2. Limit values

Limit values are limits with which we want to compare a calculated or measured exposure. Limit values for external exposure are usually established for certain effects that occur when exposures occur above this limit value, during a certain period, according to the route (inhalation, oral, dermal) for which the limit value was derived. There are different types of limit values: health limit values for which no effects occur, limit values for which no harmful effects occur, legal limit values, action values, typical limit values, ...Within the legislative framework, one will be mainly handling values by which one will show no adverse effects on human health and "the" limit value of a substance is usually based on the first adverse effect that occurs at the lowest exposure (the so-called critical effect). If we take the example of hydrogen sulfide: the first (non-harmful) effect that occurs is irritation of the nose(<1ppm), higher exposures lead to irritation of the eyes (harmful effect around 10 ppm), higher concentrations paralyze the olfactory (around 100 ppm), and the most harmful effect, of course, is death (> 800 ppm). The Belgian legally binding limit value for hydrogen sulfide in the ambient air is currently set at 5 ppm (7 mg / m³) for inhalatory exposure over 8h.People who are exposed to this concentration during 8 h, 5 days per week, will probably have no eye-irritation. In addition to the limit values for external exposure, there are also internal exposure limit values, so-called biological limit values, which are usually set at a certain concentration in a human sample (urine, blood, plasma, hair, teeth, ...) after a certain external exposure. At this moment, the only legally binding biological limit value in Belgium is the biological limit value for lead in blood (70 µg Pb/100 ml blood).