What is the least toxic insulation?

Online Answer
Non-Toxic Mineral Wool Brands Rockwool (formerly Roxul) is the most common and accessible mineral wool insulation. They have a newer version called AFB, that does not contain formaldehyde. The binder is an undisclosed polymer (a plastic, likely acrylic), it also contains mineral oil.
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Styrene is a chemical found in polystyrene plastics, better known as Styrofoam materials. Among other additions to the list are formaldehyde, aristolochic acids, and certain glass and wool fibers found in insulation.