Forecasting daily bathtub-drowning mortality in Japan: a comparative analysis of statistical, machine learning, and...

Japan reports the highest global mortality rate from drowning among older adults, predominantly owing to bathtub-related incidents. Despite sustained public health interventions, this mortality has increased over several decades. Timely warnings advising older adults to avoid unsupervised bathing during high-risk conditions may mitigate this issue; however, no nationwide forecasting model currently exists.

We integrated death certificate records from 1995-2020 (99,930 bathtub-drowning deaths) with meteorological, temporal, and demographic data across all 47 prefectures (446,359 prefecture-days). Daily mortality counts were modeled using a distributed-lag non-linear model (DLNM), extreme gradient boosting (XGBoost), and long short-term memory network (LSTM). Data were partitioned chronologically into training (1995-2015), validation (2016-2018), and test (2019-2020) sets. Predictive accuracy was evaluated using root mean square error (RMSE) and mean absolute error (MAE), whereas feature importance was quantified via Shapley additive explanations.

During the test period, DLNM, XGBoost, and LSTM exhibited comparable predictive performance (RMSE = 0.577, 0.574, 0.575; MAE = 0.345, 0.333, 0.347, respectively). The most important features across all models were daily mean temperature, prefectural population, and binary prefecture indicators. Restricting DLNM meteorological inputs to routinely forecasted variables-daily maximum and minimum temperatures-did not reduce predictive accuracy [RMSE = 0.577 (95% confidence interval, 0.566-0.590); MAE = 0.344 (95% confidence interval, 0.340-0.349)].

The DLNM-based framework provides a practical means of forecasting the daily bathtub-drowning deaths. Integration into routine meteorological broadcasts and mobile platforms may facilitate timely warnings, prompting older adults to avoid unsupervised bathing on high-risk days, thereby reducing Japan's ongoing preventable bath-related mortality.