Abstract Geological storage of the greenhouse gas CO 2 has the potential to be a widespread and effective option to mitigate climate change. As any industrial activity, CO 2 storage may lead to adverse impact on human health and the environment in the case of unexpected leakage from the reservoir. These potential impacts should be considered in a risk assessment process. We present an approach to assess the impacts on human health in case of CO 2 leakage emerging in the unsaturated zone under a building. We first focus on the migration of the CO 2 in the unsaturated zone and the foundation through numerical simulation with sensitivity analysis. Our results show that the intrusion of CO 2 into a building is substantially attenuated by the unsaturated zone and the foundation and may lead only under very specific conditions (very low ventilated parts of buildings, high flow rate and/or building situated very close to a leaking pathway) to hazardous CO 2 indoor concentrations. We have then integrated the former results in a global toolbox that provides an efficient and easy-to-use tool for decision support, which enables to assess the impacts on human health of CO 2 leakage from the reservoir to a building.