Psychiatric Disorders Mediate the Association between Floods and Dementia: A Prospective Cohort Study in the UK Biobank.

Flooding has become more frequent and severe worldwide, leading to an increased burden of psychiatric disorders (e.g., depression and anxiety). Psychiatric disorders are associated with an increased risk of subsequent dementia. However, the associations among floods, psychiatric disorders, and dementia are still unclear. Using a population cohort from the UK Biobank, we aimed to investigate the mediating role of psychiatric disorders on the associations between floods and dementia. In this study, cumulative exposure to floods over an eight-year period preceding the study baseline was assessed for each participant at residential addresses. Multivariable Cox proportional hazards regression was used to study the associations of flooding exposure with psychiatric disorders and dementia. Stratified analyses and mediation analyses were conducted to examine whether psychiatric disorders mediate the relationship between floods and dementia. During a median follow-up of 12.3 years (interquartile range: 11.6-13.0), 0.9% (2,028) of participants developed dementia and 9.5% (21,629) were diagnosed with psychiatric disorders. The flooding exposure was associated with an 8.0% increased risk of incident dementia (Hazard Ratio [HR]: 1.080, 95% CI: 1.023-1.141). The flood-dementia association was observed to be partially mediated by several subtypes of psychiatric disorders (overall proportion of mediation: 75.7%), with psychotic disorder accounting for 49.7% (indirect effect HR: 1.039, 95% confidence interval: 1.015-1.064) of flood-related dementia, followed by stress-related disorder (proportion of mediation: 18.1%), and depression (proportion of mediation: 3.9%). This study provides evidence of an increased risk of dementia associated with exposure to floods, with psychiatric disorders playing a crucial mediating role in the flood-related dementia pathway. These findings suggest that flooding exposure is a critical risk factor for dementia, and targeted interventions addressing postdisaster mental health may be crucial for dementia prevention in affected populations.