Predicted exposure concentration

Environmental risk assessment of substances is nowadays based on an evaluation of exposure pathways and concentrations on the one hand and identification and selection of sensitive endpoints on the other. The concept is operationalised by comparing real or estimated (predicted) exposure concentrations (PEC) with calculated no-effect concentrations (NEC or PNEC, predicted NEC). The comparison can be made by calculating the quotient of exposure and no-effect concentration. If the quotient is less than one, then the substance poses no significant risk to the environment. If the quotient is greater than one, the substance may pose a risk, and further action is required, e.g. a more thorough analysis of probability and magnitude of effects will be carried out. 

Similarly, an environmental risk characterization is carried out by comparing the predicted exposure concentrations (PECs) with the corresponding PNECs. If a PEC exceeds the corresponding PNEC, then the risk may be significant. A series of comparisons may be performed to assess the risks at different trophic levels, in different media, or at different scales (e.g., regional versus local). 

On the other hand, indirect methods should be considered as an alternative when analytical measurements are not feasible. Predicting methods involve extrapolating exposure estimates from existing data, e.g., previous environmental monitoring, data about emissions and chemicals production, and questionnaires. Distribution of chemicals among the different environment compartments is also a key aspect for predicting environmental concentrations. Therefore psysicochemical properties (see Sect. 4) are required inputs in these tools. 

Exposure assessment using monitoring data or fate and transport models calculate the predicted environmental concentration (PEC) in each environmental compartment. More information can be obtained from Suciu et al. [4]. 

For the ecological assessment, risk analysis was based on the traditional PEC/ PNEC ratio (Hazard Quotient) where PEC is the predicted environmental concentration (resulting from chemical analysis) and PNEC the predicted no-effect concentration. Ecological assessment for aquatic species was based on rainbow trout or fathead minnow while terrestrial assessment was based on small rodents like mice rats and rabbits. Exposures associated with HQ<1 were considered negligible. 

Besides the LCA approach, also risk assessment can be performed analysing the chemical compounds or modelling via predictive exposure models. Both types of approaches have their justification to measure environmental concentrations of chemicals in the environment with laboratory measurement is still the most reliable way for determination. But it goes along with the disadvantage of high investments concerning time and money. Besides that laboratory approaches are limited in terms of space and time, and in consequence, the survey of many micro-pollutants and their... 

The aim of the exposure assessment is to predict the concentration of the substance that is likely to be found in the environment, i.e., the predicted environmental concentration (PEC). Again it may not be possible to establish a PEC, and a qualitative estimation of exposure has to suffice. 

The principal application of PBPK models is in the prediction of the target tissue dose of the toxic parent chemical or its reactive metabolite. Use of the target tissue dose of the toxic moiety of a chemical in risk assessment calculations provides a better basis of relating to the observed toxic effects than the external or exposure concentration of the parent chemical. Because PBPK models facilitate the prediction of target tissue dose for various exposure scenarios, routes, doses, and species, they can help reduce the uncertainty associated with the conventional extrapolation approaches. Direct application of modeling includes... 

The method compares the predicted environmental concentrations (PECs), as indices of exposure, with predicted no effect concentrations (PNECs), as indices of... 

Environmental concentration Exposure analysis, biodegradation PEC Predicted environmental concentration... 

McKone (1993) demonstrated that chloroform in shower water had an average effective dermal permeability between 0.16 and 0.42 cm/hour for a 10-minute shower. The model predicted that the ratio of chloroform dermally absorbed in the shower (relative to chloroform-contaminated water concentration) ranged between 0.25 and 0.66 mg per mg/L. In addition, the McKone model demonstrated that chloroform metabolism by the liver was not linear across all dermal/inhalation exposure concentrations and became nonlinear at higher (60-100 mg/L) dose concentrations. 

Interroute Extrapolation. The Chinery-Gleason model examined two routes of exposure, inhalation-only exposure and inhalation/dermal exposure. The model was useful in predicting the concentration of chloroform in shower air and in the exhaled breath of individuals exposed by the dermal and inhalation routes. 

The indirect exposure is estimated by the use of FUSES. FUSES estimates concentrations in food and the total daily intake of a substance based on predicted environmental concentrations for (surface) water, groundwater, soil, sediment, and ambient air. The indirect exposure is principally assessed on two spatial scales locally near a point source of the substance, and regionally using averaged concentrations over a larger area. A third spatial scale, the continental scale, is... 

For risk assessment of chemicals based on a DNEL as described above, the output of the exposure assessment (Chapter 7) is usually an estimate of dose or concentration. Exposure data can either be measured or predicted. Measured exposure data are preferred if they are valid, i.e., an actual exposure estimate. However, in most cases, measured data are not available and therefore model-generated data must be used for the risk characterization, i.e., a predicted exposure estimate. 

Predictive methods of exposure assessment often rely on single values for input parameters to the exposure model that represent one point on the distribution curve of all possible values for this parameter. This point value can range from a 50th percentile, mean, median, or typical value to a worst-case estimate. In the predictive exposure assessment, a number of parameters are integrated through an algorithm to produce an output such as the predicted environmental concentration (PEC). If many worst-case values are involved, this integration can result in a PEC that has a... 

The authors concluded that neither effect addition nor potentiating interactions occurred, providing the exposure concentrations of the aldehydes are at their NOAECs. They also stated that the type of combined action or interaction found at clearly toxic effect levels was not very helpful in predicting what would happen at levels that are not toxic. 

Guidance to date supports the risk assessment principles for general chemical substances already published by the Commission (1996). Consequently, the risk characterisation simply involves a quantitative comparison of the outcome of the hazard/effects assessment with the exposure assessment. For human risk this involves the calculation of the TER (Toxicity Exposure Ratio) and comparing it with the MOS (Margin Of Safety). For environmental risk the PEC/PNEC ratio (Predicted Environmental Concentration versus the Predicted No-Effect Concentration) for the various environmental compartments. 

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