Optimal calcium intakes are desirable for prevention or treatment of osteoporosis, but the safety and efficacy of some calcium supplements used to achieve such intakes have been questioned. Lead contamination is not restricted to bone-meal and dolomite; significant amounts of lead, and also aluminum, have been found in calcium carbonate supplements labeled "oyster shell" or "natural source." While no evidence for in vivo toxicity has emerged, chronic use of these supplements may constitute unnecessary metal exposure.