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3.12.2 Identification of Data Needs
Acute-Duration Exposure.
The possibility of brief exposure of humans to plutonium exists at
hazardous waste sites or at accidental spill sites. However, no data are available for humans exposed
acutely via inhalation or oral routes. Information on the toxicity of plutonium in laboratory animals
following single high-dose inhalation exposure is extensive and indicates that the lung is the main target
organ for inhaled plutonium. Laboratory animals exposed by this route have developed pneumonitis,
fibrosis, metaplasia, and cancer. Acute exposure of laboratory animals to lower doses of plutonium
would be useful to identify possible inhalation toxicity in humans. Limited
information on adverse
effects in laboratory animals following acute oral exposure indicates that the gastrointestinal tract is the
main target organ. However, kinetic studies indicate that plutonium absorbed from the gastrointestinal
tract is distributed to the skeleton and other tissues; therefore, other organs may also be affected. Because
there are no data on humans, and animal data are insufficient, additional information
is needed on adverse
effects following acute exposure by the oral route. No data are available on adverse effects following
acute dermal exposure in humans or animals. Limited information from kinetics studies in humans and
animals indicates that there is little absorption of plutonium through intact skin. However, plutonium
deposited in wounds is absorbed and
distributes to numerous organs, including regional lymph nodes and
the liver. Since industrial accidents resulting in plutonium-contaminated wounds are known to occur,
additional information on adverse effects following this type of exposure would be helpful. One
outstanding problem with all of the existing acute exposure tests in laboratory animals is that the doses
tested are extremely high. Further single-dose studies for all exposure routes using a number of lower
exposure concentrations would be useful in determining any dose-response relationship for adverse health
effects.
Intermediate-Duration Exposure.
All of the major studies of cancer and other health end points in
animals have involved lifetime follow up of animals acutely exposed to plutonium aerosols. The
relatively long retention time of plutonium in the body produces a chronic radiation dosing of tissues that
retain plutonium. These studies have provided the bases for absorbed radiation
dose-response
relationships for inhaled plutonium compounds (e.g.,
238
PuO
2
,
238
PuO
2
, and
238
Pu[NO
3
]
4
). Toxicokinetic
studies, and models derived from these studies, allow predictions of the tissue distribution of plutonium
that would be expected with repeated exposures. These can be used to predict dose-response relationships
for repeated dosing scenarios. The feasibility of conducting studies of intermediate- or chronic-duration
exposures to plutonium compounds should be weighed against current uncertainties in predicting the
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outcomes from existing knowledge. Furthermore, repeated
exposure does not alter risk; the total dose of
alpha radiation is important, not the temporal pattern of exposure.
Few studies of the health effects of plutonium administered to animals by the oral route have been
reported. A single study in rats found profound effects on the gastrointestinal epithelium, consistent with
radiation-induced injury. Given the relatively small fractional absorption of ingested plutonium (<0.1%
of administered dose), it is very likely that repeated (or single) oral dosing studies that produce systemic
toxicity will produce lethal effects on the gastrointestinal tract. Thus, the feasibility of conducting studies
of intermediate- or chronic-duration oral exposures to plutonium compounds should be weighed against
current uncertainties in predicting the outcomes from existing knowledge.
Chronic-Duration Exposure and Cancer.
Epidemiological studies of occupational cohorts with
long-term exposure to plutonium include those established from employees at U.S. plutonium production
and/or processing facilities (Hanford, Los Alamos, Rocky Flats), as well as facilities in Russia (Mayak)
and the United Kingdom (e.g., Sellafield). Studies of Mayak cohorts provide evidence for an association
between cancer mortality and exposure to plutonium. Plutonium dose-response relationships for lung
cancer mortality have been corroborated in four Mayak studies (Gilbert et al. 2004; Jacob et al. 2005;
Kreisheimer et al. 2003; Sokolnikov et al. 2008). Studies of U.K. and U.S.
facilities have examined
cohorts of workers who had substantially lower estimated plutonium exposures and corresponding
internal radiation doses than the Mayak cohorts. Collectively, findings from these studies are not as
consistent as the Mayak studies; although significantly higher incidence of cancer mortality in certain
groups of plutonium workers has been
found in some studies, higher cancer incidence and/or risks for
tissues that received the highest plutonium radiation doses (i.e., lung, liver, bone) have not been found,
making causal connections of these outcomes to plutonium exposure more uncertain (Brown et al. 2004;
Carpenter et al. 1998; Gilbert et al. 1989b; McGeoghegan et al. 2003; Omar et al. 1999; Wing et al.
2004). Uncertainties in exposures received by each of these populations are a major contributor to the
overall uncertainty in estimates of radiation doses and dose-response relationships. For the Mayak cohort,
an extensive collaborative effort between U.S. and Russian scientists has led to many improvements in
both external and internal doses. The improved doses (Leggett et al. 2005) have not been used in
published
analyses, but are likely to be used future analyses (Leggett et al. 2005; Vasilenko et al. 2007).
Continued follow up of these cohorts, using improved exposure estimates, would extend our
understanding of dose-response relationships.