How to Calculate Predicted No-Effect Concentration (PNEC)

Little Pro on 2016-07-13

Predicted No-Effect Concentration (PNEC) is the concentration of a substance in any environment below which adverse effects will most likely not occur during long term or short term exposure. In environmental risk assessment, PNECs will be compared to actual or predicted environmental concentration (PEC) to determine if the risk of a substance is acceptable or not. If PEC/PNECs<1, the risk is acceptable.

Predicted No-Effect Concentration (PNEC) Example - Acetone

According to this source, the PNEC-fresh water of acetone is 10.6mg/L. That means that if the concentration of acetone present in fresh water (pond/lake/river) is below than 10.6mg/L, the acetone will unlikely cause adverse effects to the aquatic environment. This value is not set randomly. There is a systematic and scientific way to derive it. Please continue reading.

How to Derive Predicted No-Effect Concentration (PNEC)?

The PNECs are usually calculated by dividing toxicological dose descriptors by an assessment factor. The endpoints most frequently used for deriving PNECs are mortality (LC50), growth (ECx or NOEC) and reproduction (ECx or NOEC).

  • LC50 /EC50 (Median Lethal Concentration/Median Effective Concentration): They are  the concentrations at which 50% mortality or inhibition of a function (e.g. growth or growth rate) was observed. They are usually obtained from short-term eco-toxicology studies. 
  • NOEC (No Observed Effect Concentration): NOEC is the highest tested concentration for which there are no statistical significant difference of effect when compared to the control group. It is usually obtained from long-term eco-toxicology studies. In some studies, only LOEC (lowest observed effect concentration) can be obtained, in which case NOEC can be calculated as LOEC/2. 
  • ECx: It is the concentrations at which x % (10% for EC10) effect was observed or derived statistically when compared to the control group. It is usually obtained from long-term eco-toxicity studies. 
  • Typical units: mg/L or mg/kg.

The table below is an example of how to calculate Derive Predicted No-Effect Concentration (PNECs) for different environmental compartments by dividing dose descriptors with assessment factors. 

Compartment Eco-toxicology Dose Descriptors Assessment Factor  PNEC value
PNEC-Fresh water
  • NOEC(Algae growth inhibition): 100mg/L;
  • NOEC(Daphnia reproduction):10mg/L;
  • NOEC (Fish):20mg/L.
10 1mg/L
PNEC-STP microorganism 3h-NOEC>1000mg/L (activated sludge inhibition test) 10 100mg/L
PNEC-soil LC50 (earthworm acute toxicity) >1000mg/kg 1000 1mg/kg

In above case, PNEC-water is calculated as 1mg/L.  For aquatic environment, toxicological data (10mg/L) from the most sensitive species  (Daphnia) is used for PNEC-water calculation. An assessment factor of 10 is used to take into account of the differences between laboratory conditions and natural conditions. If the actual concentration of the substance in aquatic environment is 2mg/L, the substance will cause unacceptable risks to aquatic enviroment. 

How Many Predicted No-Effect Concentrations (PNECs) You Need to Derive?

PNECs need to be derived for various environmental compartments (water, sediment, soil, air, etc.). The table below summarizes the types of PNECs you may need to derive and how to calculate them.

Compartment PNEC How to Get
Fresh Water PNEC-fresh water
  • The lowest value of EC50, LC50, NOEC, etc from aquatic toxicity studies (algae, daphnia and fish) divided by assessment factors;
  • See aquatic toxicity studies
Marine Water  PNEC-marine water
  • Derived only when exposure to marine water is possible;  
  • Bigger assessment factor for marine water;
  • If there is not enough ecotox data on marine organisms, PNEC-marine water = PNEC-fresh water/10.
Sediment  PNEC-sediment
  • The lowest NOEC/EC10 value for sediment living organisms divided by assessment factors (10~100); or
  • Using the equilibrium partitioning method (EPM) when only aquatic toxicity data is available.
  • Larger assessment factor for marine sediment.
Soil  PNEC-soil
  • The lowest LC50/NOEC/EC10 value for soil living organisms divided by assessment factors (10~100); ; or
  • Using the equilibrium partitioning method (EPM) when only aquatic toxicity data is available.
  •  See terrestrial toxicity studies
Sewage treatment plant micro-organisms  PNEC-STP
  • The lowest NOEC/EC10/EC50 value from activated sludge inhibition test or biodegradability studies divided by assessment factors (1~100).
Air  PNEC-air
  • No standard procedure;
  • Important for gases and highly volatile substances. 
Predator  PNEC-predator
  • The lowest value of LC50bird, NOECbird or NOECmammal divided by assessment factors (30-3,000); 
  • NOECbird/mammal=NOAELbird/mammal* conversion factors(8-40 for different species) if only NOAEL is available;
  • Required when log Kow>3; or BCF >100; and there is no mitigating property such as ready biodegradability or hydrolysis.

It is not always necessary to derive PNECs for all mentioned environmental compartments. Usually PNECs are only derived for 4 compartments: fresh water, soil, STP micro-organism and sediment.

How to Choose Appropriate Assessment Factors?

Assessment factors (AFs) are used to address the differences between laboratory data and natural conditions, taking into account of interspecies differences and intraspecies differences. Assessment factors applied for long-term tests are smaller because the uncertainty of the extrapolation from labs to natural environment is reduced. More data on more species in the same environmental compartment can also reduce uncertainties, thus further decreasing assessment factors. 

The picture below summarizes common assessment factors used for PNEC calculation (from ECHA guidance on chemical risk assessment).

Predicted No Effect Concentration

*Species representing 3 trophic levels : water (algae,fish and daphnia), soil (earthworm, plants, and micro-organisms).

Equilibrium Partitioning Method (EPM) for PNEC Calculation

In the absence of any ecotoxicological data for soil organisms or sediment-dwelling organisms, the PNEC-soil and PNEC-sediment may be provisionally calculated from PNEC-water using the equilibrium partitioning method (EPM). This method might result in overestimation or underestimation of toxicity and should only be considered as a screen for identifying substances requiring further testing on soil organisms and sediment-dwelling organisms .

Important: EPM is usually not recommended for substances that may pose a high hazard potential to soil organisms (i.e, logKow/Koc>5 and LC50/EC50 to <1mg/L to aquatic species). 

Calculating PNEC-soil from PNEC-water using  EPM

The picture below shows you how to calculate PNEC-soil from PNEC-water using equilibrium partitioning method (EPM).

PNEC equilibrium partitioning method

If default values were used, the above equations can be simplified to:

PNEC soil and Koc

Note 1: Organic carbon-water partition coefficient Koc is a very important parameter for predicting PNEC-soil from PNEC-water. If Koc values are obtained from various types of soils, the mean value of Koc can be used for PNEC calculation. If you do not know what Koc is and how to get it, please click here

Note 2: If only one test result with soil dwelling organisms is available, PNEC-soil is calculated on the basis of this result using assessment factors and on the basis of the equilibrium partition method (EPM). The lowest PNEC-soil value obtained will be used for risk assessment. 

Calculating PNEC-sediment from PNEC-water using  EPM

The picture below shows you how to calculate PNEC-sediment from PNEC-water using equilibrium partitioning method (EPM).

PNEC sediment

If default values were used, the above equations can be simplified to:

PNEC sediment Koc

Note: For substances with a log Kow > 5 (or for compounds with a corresponding adsorption or binding behavior, e.g. ionisable substances, surface reactive substances), PNEC-soil and PNEC-sediment calculated need to be decreased by a factor of 10

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

You have been assigned with a task to derive PNECs for a substance (Koc=10) for 4 environmental compartments based on the available data below. 

Compartment Available data Method PNEC 
PNEC-Fresh water
  • NOEC(Algae growth inhibition): 10mg/L;
  • NOEC(Daphnia reproduction):1mg/L;
  • NOEC (Fish):20mg/L.
AF=? ?
PNEC-STP microorganism EC50(activated sludge inhibition test)=20mg/L AF=? ?
PNEC-soil LC50 (earthworm acute toxicity) =500mg/kg AF=? and EQM ?
PNEC-sediment No data EQM ?

After calculating PNECs, please answer this question: if the concentration of the substance in water is 0.05mg/L, is this risk acceptable?

View quiz answer and explanation here.

Main Reference


Good job. You have learned what PNEC is, how many PNECs you need to derive, how to choose assessment factors and how to derive PNECs. We will cover how to estimate predicted environmental concentrations in a separate article. Please subscribe our newsletter to keep updated of our new articles.

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 Tags: Topics - CRAEnvironmental Risk Assessment