Compost made from yard wastes, such as leaves and grass clippings, rarely contains any harmful materials. Problems can occur, however, when compost is made from partially sorted municipal refuse, certain industrial wastes, or sewage sludge. In those cases, unacceptable levels of toxic metals, chemicals, or harmful bacteria may be present.
To protect the public, the federal Environmental Protection Agency (EPA) sets acceptable levels for thousands of materials that might be present in compost. Each state may have its own standards as well. For municipal refuse, source separation—that is, having homeowners sort their yard wastes into separate containers rather than throw them away with the rest of their trash—is felt to be one of the most effective way to produce clean, safe compost.