Min - thank you so much for those links! Time and again I am reminded that you really need to go back to the data and how/where it was collected before making a determination about things. For example, I've never quite understood the relative risk of using manganese dioxide in a glaze, and this paragraph from the longer version is quite illuminating:
An example of ceramic glaze (and body) folklore are the reports surrounding the use of manganese (Mn) and its link to Parkinson’s Disease. Mn in a soluble form is a neurotoxin, but this data was generated from the steel industry and the conditions of exposure are dramatically different due to the vaporization of Mn during steel production (and the use of a furnace open to the local environment, that is, without a stack). Inhalation of these vapors creates a readily available reactive species for adverse interactions with the body (and internally through the lungs and mucus membranes). Even small amounts can be readily absorbed into the bloodstream. The oxides of manganese, MnO and MnO2, are both insoluble in water and do not pose a threat. The Mn is not soluble and therefore not available, cannot be absorbed through the skin or even an open cut, and even if ingested, cannot dissolve into the blood stream. Mn obtained from a salt (chloride, nitrate, or sulfate) is soluble, so if the source of Mn is a salt, exercise appropriate care.