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tubular secretion and resorption capacities (Altman and Dittmer 1974; NRC 1993; West et al. 1948).
Children and adults may differ in their capacity to repair damage from chemical insults.
Children also
have a longer remaining lifetime in which to express damage from chemicals; this potential is particularly
relevant to cancer.
Certain characteristics of the developing human may increase exposure or susceptibility, whereas others
may decrease susceptibility to the same chemical. For example, although infants breathe
more air per
kilogram of body weight than adults breathe, this difference might be somewhat counterbalanced by their
alveoli being less developed, which results in a disproportionately smaller surface area for alveolar
absorption (NRC 1993).
Numerous epidemiological studies of ionizing radiation exposures have found higher cancer risks
associated with exposures of infants and
children infancy and childhood, compared to adults (Agency for
Toxic Substances and Disease Registry 1999). Although there is no direct evidence for increased
susceptibility of children to toxicity from plutonium, several kinds of observations made in animals
suggest that immature animals may be more vulnerable to plutonium as a result of higher deposition of
absorbed plutonium on bone surfaces and higher turn-over of bone. Studies conducted in immature
beagles (inhalation exposures to
239
PuO
2
at age 2.6–3.6 months) showed that, in comparison to similar
exposures of adult beagles, a larger fraction of the initially deposited lung burden was transferred to the
skeleton (DOE 1988d, 1989). This observation is consistent with the results from injection studies. A
comparison of bone distribution of plutonium in juvenile beagles (3 months of age),
compared to young
adult (17–20-month-old) and mature (60-month-old) beagles that received a single injection of plutonium
citrate showed that skeletal deposition (percent of dose) was higher in juveniles, occurred more
extensively in growing limb bones, and within bone, a larger portion of the bone burden was associated
within bone volume (Bruenger et al. 1991a). As discussed in Section 3.5, Mechanisms of Toxicity, these
observations are consistent with the concept that plutonium preferentially deposits in regions adjacent to
red marrow, which has a wider distribution in juveniles than
in adults, and is more prominent in
trabecular bone than in cortical bone, and in bones of the axial skeleton. High bone turn-over in juveniles
may also contribute to more rapid distribution of plutonium from bone surface to bone volume as a result
of burial of surface deposits, uncovering buried deposits, and recycling of the plutonium between marrow,
bone, and blood (Bruenger et al. 1991a; Leggett 1985; Vaughan et al. 1973).
These observations suggest
the possibility that children may have a higher susceptibility to bone marrow toxicity and related
outcomes (e.g., leukemia) and skeletal toxicity of plutonium than adults, although this has not been
verified either experimentally, or in epidemiological studies. One observation that may be pertinent is
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that dogs, examined 5 years after an inhalation exposure to
239
PuO
2
at age 2.6–3.6 months,
showed a
lower incidence of radiation pneumonitis than dogs exposed as adults. This would be consistent with a
greater transfer of plutonium from the lung to the skeleton. Lung tissue growth in the younger dogs may
have resulted in some lung tissue with little or no plutonium.
Gastrointestinal absorption of ingested plutonium is higher in neonatal animals compared to mature
animals, which may reflect a more permeable gastrointestinal tract in neonates or physiological
adjustments in neonates related to nutrient (e.g., iron) absorption that affect plutonium uptake.
Absorption has been shown to be 10–1,000 times greater in
neonates compared to adults, depending on
animal species and chemical form of plutonium (Sullivan 1980a, 1980b; Sullivan and Gorham 1983;
Sullivan et al. 1985). Iron deficiency increases absorption in juvenile rats (Sullivan and Ruemmler 1988).
However, available animal data have not demonstrated that increased plutonium uptake by neonatal and
juveniles results in increased susceptibility to the toxic effects of internalized plutonium.
3.8 BIOMARKERS OF EXPOSURE AND EFFECT
Biomarkers are broadly defined as indicators signaling events in biologic systems or samples. They have
been classified as markers of exposure, markers of effect, and markers of susceptibility (NAS/NRC
1989).
A biomarker of exposure is a xenobiotic substance or its metabolite(s) or the
product of an interaction
between a xenobiotic agent and some target molecule(s) or cell(s) that is measured within a compartment
of an organism (NAS/NRC 1989). The preferred biomarkers of exposure are generally the substance
itself, substance-specific metabolites in readily obtainable body fluid(s), or excreta. However, several
factors can confound the use and interpretation of biomarkers of exposure. The body burden of a
substance may be the result of exposures from more than one source. The substance being measured may
be a metabolite of another xenobiotic substance (e.g., high urinary levels of phenol can
result from
exposure to several different aromatic compounds). Depending on the properties of the substance (e.g.,
biologic half-life) and environmental conditions (e.g., duration and route of exposure), the substance and
all of its metabolites may have left the body by the time samples can be taken. It may be difficult to
identify individuals exposed to hazardous substances that are commonly found in body tissues and fluids
(e.g., essential mineral nutrients such as copper, zinc, and selenium). Biomarkers of exposure to
plutonium are discussed in Section 3.9.1.