Quiz: Calculation of Predicted No-effect Concentration (PNEC)

Little Pro on 2016-08-11

This is the answer to a quiz on how to derive predicted no-effect concentration (PNEC) for a substance (Koc=10) for 4 environmental compartments (water, soil, STP and sediment) based on available data. The  original quiz and tutorial can be accessed here.


Compartment Available data Method PNEC 
PNEC-Fresh water
  • NOEC(Algae growth inhibition): 10mg/L;
  • NOEC(Daphnia reproduction):1mg/L;
  • NOEC (Fish):20mg/L.
AF=10 0.1mg/L 
PNEC-STP microorganism EC50(activated sludge inhibition test)=20mg/L AF=100 0.2mg/L
PNEC-soil LC50 (earthworm acute toxicity) =500mg/kg


PNEC (EQM)=0.029mg/Kg

PNEC-sediment No data EQM 0.1mg/L

Q2. If the concentration of the substance in water is 0.05mg/L, the risk is  acceptable. 


For PNEC-water, long-term results (NOEC) from three species are available. An assessment factor of 10 is assigned to the lowest NOEC value of the 3 species.

  • PNEC-water=Lowest NOEC/10=1mg/L/10 (AF)=0.1mg/L.

For PNEC-STP microorganism, only short-term EC50 from activated sludge respiratory inhibition study is available. An assesment factor of 100 is assigned to the EC50.

  • PNEC-STP microorganism=EC50/1000 (AF)=20mg/L/100=0.2mg/L.

For PNEC-soil, only one acute test result with soil dwelling organisms is available. PNEC-soil is calculated on the basis of this result using both assessment factor approach and on the basis of the equilibrium partition method (EPM).

  • PNEC-soil(assessment factor)=LC50/1000(AF)=0.5mg/kg
  • PNEC-soil(EPM)=(0.1176+0.01764*Koc)*PNEC-water=0.029mg/kg

In the end, the lowest PNEC-soil value (0.029mg/kg) will be used for soil risk assessment.

For PNEC-sediment, no test data is available. Only EPM can be used. 

  • PNEC-sediment=(0.783+0.0217*Koc)*PNEC-water=0.1mg/L

If the concentration of the substance in water is 0.05mg/L and it is less than PNEC-water, the risk is  acceptable.

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