Threshold Limit Value. A workplace value to be applied to the private sphere. Refers to vapours, dust or gases of harmful substances mixed with the ambient air and that as of a certain concentration or, as may apply, saturation have harmful effects on the health of humans. The TLV is expressed in mg/m³ of air moved (milligrams of suspended matter per cubic metre of air), or in ppm 'parts per million'. The designation 1 ppm = 1cm³/m³ = 0.0001%. Using the size of the room and the materials obtained for processing allows how often the air needs to be changed per hour to be calculated. Using a test device or the so-called tube method, certain gases - depending on the tube used and employed - can be tested and concentrations (with a measurement error of 10 - 20%) can be determined. The TLV refers to an 8 hour day and a 40 hour week