Ecological analysis of regional and temporal variations in cause specific mortality and photovoltaic deployment in Japan

The rapid expansion of photovoltaic (PV) systems has raised concerns about potential environmental and health impacts, including exposure to very low-frequency electromagnetic fields (ELF-EMFs). However, population-level evidence is limited. This study examined regional and temporal variations in cause-specific mortality related to differences in PV deployment in Japan. Ecological analyses compared standardized mortality ratios (SMRs) for major causes of death in Naka-Harima and Kobe over three periods (2006–2010, 2011–2015, and 2016–2020). PV density (kW/km 2 ) was used as a proxy variable for regional deployment intensity. Differences in SMR were assessed using descriptive and approximate statistical methods. Regional differences in SMR were evident even before the PV expansion, indicating baseline heterogeneity. In Naka-Harima, temporal variability was observed in selected mortality outcomes, with variations by cause of death and sex. Overall, while the findings of this study indicate regional and temporal variability in mortality patterns, they do not support a consistent association with PV deployment. Given the ecological study design, indirect exposure assessment, and potential residual confounding, a causal relationship cannot be established. These results should be considered exploratory and hypothesis-generating. Further research incorporating individual-level data, more detailed exposure characterization, and more rigorous analytical methods is needed to uncover potential associations. • Ecological comparison of two regions with differing photovoltaic (PV) densities as a proxy indicator of regional deployment intensity. • Temporal variation in standardized mortality ratios (SMRs) was observed across multiple disease categories. • In one region with higher PV density, changes in acute myocardial infarction mortality were observed during early expansion periods. • In later periods, elevated SMRs were noted for selected cerebro-cardiovascular diseases and female esophageal cancer, although patterns were heterogeneous across outcomes. • No causal relationship can be established, because findings are exploratory and highlight the need for improved exposure assessment and control of confounding.