While cobalt is needed in the diet and is found in trace amounts in many foods, excessive exposure may cause pulmonary distress symptoms (Asthma) and dermatitis. Therefore prudence suggests that ceramic glazes should not leach this metal. Besides, plants obtain cobalt from the soil and process it into cobalamin compounds, quite different than the cobalt materials employed in ceramics.
Inhalation of cobalt fumes during firing can cause metal fume fever.
It can cause allergies, and metabolic disturbances, especially of the thyroid in high doses.
Conclusive TLVs are lacking and it is difficult to say at what levels damage begins to occur. Cobalt and nickel are about equally toxic. One TLV number we have seen for cobalt is 0.02 mg/m3 (nickel compounds is 0.1-0.2 milligrams per cubic meter of air breathed).
There are no therapies for cobalt poisoning other than allowing time for the body to eliminate it.
Tee L. Guidotti, MD, MPH, FRCPC, CCBOM, MFOM, Professor of Occupational and Environmental Medicine, University of Alberta Faculty of Medicine, fax +1 403 492-9677, said this: "Cobalt is one of the more dangerous metal toxicities and prolonged exposure to excessive levels can cause a form of asthma and a type of heart disease. People who work with "hard metal" (a cobalt-based quasi-alloy commonly used for heavy-duty machine cutting tools) are at risk."
A study published in the Scandinavian Journal of Work, Environment and Health, Vol. 22, No. 6, 1996 compared 874 porcelain factory cobalt painting workers with Danish women in general. Little or no increased incidence of cancer was found. Another study published in 1994 (NIOSH-00221545) monitored plate painters who used soluble and insoluble cobalt from 1982-1992. This study found that chromic impaired lung function was related to airborne cobalt levels and that high levels of cobalt in the urine were associated with the use of soluble material. Uptake was higher in females. A 1993 study of decorators in Italy (NIOSH-00213091) concluded that high exposure to cobalt was at times associated with skin sensitization but not clinical dermatitis. A 1988 study in Denmark (NIOSH-00186005) examined workers after a 6 week factory shut down and found that skin, nose, throat, and mouth irritation increased when they resumed work. Cobalt concentrations in the blood increased four-fold to eight times the population average. Considering the excessive cobalt is toxic, this is a concern. In general, improved factory conditions have dramatically reduced the effects of cobalt toxicity.
Some sources say that cobalt is vitamin B-12, however B-12 is a cobalt-based vitamin. The compounds in this group are very complex and are required in the diet in this form. It is an oil-soluble vitamin, and so is stored in the liver of animals. Several year's supply can be obtained from a single meal and stored. Even vegetarians are not likely to become deficient because it is obtained from bacteria, molds in fruits and vegetables, from dairy products, and even in Comfrey.