Creating appropriate production processes also depends on understanding the kinds of materials cycles possible. In Fig. 13, the large loop represents natural cycles. Cell 2 is the human economy, which has exchanges with nature, but also material flows that are isolated from natural systems. By rights, the materials interacting with nature what McDonough & Braungart (1998; McDonough & Braungart, 2002) call biological nutrientsshould be almost completely benign and biodegradable, and those that flow continuously within the economy without cycling back to nature should be the most toxic. McDonough & Braungart call the latter technical nutrients that should be well sealed in their closed loops.