Associations between domestic hard water exposure and incident psoriasis in adults: Insights from the UK Biobank cohort study.

Although exposure to hard water is associated with various inflammatory skin conditions, the specific relationship between hard water and psoriasis has not been clearly defined. We analyzed data from 486,414 participants in the UK Biobank cohort to explore the association between domestic hard water exposure and the incidence of psoriasis. Domestic water hardness, measured in calcium carbonate concentration, was obtained in 2005 from local water providers in Wales, Scotland, and England. During a median follow-up period of approximately 14 years, a total of 4801 (1.0 %) participants reported psoriasis, and we observed that for every 50 mg/L increase in water hardness (as CaCO₃), there was a 3 % increase in the risk of psoriasis [Hazard Ratio (HR) = 1.03, 95 % Confidence Interval (CI) 1.01-1.06]. Individuals exposed to very hard water (>180 mg/L) exhibited a 20 % increased risk of psoriasis compared to those exposed to soft water (0-60 mg/L) [HR: 1.20, 95 % CI: 1.07-1.34]. A positive linear relationship was observed between water hardness and the risk of psoriasis. Furthermore, the polygenic risk score indicated a synergistic effect between hard water exposure and genetic risk factors in developing psoriasis. Subgroup analysis suggested that the effect of hard water on psoriasis is more pronounced in individuals with a low polygenic risk score, women, the elderly, non-obese individuals, those of high socioeconomic status or without dyslipidemia. Residential water hardness has been identified as a significant risk factor for psoriasis in adults. Sustained efforts to reduce exposure to hard water may alleviate the psoriasis burden.