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= PNEC = Predicted No Effect Concentration (PNEC) is an estimation from aquatic toxicity data for a concentration of xenobiotic (Human introduced) chemical that is expected to have no impact when it enters the environment. Environmental concentrations of a chemical can then be measured and compared to the PNEC to determine if the area is at risk. The PNEC helps determine when action should be taken in a contaminated area or to set regulatory guidelines.

Formula
A PNEC for a chemical is calculated by taking an effects concentration from toxicological tests and dividing by an appropriate assessment factor to come up with a conservative estimate.

For example, an LC50 from acute testing data would be divided by 1000 under EU guidlines.

$$LC50 Concentration \div 1000$$

Assessment Factors
A PNEC for a contaminant is derived using toxicological data on the most sensitive species available for a given environment. The most sensitive species that is actually in the environment may not be included because toxicology data is limited. To mitigate this the EU has implemented assessment factors. data from less sensitive species can be divided by assessment factors to predict substance concentrations where there will be no impact. These are similar to uncertainty factors used by the US EPA when extrapolating from limited data to predict chemical concentrations where harm will not occur.

A smaller assessment factor is applied when there is more confidence in the data used to derive a PNEC. Confidence increases the more toxicological data there is on effects to different trophic levels of organisms, acute and chronic effects, and lab results matching field results. The less data available or more variable it is between species and lab to field results the larger the assessment factor will be. This is to produce a more conservative PNEC and prevent harm to organisms because of poor data availability.

The EU uses an assessment factor of 1000 if only acute toxicity data is available. This assessment factor is used with lowest LC50 recorded for any species tested. If a NOEC is available for fish or daphnia an assessment factor of 100 may be applied. This drops to 50 if two NOEC’s are available for a mix of either fish, daphnia or algae. If NOEC’s for all three species exist then the assessment factor may be 10. A species sensitivity distribution HC5 can have an assessment factor ranging from 5 to 1. All of these assessment factors are open to adjustment according to each substance and data set circumstances. * NOEC's must be from a combination of fish, daphnia, and/or algae.

Application
PEC: Predicted environmental concentration (PEC) is how much of a chemical is predicted to be in the environment. PEC is determined with the most realistic data on a chemicals entry into the environment. Concentrations for a chemical can also be estimated for a worst case scenario to determine if the chemical should be investigated further.

Risk Characterization Ratio: Risk Characterization Ratio (RCR) combines PNEC and the PEC. It tells you if the concentrations currently entering in the environment are going to have an impact. If PEC/PNEC is greater than one then there is determined to be a risk and steps are taken to manage the contaminant.

Acute Data
If only acute toxicity data is available then a LC50 or EC50 may be used. Data for multiple species and trophic levels will be examined and the most sensitive species LC50/EC50 will be divided by the appropriate assessment factor, usually 1000 for acute data.

Chronic Data
Long term toxicological studies ranging from days to months depending on species and usually using lower concentrations of chemcial can produce a NOEC (No Observed Effect Concentration) and LOEC (Lowest Observed Effect Concentration). This value can be divided by an assessment factor of 100 for a PNEC. If NOEC/LOECs are available for multiple species the assessment factor can be lowered to 50-10 depending on the quality and amount of data.

Population Models
The use of population models for determining regulatory standards was under review in 2010, and a Society of Environmnetal Toxicology and Chemistry (SETAC) workshop was held in 2013. It was determined that population models could be used for regulation but require more standardized practices. The most important advancements identified were creating a standard for model selection, and more clearly defining the use of population models in regulation. Currently The majority of population models used in toxicology are meant to determine if a population should be monitored or if bioaccumulation will occur. They are not widely used to generate a PNEC for regulations.

Population model have potential for determining PNEC because they allow the use of population level endpoints such as recovery time, extinction rate, or changes in population structure. Population models can tailor a PNEC for a specific population. They can also be made to project effects at the community level. They can include factors like life history for a population of interest, and impacts from stressors like density and temperature. Population models still use data such as a NOEC or LC50 for inputs so existing toxicity data can be used.

Species Sensitivity Data (SSD)
Species sensitivity distributions (SSDs) can be applied in environmental risk assessments to estimate a PNEC for a chemical substance. For this estimation to occur, there needs to be sufficient data on species toxicity; with the European Chemicals Agency recommendation being about ten biological species and the U.S. EPA recommendation being no fewer than five biological species. SSDs are models of the variation in sensitivity of species to a particular stressor.

To generate an SSD, an empirical or statistical distribution function is fitted to the proportion of species affected as a function of a stressor concentration or dose. Typically the data needed to generate an SSD are from single-stressor laboratory toxicity tests, such as LC50s and EC50s that can be acquired from the U.S. EPA’s ECOTOX database. It is possible to make an SSD more specific for a particular area by using data from databases or literature that is specific to the site you are focusing on. This allows for an SSD to be more accurate in regards to your site .When there is the adequate data needed for an SSD they can be more accurate in estimating the PNEC compared to traditional quotient and assessment factor approaches.

Once the SSD for a chemical substance is generated the next step is to estimate the PNEC. Most commonly the 5th percentile of the SSD distribution is set as the PNEC. The choice of 5% is somewhat arbitrary but this is the percent used most often. This implies that the lowest 5% of the effect concentrations represents particularly sensitive organisms or it signifies that 5% of the data is unreliable in some way. Using 5% allows for the PNEC to remain the same even if a researcher finds a more sensitive bioassay as opposed to it being continually revised.