Zinc Health and Safety

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. 

https://en.wikipedia.org/wiki/Zinc

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/

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

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.