Oxygen, a necessity for life, like water, is best taken with nitrogen, because pure it can lead to excessive tissue oxidation, even combustion. So toxicology is generally relative. Most of the metals we are concerned about are required by the body in trace amounts. Barium, for example, seems to have a place in nerve chemistry. Sodium and potassium are ubiquitous and highly tolerated, although I tend to rip the roof off my mouth with excess salt when I eat corn on the cob.
Potassium is a key synapse carrier.
Iron, of course, is used as an oxygen carrier in the blood haemoglobin. It seems that we grew up in this soup of elements, and our bodies have evolved the means to keep most of them in proper balance, according to the concentrations normally encountered in our ecosystem. It is when the balance gets disturbed that things happen, and we get problems. Our glazes and clay bodies are concentrated substances. This is the primary source of danger.