The biological limit value, or BLV in short, describes the limit of the toxicological concentration derived from occupational health and safety of a substance, its metabolite, or indicator of effect in the appropriate biological material up to which the health of employees is generally not impaired. If the BLV of a substance is exceeded once, a health impairment does not necessarily arise. An exception to this are acutely toxic substances, which must not exceed the BLV under any circumstances. A list of biological exposure limit values is provided in the Technical Rules for Hazardous Substances (TRGS) 903. Sources: Ordinance on Hazardous Substances; TRGS 903