Risk Assessment

Risk assessment is based on comparing a predicted exposure concentration to a predicted no effect concentration. The predicted exposure resulting from the production or use of a substance should be lower than the predicted no effect level for this scenario to be considered acceptable. Exposure assessment is concerned with generating representative concentrations in environmental media and determining the amount to which human and environmental receptors may be exposed.

For environmental risk assessments the Predicted Environmental Concentrations (PEC) for various environmental compartments are compared to the relevant Predicted No Effect Concentrations (PNEC) which are derived from ecotoxicity data. For human health exposure assessments the Predicted Exposure Levels (PEL) for various exposure pathways are compared to the respective Derived No Effect Level (DNEL) which are derived from human or mammalian toxicity data. The PEC or PEL divided by thePNEC or DNEL gives the Risk Characterisation Ratio (RCR). The RCR must be below 1 for production or use of a substance to be considered acceptable.

Risk assessments generally follow a tiered approach, such that if an initial assessment using default assumptions demonstrates safe use the assessment stops at that point. If safe use is not demonstrated then the exposure assessment is refined in further tiers, which require greater levels of data and user input.

Our Expertise & Experience

wca’s environmental scientists, toxicologists and chemists are experts in risk assessment activities and provide quality assured advice and consultancy to industrial consortia and individual companies. Our staff are experienced exposure modellers who can offer clients the options of using various available modelling tools (e.g. EUSESECETOC TRACHESAR) or developing more bespoke exposure assessments for site-specific scenarios using measured emission data where they are available.

wca staff have used their exposure expertise for a variety of regulatory regimes, with exposure assessments performed for the purposes of REACHHuman and Veterinary Medicine and Biocides assessments. As well as performing the modelling, wca staff can assist clients in the collation and review of input data, design and implementation of monitoring programmes, preparing justifications for any assumptions used based on operating conditions and the application of risk management measures, and writing reports detailing the modelling and the outputs. Some examples of our exposure experience are provided below.


REACH requires that an exposure assessment is undertaken for production and each identified use of a substance registered above 10 tpa that is classified as hazardous or PBT or vPvB 1Environment Release Categories (ERC) are defined in the ECHA guidance for use at the first Tier of environmental exposure assessment and these provide default parameter values for the proportion of a substance that is released to different environmental compartments. The default values defined by ERCs are generally very conservative and various industry sectors have developed Generic Exposure Scenarios (GES). Industry has also refined environmental release estimates for typical uses and these Sector Specific Environmental Release Categories, known as SPERCs, can be used to refine the assessment.

As well as conducting the exposure assessments and the modelling, wca staff have a large amount of experience in formatting the outputs of exposure modelling into the Exposure Narratives that are presented in Section 9 and 10 of the Chemical Safety Report (CSR) and in extended Safety Data Sheets (eSDS).


wca has designed and implemented monitoring programmes to allow clients to refine their exposure assessments further and validate model assumptions and predictions in order to demonstrate safe use. These monitoring programmes have included work to:

  • Establish background concentrations of the substance in various compartments
  • Establish typical concentrations released from municipal sewage treatment works
  • Measure environmental parameters that affect the bioavailability of metals
  • Establish reasonable worst case release fractions from manufacturing sites and downstream user sites

These monitoring programmes have successfully allowed clients to refine the output of estimated/ modelled exposure assessments, and undertake a more detailed and relevant risk assessment in order to demonstrate safe use. Several such monitoring programmes have been jointly undertaken by industries and environmental regulators. This has the advantage of ensuring that data gathered is applicable to multiple uses, such as exposure assessments under REACH and assessing the need for setting water quality standards under the Water Framework Directive.

Probabilistic Modelling Of Exposure

Standard exposure assessments predict a single value for the estimated concentrations of a chemical in different environmental compartments for a given set of input parameters. However, in reality one or more of these parameters will have an associated uncertainty or variability, which can be important to take into account in the assessment. Probabilistic modelling allows the risk assessor to demonstrate the influence of this variability on predicted exposure concentrations with defined probability distributions – allowing for a more realistic understanding of the likely environmental impact of the substance. Sensitivity analysis highlights which uncertainties contribute most to uncertainty/variability in the output, allowing for targeted refinement of the input data and any assumptions.

Socio-Economic Analysis

Socio-Economic Analysis (SEA) seeks to quantify as far as possible the social, economic and human health costs and benefits of different potential scenarios so that rational, transparent and consistent decisions can be made. SEA is an important process under REACH if Authorisation or Restriction is being considered for the substance of concern. In the context of SEA, wca have used modelling techniques to construct comparative Joint Probability Curves for different risk management options. Furthermore, defining confidence limits on environmental effects helps determine whether an environmental risk management measure, such as a substitution, is predicted to have a statistically significant impact.

1 Persistent, bioaccumulative and toxic (PBT) or very persistent and very bioaccumulative (vPvB)

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