Basaltic Geological Context and Regional Variation in Dry-Eye Medication Utilisation in Japan: An Exploratory Ecological Study

Basaltic geological environments represent spatially heterogeneous regional contexts encompassing diverse mineralogical, hydrological, and geochemical characteristics. This nationwide exploratory ecological study examined whether basalt area ratio (BAR) was associated with dry-eye medication prescription volume across the 47 prefectures of Japan. The outcome was the annual number of bottles dispensed per 1000 population for three dry-eye medications in 2022. BAR was calculated using geographic information system data. Associations were evaluated using LOESS, covariate-adjusted restricted cubic splines, univariable and multiple linear regression, and sensitivity analyses using alternative denominators. Neither the overall spline association nor its nonlinear component was statistically significant. BAR was also not significantly associated with medication prescription volume in either the univariable or multiple linear regression analysis. Although the smoothing curves showed descriptive variation across the observed BAR range, the apparent nonlinear pattern was not statistically supported or consistently reproduced across analytical methods. Thus, the available prefecture-level data provided no statistical evidence of an association between basalt coverage and dry-eye medication utilisation. BAR should be interpreted as an indicator of regional geological context rather than as a measure of natural H2 production or human exposure. Future studies should incorporate direct environmental measurements, finer spatial resolution, seasonal data, and clinical dry-eye outcomes.