Within the framework of the REEEM project this report analyses health impacts and external costs due to air pollution as one indicator of a sustainable energy system as part of the REEEM Work Package 5 on Environment, Health and Resources. To achieve clean air and a low-carbon society, climate change mitigation and air pollution control policies need to be considered simultaneously in an integrated assessment since both interact with each other. For this task the energy system model is linked with an impact assessment model. In a comparative scenario analysis based on detailed estimations of impacts and external costs due to air pollution interactions and co-benefits between climate change mitigation and air pollution control are identified The three considered decarbonisation pathways are all characterized by ambitious decarbonisation targets, with one also having a dedicated target for a high share of renewable energy sources. To be also able to analyse the effect of air pollution control on the European energy system and its decarbonisation, marginal damage costs are estimated for several air pollutants and fed back to the energy system model as unit cost factors in the third pathway.