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PLUTONIUM
2. RELEVANCE TO PUBLIC HEALTH
intestine in neonatal rats that received an oral dose of 5,300 kBq
238
Pu/kg; total obliteration of epithelial
cells and crypts, combined with intestinal hemorrhaging, were noted in rats that received 17,400 kBq
238
Pu/kg. Increased neutrophils were noted on the surface epithelium and superficial cellular layers of the
large intestine in adult rats given 155 μCi
238
PuO
2
/kg (5,740 kBq/kg). This effect was noted at 3 (but
not 6) days posttreatment.
2.3 MINIMAL RISK LEVELS (MRLs)
Inhalation MRLs
No acute-, intermediate-, or chronic-duration inhalation MRLs were derived for plutonium due to the lack
of suitable human or animal data regarding health effects following inhalation exposure to plutonium.
The strongest evidence for plutonium exposure-response and radiation dose-response relationships in
humans is for cancers of the lung, liver, and bone. Although non-neoplastic lesions
have been observed
in animals exposed to
238
PuO
2
,
239
PuO
2
, and
239
Pu(NO
3
)
4
, these lesions occurred in association with acute
exposures that also resulted in fatal cancers.
Oral MRLs
No acute-, intermediate-, or chronic-duration oral MRLs were derived for plutonium due to the lack of
suitable human or animal data regarding health effects following oral exposure to plutonium. No data are
available on exposure- and radiation dose-response relationships in humans for oral exposures to
plutonium. Animal studies of health effects of oral exposures to plutonium have not examined major
health outcomes that would be expected to occur from absorbed plutonium (e.g., effects on skeleton, liver,
and lymphopoietic systems).
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PLUTONIUM
3. HEALTH EFFECTS
3.1 INTRODUCTION
The primary purpose of this chapter is to provide public health officials, physicians, toxicologists, and
other interested individuals and groups with an overall perspective on the toxicology of plutonium. It
contains descriptions and evaluations of toxicological studies and epidemiological investigations and
provides conclusions, where possible, on the relevance of toxicity and toxicokinetic data to public health.
A glossary and list of acronyms, abbreviations, and symbols can be found at the end of this profile.
Plutonium (Pu) is a radioactive element and a member of the actinides in the periodic table. Although
trace amounts of plutonium exist naturally in the environment, the plutonium in the environment today
has been (and continues to be) formed primarily from anthropogenic activity related to nuclear fission.
Environmental plutonium levels are generally low and not of significant health concern.
Anthropogenic
isotopes with masses ranging from 228–247 have been produced and recorded on the chart of the
nuclides; however,
238
Pu and
239
Pu, in their oxide and nitrate forms, are the plutonium isotopes most
widely used in health effects studies. They are also the dominant isotopes that contribute to
environmental and occupational exposure. Plutonium nitrates are associated with dissolving uranium-
plutonium metal matrices after plutonium is produced in a nuclear reactor or by an accelerator.
Plutonium oxides form on the surface of plutonium metal and are released through the machining of
plutonium metal parts or the incomplete fissioning of plutonium during weapons detonation.
Most plutonium isotopes emit a high energy (generally >5 MeV) alpha particle and low energy (<20 keV)
gamma and x-rays as they transform into uranium. The others (
241
Pu and
243
Pu) undergo beta minus
decay and transform into isotopes of americium. The radiation dose from plutonium can be designated as
either external (if the material is outside the body) or internal (if it is inside the body). The total radiation
dose is the sum of external and internal radiation doses. The external dose from most plutonium isotopes
is low because the x- and gamma-rays are of very low branching intensity and energy
and the high energy
alpha particles travel only very short distances and can only affect the outermost (epidermal) layers of
intact skin even when in direct dermal contact. External beta emissions from isotopes such as
241
Pu can
travel slightly farther and may even penetrate the outer dermal layers, but are generally not of significant
health concern unless a beta-emitting plutonium isotope comes into direct contact with the skin. Extreme
skin contamination from plutonium-produced alpha and beta radiation, which could potentially occur in
accidents
or the workplace, might induce dermal and subdermal effects such as erythema, ulceration, or
20
PLUTONIUM
3. HEALTH EFFECTS
even tissue necrosis. Internally deposited plutonium, however, possesses the potential to produce
significant health effects via transfer of energy from alpha particles to nearby cellular molecules. Once
plutonium is internalized, the distribution, retention, and excretion kinetics, paired with the plutonium
decay and energy deposition parameters, determine how the radiation dose increases over time.
In
radiation biology, the term absorbed dose refers to the amount of energy deposited by radiation per unit
mass of material (e.g., tissue), and is expressed in units of rad or gray (Gy) (see Appendix D for a detailed
description of principles of ionizing radiation). One Gy is equivalent to 100 rad. Because alpha radiation
is more biologically damaging internally than other types of radiation (i.e., x-rays, gamma rays, beta
particles), a given absorbed dose (rad or Gy) is multiplied by a radiation weighting factor of 20 for alpha
radiation or 1 for x-rays, gamma rays, and beta particles to obtain a quantity that expresses, on a common
scale
for all ionizing radiation, the biological damage (dose equivalent in units of rem or Sievert [Sv]) to a
particular tissue. One Sv is equivalent to 100 rem. The committed dose equivalent is typically the
radiation dose to a particular organ or tissue that is received from an intake of radioactive material by an
individual during the 50-year period following the intake. The internal dose from plutonium is estimated
using the quantity of material entering the body (via inhalation, ingestion, or dermal absorption), the
biokinetic parameters for plutonium (distribution, retention, and excretion), the energies and intensities of
the various radiations emitted, and the parameters describing the profile of absorbed energy within the
body. For example, for a person who
inhales a given activity of
239
Pu (measured in becquerel [Bq] or
curies [Ci]), a certain portion is retained and the body will absorb all of the alpha and beta energy emitted
and some of the gamma energy in a pattern reflecting the temporal and spatial (tissue) distribution of the
239
Pu (which might be a function of age), the isotope decay rate, the production and decay rates of the
progeny radionuclides, and radiation energy absorption factors. Each tissue, therefore, receives a tissue-
specific radiation dose. The effective dose reflects the integration of dose
over the time interval of
interest and a tissue weighting factor scheme based on the relative sensitivities of the tissues and organs.
Radiation-induced adverse health effects are related to the extent of molecular damage resulting from
both direct ionization of atoms within range of the emitted radiation energy and interaction of radiation-
produced free radicals with nearby molecules. Tissue damage occurs when the molecular damage is
sufficiently extensive and insufficiently repaired in a timely manner.
Uptake-to-dose conversion factors (dose coefficients) are typically expressed in terms of committed dose
equivalent per unit intake of activity (Sv/Bq). Age-specific dose coefficients for isotope-specific
inhalation and/or ingestion are available in U.S. EPA Federal Guidance Report Number 11 (EPA 1988b);
U.S. EPA Federal Guidance Report Number 13 (EPA 1999) and supplemental CD (EPA 2002);