Within the meaning of the Labour Protection Act (ArbeitnehmerInnenschutzgesetz, ASchG), limit values of work substances are expressed as MAC (maximum allowable concentration) and TAC (technical approximate concentration) values. TAC values have been defined, in particular, for carcinogenic work substances, which, even in small concentrations, can have harmful effects on human health. MAC values for work substances and materials have a dosage / effect ratio. In both cases, the mean values are used for determining the concentration of hazardous substances over a certain time period serving as a basis for the assessment, usually the time period is an eight-hour workday. If it can be assumed with regard to MAC values that even long-term exposure will not cause harm to employees provided that the limit values are not exceeded, this rule does not hold true for carcinogenic substances for which the TR values have been defined. The MAC and TAC values are set forth in Annex One to the Limit Value Regulation (Grenzwerteverordnung, GKV). The Limit Value Regulation (GKV) contains the MAC values (maximum allowable concentration at work) for a number of pure substances. If a substance is not shown on the MAC list, it cannot automatically be concluded that it has no harmful impact on human health.