concentration of the substance below which adverse effects in the environmental sphere of concern are not expected to occur. PNEC is relevant for an ecosystem, e.g. aquatic or terrestrial. PNEC is generally calculated from the ecotoxicity test results of testorganisms of three different trophic levels by factorial extrapolation, using safety factors or by statistical extrapolation.
Applying factorial extrapolation the following assessment factors are applied according to uniform protocols:
f=1000: at least one short-term EC50 from each of three trophic levels,
f = 100: long-term NOEC from one trophic level besides 2 acute,
f = 50: long-term NOEC from species representing two trophic levels besides one acute,
f = 10: long-term NOEC from at least three trophic levels,
f = 1: PNEC can be directly measured in microcosms, mesocosms or ecosystem-field-testing.
Another method to determine a PNEC value is the use of statistical extrapolation methods using the variation in species sensitivity. If a large data set with NOECs from long-term experiments for different taxonomic groups is available, these values can be used to draw a distribution. This distribution that describes the variability of hazard of a substance to organisms is called a Species Sensitivity Distribution (SSD). This distribution can be presented as a frequency distribution (cumulative normal distribution curves or other similar distribution curves) of NOEC values for species. From this curve we can read Xm, the mean toxicity expressed as the mean NOEC value of a substance. The Sm represents the toxicity range or variation in sensitivity of a substance.
The main assumption on the use of SSDs in risk assessment is that the distribution based on a selection of species (tested in laboratory experiments) are representative for all species (in the field).
Statistical extrapolation methods may be used to derive a PNEC from a SSD by taking a prescribed percentile of this distribution. For pragmatic reasons it has been decided that the concentration corresponding with the point in the SSD profile below which 5% of the species occur, should be derived as an intermediate value in the determination of a PNEC. This 5% point in the SSD is also identified as a hazardous concentration (HC) at which a certain percentage (in this case 5%) of all species is assumed to be affected.