TIN
EXPLANATION
Tin was previously evaluated for tolerable intake for humans
by JECFA in 1966, 1970, 1971, 1975, 1978, and 1982 (Annex 1,
references 11, 22, 26, 38, 47, and 59). A provisional maximum
tolerable daily intake of 2 mg of tin per kg bw was established by
the committee. The committee also noted that 200 ppm or more of tin
in foods and beverages can be expected to produce acute toxic
effects, including abdominal cramps, nausea, and/or diarrhea.
INTRODUCTION
The Codex Committee on Food Additives (1986) indicated that
there is a need to clarify the information presented in the 26th
report, as well as to provide more information about the chemical
forms of tin which cause gastric disturbances. The following is a
reexamination of available human data. Although no very recent
studies are available, several pertinent reports published in 1975
or earlier are summarized. Because of the nature of this problem,
the information presented is limited to data on humans.
DIETARY EXPOSURE
Approximately 50% of the world production of tin is used for
plating. Tin coatings are generally corrosive resistant and easy to
solder and hence have been used extensively for food containers and
food processing equipment. Tin has also been used in the production
of tin alloys, such as solders, bronzes and pewters which may be
used in the manufacture of food containers. Inorganic tin compounds
are used as pigments in the ceramic and textile industry. Organo-
tin compounds are used as heat stabilizers in the production of PVC
plastics, curing agents for silicon rubber, and catalysts in the
production of polyurethane. Triorganotins are used as fungicides,
miticides, insecticides and bactericides (Friberg et al., 1986;
Reilly, 1980; WHO, 1980).
The combustion of fossil fuels contributes to the amount of
tin in air. Excluding point source industrial emissions, tin levels
in air have been found to be generally less than 0.3 µg/m3 and the
intake of this metal from air would therefore be less than
6 g/person/day (Friberg et al., 1986; WHO, 1980; Ministry of
Agriculture, Fisheries and Food, 1985).
For the general population, drinking water is not a
significant source of tin. When found, tin levels averaged 6 µg/l
and consumption of 1.5 to 2.0 l/day would result in a daily intake
of 9 to 12 g tin/person (Friberg et al., 1986; WHO, 1980;
Ministry of Agriculture, Fisheries and Food, 1985).
In most unprocessed foods, tin levels are generally less than
1 µg/g. Higher concentrations are found in canned foods as a result
of dissolution of the tin coating or tin plate. The concentration
of tin in canned foods depends on a number of factors, including
the type and acidity of the food, time and temperature of storage,
and the presence of air in the can headspace. Oxidizing agents such
as nitrates and iron and copper salts, certain pigments, and
sulphur compounds all accelerate dissolution of tin. Other
substances, such as tin salts, sugars, and colloid like gelatin,
retard detinning. Lacquering of cans also reduce corrosion and
prevent detinning. Tin concentrations in foodstuffs in unlacquered
cans frequently exceed 100 µg/g, while in lacquered cans, tin
levels are generally below 25 µg/g. Storing foods in opened
unlacquered cans results in substantial increases in the tin
concentration of the food. Given all of these factors, it becomes
apparent why tin levels found in canned foods are quite variable
(Reilly, 1980; Ministry of Agriculture, Fisheries and Food, 1985;
Tipton et al., 1966; Schroeder et al., 1964; Dick et al., 1978).
The intake of tin by different segments of the population is
dependant on the type(s) and amount of canned food ingested and its
tin level. For example, a 60 kg adult consuming 1 (one) litre of
juice containing 100 µg/g tin, would ingest 1.7 mg/kg bw of tin
whereas a 20 kg child consuming 1/2 litre of the same juice would
ingest 2.5 mg/kg bw.
In summary, food and in particular canned food, represents the
major route of human exposure to tin, Intake levels from this
source can vary widely and for some segments of the population can
reach several mg/kg bw.
BIOLOGICAL DATA
Observations in man
Warburton et al. (1962) reported that 31 persons suffered
from nausea, abdominal cramps and/or vomiting within 1-2 hours
after drinking a fruit punch containing 2,000 ppm tin. The
reconstituted pineapple-grape-fruit juice used to make the punch
had been delivered in a 5-gallon retinned milk container. No
information was available as to when the juice was processed or how
long and in what kind of container it was stored before delivery.
However the lining of the container in which the juice was
delivered showed obvious signs of corrosion, probably due to the
unusually high acidity (pH=3) of the juice. The illness reportedly
lasted from 2 to 48 hours after onset. Omori (1966) reported that
a canned orange-based drink containing 425 ppm tin produced a
similar outbreak of intoxications, manifested by nausea, vomiting,
diarrhea, fever and headache. Most of the 1,838 affected persons
recovered within a day or two. The inside surface of the can in
which the juice was stored was covered by a thin black rusty layer
indicative of corrosion (Omori et al., 1973).
Nausea, vomiting and diarrhea were observed in a large,
unspecified number of persons in Kuwait who consumed formulated
orange juice and apple juice containing 250-385 ppm tin (Metal Box
Co., Ltd., unpublished report, 1967, cited in Benoy et al.,
1971). Kojima (1969) reported 8 cases of poisoning from tomato
juice containing 247 ppm tin. Similarly, vomiting, diarrhea and
other signs of distress were observed in 15 students who had
consumed a canned orange beverage containing 100-494 ppm tin, and
in 8 other persons that had consumed tomato juice containing
156-221 ppm tin (Horio et al., 1967).
In 97 well documented cases, severe abdominal bloating,
vomiting and diarrhea were noted after the consumption of canned
tomato juice with tin levels ranging from 141 to 405 ppm (Barker &
Runte, 1972). The mean tin concentrations ranged from 245 to 363
ppm in the various lots implicated as the cause of these
intoxications. The cans were visibly detinned, an effect which was
attributed to unusually high nitrate levels on the tomatos used to
make the juice. Excessive nitrate fertilization of tomato plants
was related to complete corrosion of the can lining within 6 months
of storage, yielding tin levels of 381-477 ppm (mean = 418 ppm),
compared to 29-81 ppm (mean = 50 ppm) in juice from non-corroded
cans.
Similar cases have been reported after consumption of canned
cherries, asparagus, herrings, and apricots (Schryver, 1909). For
example, Luff & Metcalfe (1890) found that canned cherries
containing tin at 3,430 ppm or more produced gastrointestinal
distress in 4 persons within 0.5-2 hours after consumption. The
juice of the canned cherries was strongly acid due primarily to the
presence of malic acid. Pickled herrings preserved in vinegar and
implicated in another outbreak were found to contain 1030 ppm tin
in the solids and 316 ppm tin in the liquid (Gunther, 1899, cited
in Schryver, 1909). Savage (1939) cited a report of nausea, vomiting
and abdominal pain in four members of one family occurring 0.5-3
hours after the consumption of canned apricots containing 800 ppm tin.
Nausea and vomiting were noted after consumption of peach
preserves containing 563 ppm tin (Nehring, 1972, cited in WHO,
1980). The food sample also contained 93 ppm nitrate, as well as
1.7 ppm nitrite, 1.5 ppm zinc, 0.1 ppm cadmium, 0.16 ppm lead, 1
ppm copper, and 115 ppm chloride. Also, about half of the 85
persons answering a questionnaire reported nausea, vomiting and
diarrhea occurring within 1 hour after consuming canned peaches in
which the fruit contained 413-597 ppm tin (mean = 533 ppm) and the
juice contained 298-405 ppm tin (mean = 369 ppm) (Svensson, 1975,
cited in WHO, 1980). Ungar & Bodlander (1887, cited in Schryver,
1909) found that canned asparagus containing 300-4000 ppm tin was
associated with gastrointestinal symptoms. Furthermore, Kwantes
(1966, cited in Benoy et al. 1971) reported a few small outbreaks
of poisoning related to the consumption of solid foods (salmon,
fruit salad and rhubarb) in which tin levels ranged from 250 to 650
ppm. Canned fruit containing as much as 250 ppm (212-250 ppm) tin
reportedly produced no ill effects in 4 subjects (Momontova, 1940,
cited in WHO, 1980).
In contrast to the findings described above, Dack (1955)
reported that no ill effects were produced in 4 subjects consuming
canned pumpkin containing 383-476 ppm tin and canned asparagus
containing 361 ppm tin. The average daily ingestion of tin in this
study ranged from 426 to 490 mg.
No toxic effects were observed in several studies in which
the juice or food was processed specifically to raise tin
concentrations up to 703 ppm. For example, Benoy et al. (1971)
pooled orange juice samples implicated in the Kuwait incident,
reportedly containing 250-385 ppm tin, and then concentrated the
juice under vacuum to yield a tin level of 498 ppm. Five volunteers
showed no toxic effects after drinking a volume of the concentrated
juice which provided 1.59-2.44 mg of tin per kg bw. Likewise, no
toxic effects were produced after these volunteers consumed tin at
1.73-3.58 mg/kg bw in orange juice in which the tin levels were
increased to 540 ppm or 730 ppm by repeatedly aerating the juice,
adding it cold to cans with a large headspace and then pasteurizing
the cans in a rotating retort. Adding 50 ppm nitrate to the juice
containing 730 ppm tin did not enhance its toxicity. However,
orange juice in which the tin level was increased to 1370 ppm by
the pasteurization method and consumed in a volume which provided
4.38-6.71 mg tin per kg bw, produced nausea and/or diarrhea. Only
one case of nausea recurred when the trial was repeated 1 month
later. The apparent development of tolerance observed in this study
remains to be explained.
No adverse effects were noted in 9 volunteers fed a C-ration
diet consisting of canned fruits and meats which had been stored
for 20 months at 37°C (Calloway & McMullen, 1966). The tin content
in the food ranged from 254 to 538 ppm, expressed on the basis of
dry solids. It was estimated that these subjects consumed an
average of 163 mg of tin per day (ranging from 116 to 203 mg/day)
and that virtually all of the ingested tin was recovered in the
feces. Nitrogen absorption from the gastrointestinal tract was
decreased, suggesting that the tin in the food was associated with
protein as an indigestible complex, as previously proposed by Goss
(1917). C-rations stored at 1°C for 20 months contained less than
50 ppm tin, and also produced no adverse effects in the volunteers.
COMMENTS AND EVALUATION
It is clear that different types of canned foods are consumed
in varying amounts and that levels of tin are higher in unlacquered
cans than in lacquered cans. Infants and small children, on a body
weight basis, are more likely than adults to consume higher levels
of tin from a single source, i.e. canned juice. Furthermore, it is
acknowledged that factors other than tin levels may play a role in
potentiating adverse effects and yet others may serve to moderate
potential toxic effects. In the absence of more specific details
pertaining to these factors as well as information on the chemical
forms of tin which cause acute gastric irritation, it was not
possible to incorporate such considerations into the evaluation of
this matter. It was concluded from the limited human data available
that tin concentrations as low as 150 µg/g in one incident
involving canned beverages and 250 µg/g in other canned foods may
produce acute manifestations of gastric irritation in certain
individuals; however, it was also noted that some canned products
containing levels up to 700 µg/g of tin produced no detectable
toxic effects.
Therefore, the Committee recommended that efforts be made to
keep tin levels in canned foods as low as practicable. In this
regard, tin concentrations in canned foods should be limited to
those consistent with the application of good manufacturing
practices (FAO, 1986). The Committee converted the previously
established tolerable daily intake of 14 mg of inorganic tin per kg
of body weight into a PTWI and emphasized that this value was
applicable to chronic tin exposure.
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