Cobalt is a known carcinogen and presents a challenge to the immune system. Due to its hardness, cobalt may be used for strengthening dental stainless steel. It may be found in metallic dental appliances such as braces and the chromium-cobalt alloy used in partial dentures. Cobalt oxide may be used as a replacement for cadmium (as a pigment) in pink dental acrylic used in dentures. As well as being carcinogenic, cobalt is a poison. It is slowly released from dental appliances and high blood serum levels can cause long-term cardiovascular disease. Similar health problems to those caused by mercury may also occur. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook.