Cobalt - Toxicology of Metals - page 503

cleared within 3 days with a biological half-time of 53
hours; the second phase cleared with a half-time of 156
hours.
Clearance from the lung depends on solubility. Krey-
ling et al . (1990) studied in vitro dissolution of cobalt
oxide particles by human and canine alveolar macro-
phages. Smaller particles had faster dissolution rates.
In vitro dissolution rates were found to be similar to
in vivo translocation rates previously found for human
and canine lung. Dissolution of ultrafi ne cobalt pow-
der in simulated body fl uid was six times higher than
that of standard cobalt powder (Kyono et al ., 1992).
In vivo , nontoxic doses of cobalt metal were adminis-
tered intratracheally to rats either alone (0.03 mg cobalt
/100 g body weight [bw]) or mixed with tungsten
carbide (0.5 mg WC-Co containing 6% of cobalt metal
particles/100 g bw). Sequential measurements of cobalt
in the lung and urine demonstrated that the retention
of the metal in the lung was longer in cobalt than in
WC-Co-treated animals (12 vs 5

5.1.2 Humans

5.1.2.1 Inhalation
There are few data on the respiratory absorption
of inhaled cobalt-containing materials in humans.
Increased excretion of the element in postshift urine
of workers exposed to soluble cobalt-containing
particles (cobalt metal and salts, hard-metal parti-
cles) has been interpreted as an indirect indication
of rapid absorption in the lung; in contrast, when
workers were exposed to less-soluble cobalt oxide
particles, the pattern of urinary excretion indi-
cated a lower absorption rate and probably a longer
retention time in the lung (Lison et al ., 1994). The
importance of speciation and solubility for respira-
tory absorption has also been highlighted by Chris-
tensen and Poulsen (1994), who found that cobalt
in blood and urine was increased in pottery plate
painters who used a soluble pigment, whereas only
slightly increased values were found in those using
an insoluble pigment (see Section 6).

g cobalt/g lung after
24 hours) (Lasfargues et al ., 1992). This increased bio-
availability of cobalt from hard-metal particles has
been interpreted as the result of the physicochemical
interaction between cobalt metal and tungsten carbide
particles (Lison et al ., 1995; Section 1).
By use of autoradiography and gamma- spectrometry,
Persson et al . (2003) examined the transport of radiola-
beled Co(II) in the olfactory system after intranasal
administration as cobalt chloride in rats. The results
showed an uptake of the metal in the olfactory mucosa
and a transport to the olfactory bulbs of the brain.

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5.1.2.2 Digestive
The gastrointestinal absorption of cobalt compounds
is reported to vary from 5-45% in different individuals.
Ripak (1961) studied cobalt balance in three school chil-
dren, and their daily intake was calculated to be 41 g.

Of this fi gure, 92-95% was found in feces and 4-5% in
urine. When 60 Co as CoCl 2 was given orally to humans,
the average absorption was calculated at 5-44%; these
fi gures are based on the amount of unabsorbed cobalt
remaining in the feces (Smith et al ., 1972; Valberg et al .,
1969). On the basis of 24-hour urinary excretion stud-
ies after oral administration of 60 Co, absorption has
been estimated to be on average 18%, individual val-
ues ranging from 9-42% (Sorbie et al ., 1971). Absorp-
tion and/or excretion is infl uenced by the amount of
cobalt given and by nutritional factors. For example,
the 48-hour urinary excretion of labeled cobalt chloride
given orally ranged between 1 and 16%, depending
on the amount of cobalt given, the fasting or nonfast-
ing state of subjects, or the addition of albumin to
the diet (Paley and Sussman, 1963). In a short-term
crossover study in volunteers, the gastrointestinal
uptake of soluble cobalt chloride was found to be con-
siderably higher than that of insoluble cobalt oxide
(urine ranges, <0.17-4373 and <0.17-14.6 nmol/mmol
creatinine, respectively). It was also shown that inges-
tion of controlled amounts of soluble cobalt compounds
resulted in signifi cantly higher urinary cobalt levels
( P < 0.01) in women (median, 109.7 nmol/mmol creati-
nine) than in men (median, 38.4 nmol/mmol creatinine),
suggesting that the gastrointestinal uptake of cobalt is
higher for women than men (Christensen et al ., 1993).

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5.1.1.2 Digestive
Gastrointestinal absorption of cobalt depends on
the type of compound given, dose, and nutritional
status. Less than 0.5% of CoO given orally to ham-
sters at a dose of 5 mg is absorbed (Wehner and Craig,
1972), whereas gastrointestinal absorption of cobalt
chloride given to rats is estimated to be approximately
30% (Carlberger, 1961; Comar and Davis, 1947; Taylor,
1962). The gastrointestinal absorption of radiolabeled
cobalt chloride in rats was found to vary between
11 and 34%, depending inversely on the administered
dose (0.01-1000 g/rat) (Taylor, 1962). Both amino

acids and sulfhydryl groups complex with cobalt
ions and reduce absorption. Iron defi ciency increases
the absorption of cobalt, and simultaneous adminis-
tration of iron and cobalt reduces cobalt absorption
(Schade, 1970). Some data also indicate that the gas-
trointestinal absorption is more effi cient in young rats
(98 and 85% at days 1 and 20, respectively) (Taylor
and Harrison, 1995). After oral administration, the
largest concentrations are found in liver, kidney, and
spleen (Domingo, 1989).

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