The Impact Pathway Approach

The impact pathway approach was developed within the ExternE project series and represents its core. Impact pathway assessment is a bottom-up-approach in which environmental benefits and costs are estimated by following the pathway from source emissions via quality changes of air, soil and water to physical impacts, before being expressed in monetary benefits and costs. An illustration of the main steps of the impact pathway methodology applied to the consequences of pollutant emissions is shown in the following diagram.

Impact Pathway

Two emission scenarios are needed for each calculation, one reference scenario and one case scenario. The background concentration of pollutants in the reference scenario is a significant factor for pollutants with non-linear chemistry or non-linear dose-response functions. The estimated difference in the simulated air quality situation between the case and the reference situation is combined with exposure response functions to derive differences in physical impacts on public health, crops and building material. It is important to note, that not only local damages have to be considered - air pollutants are transformed and transported and cause considerable damage hundreds of kilometres away form the source. So local, European wide and hemispheric modelling is required.

Regarding dispersion, with NewExt, not only atmospheric pollution is analysed, but also pollution in water and soil. Human exposure to heavy metals and some important organic substances (e. g. dioxins), which accumulate in water and soil compartments and lead to a significant exposure via the food chain, is represented in further models.

As a next step within the pathway approach, exposure-response models are used to derive physical impacts on the basis of these receptor data and concentration levels of air pollutants. The exposure-response models have been compiled and critically reviewed in ExternE by expert groups.

In the last step of the pathway approach, the physical impacts are evaluated in monetary terms. According to welfare theory, damages represent welfare losses for individuals. For some of the impacts (crops and materials), market prices can be used to evaluate the damages. However, for non-market goods (especially damages to human health), evaluation is only possible on the basis of the willingness-to-pay or willingness-to-accept approach that is based on individual preferences. The monetary values recommended in ExternE by the economic expert group have been derived on the basis of informal meta-analysis (in the case of mortality values) and most recent robust estimates.

Updated values from the INTARESE and HEIMTSA projects can be found at IEHIAS.