Scary about the welding incident.
I probably would not use the product, given what is known about the risks, except that I have used it many times before with no side effects.
But it makes you wonder if there could be a cumulative problem.
Even though zinc is an essential requirement for good health, excess zinc can be harmful. Excessive absorption of zinc suppresses copper and iron absorption. The free zinc ion in solution is highly toxic to plants, invertebrates, and even vertebrate fish. The Free Ion Activity Model is well-established in the literature, and shows that just micromolar amounts of the free ion kills some organisms. A recent example showed 6 micromolar killing 93% of all Daphnia in water.
The free zinc ion is a powerful Lewis acid up to the point of being corrosive. Stomach acid contains hydrochloric acid, in which metallic zinc dissolves readily to give corrosive zinc chloride. Swallowing a post-1982 American one cent piece (97.5% zinc) can cause damage to the stomach lining due to the high solubility of the zinc ion in the acidic stomach.
There is evidence of induced copper deficiency at low intakes of 100300 mg Zn/d; a recent trial had higher hospitalizations among elderly men taking 80 mg/day. The USDA RDA is 15 mg Zn/d. Even lower levels, closer to the RDA, may interfere with the utilization of copper and iron or to adversely affect cholesterol. Levels of zinc in excess of 500 ppm in soil interferes with the ability of plants to absorb other essential metals, such as iron and manganese. There is also a condition called the zinc shakes or “zinc chills” that can be induced by the inhalation of freshly formed zinc oxide formed during the welding of galvanized materials.