Perhaps because they come from the world’s biggest toxic dump site - the ocean...
Here you go:
This sculptor got heavy metal poisoning from working with mussel shells
https://www.cbc.ca/radio/asithappen...ing-from-working-with-mussel-shells-1.4930128
More:
Transplanted or caged mussels have previously been used in Argentina, the Mediterranean Sea, Boston Harbour, and the South Pacific to identify areas of metal pollution.
Overall, the assessment that the investigated toxic elements may pose a health risk to heavy mussel consumers, especially related to the levels of Pb and Cd in M. galloprovincialis from hot spots in the all investigated Seas. Although consumption of these mussels provides proteins, essential minerals and vitamins, and thus, some protection from certain diseases, the risks and benefits of their consumption are still hard to assess because of the metals bioacumulated from the marine environment, with their reviewed toxicity. The final conclusion of this review is that M. galloprovincialis is an excellent candidate for aquaculture but that extreme care must be taken when choosing the location of farms. Care should also be taken by coastal populations when consuming wild mussels from contamination hot spots.
I don't think oyster shells are suitable for calcium supplementation, due to heavy metals in them.
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