The impact of humidex on other infectious diarrhea: a study of seasonal heterogeneity and lagged effects using a distributed lag non-linear model

Global climate changehas led to frequent extreme hot and humid events. The Humidex, as a comprehensive temperature and humidity indicator, is closely related to infectious diseases. Other infectious diarrhea is an important public health issue, but the expode-response relationship between it and Humidex, especially the seasonal effect modification and lag effect patterns, still lacks systematic research. This study aims to evaluate the impact of Humidex on the incidence of other infectious diarrhea and explore its seasonal heterogeneity and time lag effect. Using daily OID case and meteorological data from Guangdong Province (2014–2023), a Distributed Lag Non-linear Model was applied to assess the cumulative effects of Humidex, adjusting for season, day of the week, and holidays. Analyses included establishing the overall exposure-response relationship, stratifying by warm (May–October) and cold (November–April) seasons to examine effect modification, and conducting sensitivity analyses to verify robustness. At the extreme Humidex exposure level (P95, 42.27), the cumulative relative risk (CRR) for OID was 1.183 (95% CI: 1.057–1.323), indicating a significant 18.3% increase in risk, with an attributable fraction (AF) of 15.4%. Conversely, a protective effect was observed at the low Humidex level (P5, 11.3) (RR = 0.832, 95% CI: 0.748–0.926). Lag analysis at high Humidex levels (e.g., P75, P90) showed that the highest risk occurred within 0–2 days after exposure, indicating a short-term lag effect. Seasonal stratification revealed significant effect modification: during the cold season, extreme Humidex showed a protective effect (RR = 0.523, 95% CI: 0.319–0.856), while the effect was non-significant in the warm season (RR = 0.957, 95% CI: 0.880–1.040), with a seasonal effect ratio of 1.83. Sensitivity analyses confirmed the robustness of the findings to variations in lag periods and degrees of freedom for splines. Humidex is significantly associated with OID incidence, showing a non-linear exposure-response relationship and clear seasonal effect modification. While extreme Humidex generally increases OID risk, anomalously high levels during the cold season exhibit a protective effect. These findings support the development of season-specific early warnings and targeted public health interventions based on the Humidex index.