INTERNATIONAL PROGRAMME ON CHEMICAL SAFETY
WORLD HEALTH ORGANIZATION
SUMMARY OF TOXICOLOGICAL DATA OF CERTAIN FOOD ADDITIVES
WHO FOOD ADDITIVES SERIES NO. 12
The data contained in this document were examined by the
Joint FAO/WHO Expert Committee on Food Additives*
Geneva, 18-27 April 1977
Food and Agriculture Organization of the United Nations
World Health Organization
* Twenty-first Report of the Joint FAO/WHO Expert Committee on Food
Additives, Geneva, 1977, WHO Technical Report Series No. 617
SILVER
EVALUATION FOR ACCEPTABLE DAILY INTAKE
BIOLOGICAL DATA
BIOCHEMICAL ASPECTS
Silver does not occur regularly in animal and human tissues but
is present in man's environment in air, water, soils and food as well
as in specific products. In some marine species silver tends to
accumulate in soft tissue. The shells and soft tissues of
approximately 50 oysters (Crassostrea virginica Gmelin) analysed were
for silver and other elements. The oysters were collected from
10 stations of various salinity ranges along the Georgia coast.
Analysis was carried out by atomic absorption spectrophotometrically.
The precision of the analysis was about ±5. Silver was below
detectability in the shells (i.e. below 1 ppm) while the soft tissues
was 28-82 (±10-20) ppm (Casarett and Doull, 1975; Windom and Smith,
1972).
Silver can be absorbed by the gastrointestinal tract. Retention
is apparently greatest in the reticulo-endothelial organs. After
intravenous injection the concentrations were present in decreasing
order in spleen, liver, bone marrow, lungs, muscle and skin (Browning,
1969).
Various studies and clinical observations indicate that silver
salts can be absorbed from the lungs, gastrointestinal tract and such
insured epithelia as nasal mucosa, conjunctiva, and skin. Absorbed
silver is then stored in the reticulo-endothelial cells of the skin,
mucous membranes, liver, spleen, possibly bone marrow, in basement
membranes, especially those of the renal glomerulus, and presumably in
muscles (Ham and Tangue, 1972; Kanai et al., 1976; Bader, 1966;
Anderson, 1966; Voldrich et al., 1975).
Radiosilver (110mAg) administration to mice, rats, monkeys and
dogs by oral intravenous and intraperitoneal routes was excreted for
more than 90% in the faeces, 90% or more of oral doses were not
absorbed. Whole body retention in mice, rats and monkeys was less than
1% of the initial dose after one week. In the same period less than
10% was retained in dogs (Fnrchner et al., 1968).
The major route of excretion is via the gastrointestinal tract,
predominantly through desquamation of silver containing cells of the
alimentary tract. Urinary excretion has not been reported to occur
even after intravenous injection (Casarett and Doull, 1975; Kent and
Mc Cance, 1941). It seems that even mild degrees of liver damage
considerably impair the ability of the liver to excrete quite small
doses of silver (Petering, 1976). Unlike lead or mercury there is no
evidence that silver is a cumulative poison (Petering, 1976).
No information was obtained on the biotransformation of silver in
the animal body except that absorbed ionic silver is transformed into
metallic while being deposited in tissues (Petering, 1976).
Numerous enzymes were inhibited in vitro by silver ions. High
affinity to sulfhydryl and histidine imidazole groups was observed.
Silver ions compete with molecular oxygen as hydrogen acceptor,
resulting in inhibition of glucose oxydase (Nakamura and Ogura, 1968).
Protargol, a silver-protein complex containing 8% silver
inhibited the in vitro prostaglandin E2 synthesis by bull geminal
vesicles even at concentrations of 10-7M (Deby et al., 1973).
Glutathione peroxydase activity in the liver of rats treated with
76 and 751 ppm silver (as silver acetate) for seven weeks was
respectively 30% and 4% of the control values (Swanson et al., 1974).
After a single s.c. injection (3 mg silver/kg bw) AgNO3 induced
the synthesis of a low molecular weight protein in the liver of rats,
with the characteristics of metallothionein induced by cadmium, zinc
or mercury salts (Winge et al., 1975).
Silver ion is a very toxic substance when viewed from the
standpoint of its action of an inhibitor of enzymes and as a metabolic
inhibitor of lower forms of life. Biochemically, the silver ion (Ag+)
can act as potent enzyme inhibitor (Chambers et al., 1974). It has
been reported (Wagner et al., 1975) that in vitro administration of
silver dramatically decreased liver glutathione peroxidase in rats fed
Se-supplemented diets with or without vitamin E. It seems therefore
that silver acetate exerts its antagonistic effects on Se (silver
induces Se deficiency signs) through an effect on the activity of
biosynthesis of glutathione peroxidase.
Much of the biologic action of silver can be attributed to the
reaction of silver ion with sulfhydryl groups to produce stable silver
mercaptide (Petering, 1976).
Cooper and Jolly (1970) in a review of the ecologic effects of
silver have pointed out that the current experimental practice of
seeding clouds with silver iodide to promote rainfall may lead to new
hazards for both man and natural biologic systems if the practice is
extended (Petering, 1976).
TOXICOLOGICAL STUDIES
Special studies on carcinogenicity
Sarcomas, malignant fibrosarcomas, fibromas, fibro-adenomas and
invasions of muscle with corrective tissue were observed after
implantation of foil, platelets and pellets made of silver or dental
alloy under the skin of mice and rats (Oppenheimer et al., 1956;
Shubik and Hartwell, 1969).
Special studies on mutagenicity
No DNA damaging capacity was observed in a recombination-assay
with AgCl in a Bacillus Subtilis strain (Nishioka, 1975).
Acute toxicity studies
Oral administration of 50 mg AgNO3/kg bw to mice caused death in
50% of the animals in a 14 day observation period (Goldberg et al.,
1949).
Intraperitoneal administration of 2 ml of an aqueous solution
containing 0.239 M AgNO3 to guinea pigs (0.216 g AgNo3/kg bw) was
fatal in 6/10 animals after seven days (Wahlberg, 1965).
Intraperitoneal injection of 20 mg AgNo3/kg bw in rabbits caused
death accompanied by degeneration of liver parenchyma and kidney
tubules. Silver granules were observed in these organs (La Torraca,
1962).
Subcutaneous injection of 7 mg AgNO3/kg bw to rats affected
testis histology and spermatogenesis. After 18 hours the peripheral
tubules were affected and some central tubules were completely
degenerated. Some tubules recovered but not the duct system (Hoey,
1966).
A single dose of 500 mg of colloidal silver was lethal to dogs in
12 hours (Shouse and Whipple, 1931). Prior to death there was
anorexia, weakness, loss of weight, and anaemia. Death was due to
pulmonary congestion and oedema,
Short-term studies
Rats
Rats (90-100 g) were given a 0.25% solution of AgNO3 in
distilled water as drinking water for a period ranging from 1 to 12
weeks. Rats were killed at 1, 2, 3, 4, 8 and 12 weeks and at 1, 2, 3,
6, 10 months and also 16 months after silver administration had
stopped. Deposition of silver in the glomerular basement membrane was
noticed one week after the initiation of treatment electron
microscopically (Ham and Tange, 1972).
1500 ppm Ag1 (as acetate) in drinking water for two to four
weeks caused liver necrosis and death in vitamin E deficient rats. The
effect was prevented by 120 ppm D- -tocophirylacetate and partially by
1 ppm Se (Diplock et al., 1967).
Addition of silver acetate to the diet (130-1000 ppm) or drinking
water (1500 ppm) of weaning rats fed a vitamin E deficient diet,
precipitated a rapidly fatal hepatocellular necrosis and muscular
dystrophy on day 14 of the treatment or subsequently. No changes were
observed in liver of rats given silver acetate and vitamin E
supplements. The mitochondrial changes possessed some of the features
seen in rats fed a diet deficient in vitamin E and selenium. A reduced
availability of selenium by silver in vitamin E deficient rats is
postulated (Grasso et al., 1969).
Rats fed a casein-based diet were given 0.76 and 751 ppm silver
(as acetate) in drinking water for a period of seven weeks. Dietary Se
(0.5 ppm as Na2SeO3) prevented growth depression observed in rats
receiving 76 ppm silver and markedly improved growth and survival of
those given 751 ppm, but increased liver and kidney silver levels.
Liver glutathione peroxidase activity of the treated groups
supplemented with selenium was respectively 30% and 4% of the
controls. Glutathione peroxidase of erythrocytes was not affected
(Swanson et al., 1974).
Cyanocabalamine (3 ppm), vitamin E and selenium (0.05 and 1 ppm)
were found to antagonize silver-induced liver necrosis in rats (Bunyan
et al., 1968).
Rats (six per group) were treated with drinking water containing
0.5, 2 and 20 mg Ag/l for 6-12 months. 2 mg Ag+/l decreased the
nucleic acid level in brain and liver after one year and 20 mg Ag+/l
increased RNA and DNA contents of the brain after six months and
caused dystrophic changes in the brain accompanied by a decrease in
nucleic acid level after 12 months. The liver was less sensitive
towards silver than the brain (Kharchenko et al., 1973).
Groups of eight rabbits received 0, 0.00025, 0.0023, 0.025 and
0.25 mg Ag/kg via their drinking water during 11 months. Marked
effects on immunological capacity (measured as phagocytosis) and
histopathological changes of nervous, vascular and glial tissue of the
encephalon and medulla were observed in the groups receiving 0.025 and
0.25 mg Ag/kg bw. Treatment had no effects on haemoglobin, R.B.C.,
differential W.B.C., proteinogenic function of the liver and serum SH
groups. Rats treated with same amounts of silver showed affected
conditioned reflexes (Barkov and El piner, 1968).
Groups of 20 chicks received 0, 10, 25, 50, 100 and 200 ppm
silver during four weeks in combination with 0, 10 or 25 ppm copper in
the diet. Silver at 100 ppm reduced growth in the copper deficient but
not in the control chicks. At 50 ppm mortality was increased in the
copper deficient group, but not in those receiving copper. 10 ppm
silver reduced the haemoglobin concentration and the elastin content
in the aorta in deficient chicks. These effects were completely
overcome by the addition of copper to the diet (Hill and Matrone,
1970),
Turkey poults given dietary silver (900 ppm of added silver
nitrate) exhibited reduced body weight gain, haemoglobin, packed cell
volume, and aortic elastin content, as well as significantly increased
ratio of wet heart weight to body weight. The enlarged hearts were
attributed to a copper deficiency induced by the dietary silver.
Adding extra copper offset the silver-induced condition (Peterson et
al., 1973; Jensen et al., 1974),
OBSERVATION IN MAN
Absorption of silver resembles whole body retention. It is
retained in all body tissues (Hamilton et al., 1972a; Tripton et al.,
1966). The silver content of the miocardium, aorta and pancreas tends
to decrease with age (Bala et al., 1969) although the amount of silver
in the body increases with age (Hill and Pillsbury, 1939). The
concentration of silver in healthy human tissues from the United
Kingdom was 1-9 µg/kg ash was found. The average silver contents in
wet tissue of normal Americans was about 0.05 µg/kg (Tripton, 1963).
The intake from the diet is estimated at 27 µg/day (Hamilton and
Minski, 1972) up to 88 µg/day (Kehoe et al., 1940).
Silver toxicity is manifested in a variety of forms, some proven
others suspected. Proven forms include: argyria, gastrointestinal
irritation, renal and pulmonary lesions. Suspected forms include,
among others (ill-defined) arteriosclerosis (Casarett and Doull,
1975).
Argyria denotes the slate blue colour observed in parts of the
body of persons exposed chronically to silver (Anderson, 1966).
Epidemiologically, two types of argyria are recognized: industrial
argyria and iatrogenic argyria.
Regardless of type there are two forms of argyria, local and
generalized. The local form involves the formation of grey blue
patches on the skin or may manifest itself in the conjunctiva of the
eye. In generalized argyria the skin shows widespread pigmentation,
often spreading from the face to most uncovered parts of the body. In
some cases the skin may become black with a metallic lustre. Heavy
pigmentation of the eye structures can interfere with vision (Casarett
and Doull, 1975). Except for this adverse effect argyria is solely a
cosmetic problem. The slate blue colour of argyria is not entirely due
as one might suspect, to the deposition of metallic silver (Petering,
1976), but largely to an increased deposition of melanin. Silver has a
melanocyte-stimulating property (Rich et al., 1972). Cases of
generalized argyria have occurred after ingestion or chronic medicinal
application of gram quantities of silver. Silver was absorbed during
prolonged (nine months) nasal application of Targesine (silver
solution). It was calculated that during this time 7000 ml of solution
containing 210 g silver had been used (Voldrich et al., 1975).
After chronic medical and occupational exposure to silver,
argyria and argyrosis are the most common findings. Although
intravenous administration of a total of 0.91-7.6 g (average 2.39)
silver as silver arsphenamine in a period of two to nine years has
caused argyria, hundreds of patients have received up to 1.7 g Ag
(as arsphenamine) without developing argyria.
In argyria silver is regularly deposited in blood vessels,
connective tissue, skin, glomeruli of the kidney, choroid plexus,
mesenteric glands and thyroid. Adrenals, lungs, dura mater, bones,
cartilage muscle and nervous tissue are minimally involved as
deposition sites for silver.
In workers argyrosis of the cornea may be accompanied by
turbidity of the anterior lens capsule and disturbance of the dark
adaptation, usually not resulting in loss of vision.
Argyria is observed only in connexion with occupational medical
exposure or after cosmetic application of silver (Hill and Pillsbury,
1939).
The systemic effects of silver are not extensive because of the
poor absorption of silver compounds from the intestinal tract
(Petering, 1976). It is considered that 10 g of silver nitrate taken
orally is a lethal dose of man, although recovery from smaller doses
has been reported (Cooper and Jolly, 1970). The systemic effects of a
lethal dose are preceded by severe haemorrhagic gastroenteritis and
shock. According to Goodman and Gilman (1965) the silver ion seems
first to stimulate and then depress structures in the brain stem.
Central vasomotor stimulation results in a rise in blood pressure. At
the same time there is bradycardia due to central vagal stimulation.
Death eventually results from respiratory depression.
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