Threshold limit value. This value is the highest permissible concentration of a working material as a gas, steam or suspended matter in the air at the working place. According to the present state of knowledge, this concentration generally does not affect the health of the workers or cause undue nuisance even with repeated and long-term, as a rule eight-hour daily effect when observing average weekly working hours. Apart from practical circumstances of the working processes, the fixing of threshold limit values above all takes effect characteristics of the material concerned into account. The decisive elements are scientifically sound criteria for health protection and not the technical and economic implementation possibilities in practice.