So is just about everything Rob,even water is sufficient levels of ingestion.
For industrial purposes materials / chemicals have an LD50 rating. a figure at which such a level of poisoning with result in the death of 50% of those affected.(the median mortality rate or Lethal Dose rate)
It's usually expressed in weight of chemical per unit of body mass, for example Caffiene is around 150 Mg per KG of body mass (about 100 cups in a day)
For ZnSe it's 5,000 Mg per KG. It's thermal breakdown only occurs above 400 degrees C and at that point it produces irritant fumes.
If you need gloves to handle lens's then your going to need a full NBC suit along with a HazMat team to open a jar of coffee