Some environmental factors have a clear role in triggering autoimmune diseases. For example, procainamide can cause lupus-like syndrome in genetically susceptible individuals, and when the drug is discontinued, the symptoms disappear. Some other drug-induced autoimmune diseases include hemolytic anemia and thrombocytopenic purpura. Studies on the role of metals in autoimmune diseases have been conducted mainly in the laboratory and in animals. Typically, metals inhibit the proliferation and activation of immune cells, although there are exceptions, such as mercury, gold, and silver that induce lymphocyte proliferation and autoimmune responses. Lupus-like syndrome occurs in genetically susceptible rats given mercury, gold or silver. However, the mechanisms of autoimmune disease that occur after exposure to different metals are different. It is possible that the abnormal immune response is due to a deficiency of a substance. For example, selenium deficiency has been associated with autoimmune thyroiditis and cardiomyopathy, and when some patients were supplemented with selenium, there was improvement in symptoms. Some epidemiological studies suggest that certain factors are associated with disease, such as dietary iodine and thyroiditis, silica and scleroderma, and lupus erythematosus. Polychlorinated biphenyls have been associated with scleroderma and lupus erythematosus. However, there is little evidence for trichloroethylene. Others such as dioxins, pesticides, and estrogens still need further evidence. Others such as UV light and infection are also important causative factors.