Climate change mitigation and adaptation policies in the built environment can reduce greenhouse gas emissions and also bring ancillary public health benefits by reducing coldrelated mortality and indoor air pollution. However, increased airtightness of dwellings may have some negative effects on health related to thermal stress and chemical and biological contamination, if adequate ventilation is not maintained. A modelling case study focusing on indoor PM2.5 in London dwellings shows that personal exposure and related health impacts can be substantially reduced if energy efficiency measures, combined with mechanical ventilation with heat recovery (MVHR), are implemented. These findings have implications for policies aiming to mitigate climate change and improve energy efficiency of dwellings.