Exposure to cobalt can give rise to adverse health effects related to various organs or tissues, such as the respiratory organs, the skin, the hematopoietic tissues, the myocardium, the thyroid gland, as well as teratogenic and carcinogenic effects. In occupational and environmental health, it is important to identify effects that occur at relatively low exposure, i.e. the critical effects which are crucial for preventive action. The limited database available on cobalt toxicity makes it difficult to select critical effects. For dermal exposures, allergic dermatitis may be regarded as such an effect, even if this is a reversible condition and dose-response relationships need to be defined. For inhalation exposures, effects on the respiratory tract can be considered as critical effects. Evidence of cobalt carcinogenicity in humans is inadequate and cannot be evaluated quantitatively but an extra safety factor to be added on exposures estimated from other effects may well be needed. Risks for development of pneumoconiosis are likely to be dependent on physical and chemical characteristics of airborne cobalt containing particles, but such information is lacking in most studies. It has been recognised for many years that there is an excess risk of pneumoconiosis when exposure to cobalt containing dust exceeds 100 micrograms/m3. Recently considerably lower concentrations occurring in the cobalt diamond industry have given rise to such effects. Irritation of mucous membranes may occur at 5-10 micrograms/m3.