Geological storage of the greenhouse gas CO2 has the potential to be a widespread and effective option to mitigate climate change. As any industrial activity, CO2 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 CO2 leakage emerging in the unsaturated zone under a building. We first focus on the migration of the CO2 in the unsaturated zone and the foundation through numerical simulation with sensitivity analysis. Our results show that the intrusion of CO2 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 CO2 indoor concentrations. We have then integrated the former results in a global toolbox that provides an efficient and easy-touse tool for decision support, which enables to assess the impacts on human health of CO2 leakage from the reservoir to a building.