@jrobertson@microca.st good point. The FAQ at https://www.kickstarter.com/projects/546469190/the-urban-death-project-laying-our-loved-ones-to-r includes

We are currently working with our soil scientist advisors to better understand how heavy metals, pharmaceuticals, and prion diseases can be safely addressed within our system.
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Non-organic materials such as prostheses, metal plates, or amalgam fillings will be screened out and recycled.