committees that considered the proposals would never have allowed the test to go ahead.

The earlier trials began in 1993 at the UK Atomic Energy Authority’s Harwell laboratories. The first injected were the same two involved in the most recent experiment. Another 10 people were later included.

The name of one of the first two subjects, Eric Voice, had been leaked to a newspaper after the first experiment. The press approached him about the recent tests. Voice says, however, he does not recognise many of the statements attributed to him which led to the distorted reports.

The first experiments involved intravenous injections of plutonium, as this was the only intake mechanism that could be accurately monitored at the time. The excretion patterns and the movement of plutonium in blood, tissue, liver and bone were followed.

Some of the results were described in the scientific literature as early as 1994 (see panel). In particular, they were seen to discredit the “Gardner” allegation that there was a link between gonadal deposits and leukaemia in the children of reprocessing workers.

The latest test was carried out by the UK National Radiological Protection Board (NRPB) which expects to issue an interim report later this year. This work has received funding from the European Commission which wanted a study of the intake of plutonium by inhalation as this would be the main pathway of an accidental release from some actual facility, such as a chemical reprocessing plant. The experiment involved the inhalation of aerosols of plutonium-containing particulates.

Human testing – a technological breakthrough

According to Eric Voice, because of the risks involved in testing, our understanding of the health effects in humans of plutonium intake is very limited. Basically, we try to calculate the statistical probabilities of cancers by comparison with other known radiocarcinogens. This obviously gives rise to many uncertainties.

Testing on humans, which only became practical as a result of technological advances of the past decade, will help us answer many crucial questions regarding plutonium, such as what are its rates of excretion, its distribution in the various organs, and its long-term behaviour.

The recent developments involve two unusual isotopes of plutonium – Pu-237, a short lived isotope, and Pu-244, a long-lived isotope. The use of these, coupled with advanced measuring techniques, means that only negligible amounts of plutonium are needed. This ensured that the ethics committees were satisfied that the dose to the volunteers was small compared with natural sources of internal irradiation.

The first test was made possible when scientists identified Pu-237, a soft gamma emitter with a half-life of 45 days, which can be traced throughout the body from the outside with some precision over several months. After a year it will have virtually disappeared from the body.

In this case, the Medical Ethics Committee for Human In-Vivo Experiments approved the experiment, but only with certain conditions. While accepting the case for the use of trace amounts of Pu-237, it insisted that the isotope be very pure. At first this seemed a major stumbling block as there appeared to be no sources of highly pure isotope. However, workers in Western Europe learned that the Joint Institute for Nuclear Research at Dubna in Russia had the capability to manufacture highly pure Pu-237. Voice paid a visit to Dubna and helped convince the Russians to provide it.

The second development was the availability of advanced mass spectrometers which can identify atoms at levels as low as 10-15 g. This meant that the isotope Pu-244, with a half life of about 80 million years, can be used in the tests. Although remaining in the body, the level is so minute that it poses a negligible risk.

So early in 1998, Voice and the other original volunteer breathed in an aerosol of nitrates of Pu-237/244 about the size expected from an accident at a reprocessing plant. Over the next two to three months, the path of the plutonium was followed through the men’s bodies using whole body and collimated monitoring.

The excreta is also being collected and sent for analysis to a specialist laboratory in Canberra, Australia. The monitoring of excreta is continuing indefinitely.

Experimenting on people

While work has been done on animals, relatively little is known of human Pu metabolism despite the continuing programmes which monitor Japanese bomb victims and those tracking some of the original weapons development workers at Los Alamos.* One of the few experiments on humans occurred just after the second World War, when the US military conducted experiments on about 18 people with advanced cancers and other illnesses who were dying in hospital. These results are not considered reliable, due to the subjects abnormal metabolism. Since that time, this type of test has not been allowed. For the recent experiments, approval had to be obtained both from AEA Technology's ethics committee (whose external members have no other connection with the company), and also, as a legal requirement, from a committee of the UK Department of Health. Both committees were satisfied that the radiation dose to the volunteers was small compared with that received annually from natural sources of internal irradiation. Following the good results on the first two volunteers, the number was increased to 12, including six women, whose inclusion was considered necessary as their metabolism differs from men in several ways. The ethics committees also decided that younger volunteers could participate - for the initial experiment, volunteers had to be in their sixties. The latest inhalation experiment, however, has so far only involved the original two volunteers as their metabolisms were already well known. Some initial results Results of the first series of studies have been reported in a number of scientific papers. The experiments found that: Uptake of plutonium by testes was no larger than had previously been assumed, discrediting allegations of a link between gonadal deposits and leukaemia in the children of reprocessing workers. Women tended to excrete plutonium more rapidly than men. Initial deposition of plutonium was predominantly in the liver, rather than in the skeleton, so that in occupational exposure irradiation of bone would be less than had previously been assumed. * Since the first work with plutonium was undertaken at Los Alamos 25 men who had large plutonium intakes during the 1940s have been studied. Of these, only three have died, none due to radiation related illness.

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