The Mayo Clinic classifies cobalt values of 5.0 ng/mL or above as "toxic," and levels of 10.0 ng/mL or more as "indicating considerable prosthesis wear with assumed serious tissue loss." As more people are diagnosed with metallosis as a result of failing metal hip implants, there have been several cases reported where patients have suffered severe neurological problems as a result of having high levels of cobalt in their systems. Cobalt is a metal that can become toxic if it is released into the body. It is found in small amounts in many products used daily by most people. However healthy individuals retain relatively little cobalt into adulthood, with an average concentration of about 1.5 mg per liter of blood. Higher concentrations are found in people who work with metals for their jobs (e.g., welders, mechanics), or who are exposed to higher levels of air pollution. The health effects of low levels of cobalt exposure are not known, but there are concerns about its toxicity at higher doses. Hip replacement surgery uses metal materials to replace the hip joint bone and muscle. Metal ions from these devices can leak into the surrounding tissues over time. These metal particles can be carried through the blood stream to other parts of the body, causing adverse reactions there. In particular, metal particles from hip replacements have been found in large quantities in the liver of patients who have them replaced. This problem is called "metallosis".