Quantifying the temporal trends of the combined effect of temperature and relative humidity on hand, foot, and mouth disease in Yunnan, China

Background Hand, foot, and mouth disease (HFMD) poses a significant risk to children. While most studies focus on the individual effects of temperature or relative humidity, the combined effect of these factors and their temporal variations remain unclear. Understanding these effects is essential for designing effective public health interventions. Methods Using daily meteorological and HFMD case data collected from 2010 to 2019 in 16 cities in Yunnan Province, China, we compared three composite indices (Humidex, heat index, and temperature–humidity index) to identify the indices that best captured the combined effect of temperature and humidity on HFMD risk. An extended time-varying distributed lag nonlinear model (DLNM) was used to examine how these effects shifted over time across population subgroups. Relative risk (RR) values at the 1%, 25%, 75%, and 99% quantiles were extracted to represent effects at extremely, moderately low, moderately, and extremely high levels. Results The THI a8 demonstrated a monotonic upward exposure–response curve with narrower confidence intervals, more consistent relationships across cities, and the best model fit (Quasi-Akaike information criterion (QAIC) = 283564.2, Akaike information criterion (AIC) = 45.46, and Bayesian information criterion (BIC) = 62.30). HFMD risk decreased at extremely low ( RR = 0.677, 95% CI: 0.632, 0.724) and moderately low THI a8 levels ( RR = 0.766, 95% CI: 0.713, 0.823) but increased at moderately high ( RR = 1.121, 95% CI: 1.084, 1.159) and extremely high THI a8 levels ( RR = 1.478, 95% CI: 1.300, 1.680). Temporal analysis revealed a decreased HFMD risk at extremely low THI a8 values from 2010 to 2019, weakened protective effects at moderately low THI a8 values and an increased risk at extremely high THI a8 values. Subgroup analyses revealed that kindergarten children (3 ≤ age < 6 years) and females were particularly vulnerable. Conclusion The THI a8 effectively captures the combined effect of temperature and relative humidity on HFMD risk revealing temporal variations. Adaptive public health strategies are needed to mitigate HFMD transmission under changing environmental conditions.