So much is said
about the toxicity of zinc, I thought to look up some facts.
As there is significant concern about health issues, it is useful to look in
detail at the health and safety issues around the use of zinc at elevated
temperatures. Zinc is absorbed into the
body by inhalation of fumes and consumption of zinc containing materials.
Toxicity
Although zinc is an essential requirement for good health, excess zinc
can be harmful. Excessive absorption of zinc suppresses copper and iron
absorption … [which results in the symptoms of zinc intoxication]. Stomach acid contains hydrochloric
acid, in which metallic zinc
dissolves readily to give corrosive zinc chloride. … The U.S. Food
and Drug Administration
states that zinc damages nerve receptors in the nose, causing [loss of smell].
Evidence shows that people taking
100–300mg of zinc daily may suffer induced copper
deficiency. … Levels of 100–300mg may interfere with the utilization
of copper and iron or adversely affect cholesterol. … A condition called
the zinc shakes or
"zinc chills" can be induced by inhalation of zinc fumes while brazing or
welding galvanized materials.
Poisoning
Consumption of zinc can result in
death, but requires large amounts (over 1 kg in one case). Smaller amounts result in lethargy and gross lack of
coordination of muscle movements or apparent intoxication. https://en.wikipedia.org/wiki/Zinc
Research and W.H.O. Information
The Essential Toxin: Impact of
Zinc on Human Health, by Laura M. Plum, Lothar Rink, and Hajo Haase*
Compared
to several other metal ions with similar chemical properties, zinc is
relatively harmless. Only exposure to high doses has toxic effects, making
acute zinc intoxication a rare event. In addition to acute intoxication,
long-term, high-dose zinc supplementation interferes with the uptake of copper.
Hence, many of its toxic effects are in fact due to copper deficiency. While systemic
[balance] and efficient regulatory mechanisms on the cellular level generally
prevent the uptake of [cell destructive] doses of [environmental] zinc, … zinc [within
the body] plays a significant role in cytotoxic [death of individual cells] events
in single cells. … One organ where zinc is prominently involved in cell death
is the brain, and cytotoxicity in consequence of [inadequate blood supply] or
trauma involves the accumulation of free zinc.
Rather
than being a toxic metal ion, zinc is an essential trace element. Whereas
intoxication by excessive exposure is rare, zinc deficiency is widespread and
has a detrimental impact on growth, neuronal development, and immunity, and in
severe cases its consequences are lethal. Zinc deficiency caused by
malnutrition and foods with low bioavailability, aging, certain diseases, or
deregulated homeostasis [equilibrium] is a far more common risk to human health
than intoxication.
Conclusions
Zinc is an essential trace element, and the human body
has efficient mechanisms, both on systemic and cellular levels, to maintain [balance]
over a broad exposure range. Consequently, zinc has a rather low toxicity, and
a severe impact on human health by intoxication with zinc is a relatively rare
event.
Nevertheless, on the cellular level zinc impacts
survival and may be a crucial regulator of [the death of cells
occurring as a normal and controlled part of an organism's growth or
development] as well as neuronal death following brain
injury. Although these effects seem to be unresponsive to nutritional
supplementation with zinc, future research may allow influencing these
processes via substances that alter zinc [balance] instead of directly giving
zinc.
Whereas there are only anecdotal reports of severe
zinc intoxication, zinc deficiency is a condition with broad occurrence and
potentially profound impact. Here, the application of “negative zinc”, i.e., substances or
conditions that deplete the body of zinc, constitute a major health risk. The
impact ranges from mild zinc deficiency, which can aggravate infections by
impairing the immune defence, up to severe cases, in which the symptoms are
obvious and cause reduced life expectancy.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2872358/
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| Zinc came Credit: leadandlight.co.uk |
World Health Organisation Document
10.2.2 Occupational exposure
Occupational exposure to dusts
and fumes of zinc and zinc compounds can occur in a variety of settings in
which zinc is produced, or in which zinc and zinc-containing materials are
used. Typical airborne exposures observed include 0.19–0.29 mg/m3 during the
smelting of zinc-containing iron scrap, 0.90–6.2 mg/m3 at non-ferrous foundries
and 0.076–0.101 mg/m3 in hot-dip galvanizing facilities. Far higher exposures
are possible during particular job activities, such as welding of zinc-coated
steels in the absence of appropriate respiratory protection and/or fume
extraction engineering controls.
Occupational exposure to high
levels of zinc oxide and/or nonferrous metals is associated with metal-fume fever. [a condition in which the sufferer has influenza type symptoms - a
raised temperature, chills, aches and pains, nausea and dizziness. It is caused
by exposure to the fume of certain metals - commonly zinc]. This is usually a short-term, self-limiting
syndrome…. Induction of metal-fume fever is most common with ultra-fine
particles capable of deep lung penetration under conditions of exposure.
Studies on volunteers conducted under short-term exposure conditions (77–153
mg/m3 for 15–30 min) have detected pulmonary inflammation responses (including [inflammation]
induction) which are consistent with manifestations of metal-fume fever and
support an immunological [cause] for this acute reversible syndrome.
Evaluation
Based on the available
information, it is not possible to define a no-effect level for pulmonary
inflammation from exposure to zinc oxide fume.
10.2.4 Risks of zinc excess
Toxic effects in humans are most
obvious from accidental or occupational inhalation exposure to high
concentrations of zinc compounds, such as from smoke bombs, or metal-fume
fever. Modern occupational health and safety measures can significantly reduce
potential exposure. Intentional or accidental ingestion of large amounts of
zinc leads to gastrointestinal effects, such as abdominal pain, vomiting and
diarrhoea.
In the case of long-term intakes
of large amounts of zinc at pharmacological doses (150–2000 mg/day), the
effects (sideroblastic anaemia [inability to make haemoglobin], leukopenia [low
white cell quantities] and hypochromic microcytic anaemia [iron deficiency])
are reversible upon discontinuation of zinc therapy and/or repletion of copper
status, and are largely attributed to zinc-induced copper deficiency.
High levels of zinc may disrupt
the [balance] of other essential elements. For example, in adults, subtle
effects of zinc on copper utilization may occur at doses of zinc near the
recommended level of intake of 15 mg/day and up to about 50 mg/day. Copper requirements
may be increased, and copper utilization may be impaired with changes in
clinical chemistry parameters, but these effects are not consistent and depend
largely upon the dietary intake of copper. Distortion of lipoprotein metabolism
and concentrations associated with large doses of zinc are inferred to be a
result of impaired copper utilization. In groups with adequate copper intake,
no adverse effects, with the exception of reduced copper retention, have been
seen at daily zinc intakes of [less than] 50 mg/day. There is no convincing evidence
that excess zinc plays a [casual] role in human carcinogenesis. The weight of
evidence supports the conclusion that zinc is not genotoxic [damaging of
genetic information in cells] or teratogenic [affecting the development of
embryos]. At high concentrations zinc can be cytotoxic [toxic to cells]. https://www.who.int/ipcs/publications/ehc/221_Zinc_Part_3.pdf?ua=1
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| zinc sheet Credit: Belmont Metals |
Use and Risks of Zinc in Kilnforming
Zinc melts at
420°C and boils at 907°C, so any fumes will be emitted only around and above
the full fusing temperature of glass.
The main problem
in kilnforming is that the metal melts at such a low temperature that it is not
useful for containing the glass.
There is anecdotal
evidence to indicate that firing zinc contaminates the kiln, leading to subsequent
devitrification issues. This can be
cleared by firing bentonite at high temperature in the kiln to absorb the zinc.
It is not a high-risk
metal, even if it were to vaporise (above 900°C).
Research papers
show zinc poisoning to be extremely rare. It is usually associated with taking
too large daily doses of zinc as a dietary supplement, or swallowing USA
pennies - made largely of zinc - which dissolves in stomach acid and creates
large problems for the digestive system. Where zinc intoxication occurs,
it is largely reversible.
Conclusion
The idea that
zinc will poison you in kilnforming conditions is simply not correct.
