This thesis examines the long-term health impacts of flood exposure on mortality, dementia, and biological age, exploring potential mediating mechanisms. A review highlights the global rise in flood exposure and the need for improved exposure assessments. For original studies introduce a novel flood index to assess flooding exposure for each participant in the UK Biobank. Findings show increased risks of mortality and dementia, with psychiatric disorders partially mediating the flood-dementia link. Flood exposure also accelerates biological ageing. These results provide novel insights into flood-related health risks, emphasizing the urgency of addressing long-term consequences and informing public health strategies for flood-affected populations.