Comparing the role of relative and absolute humidity in heat-related mortality: A case time series study in South Korea.

BACKGROUND Despite strong physiological plausibility, epidemiology studies have reported inconsistent associations of humidity with heat-related health outcomes. In this regard, there has been a heated debate on which humidity metric to use in epidemiological research. OBJECTIVES This study aimed to compare the role of two common humidity metrics, relative and absolute humidity, in heat-related mortality in summer using a nationwide mortality dataset. METHODS We applied a case time-series design for summer (June to September) mortality across the entire 229 districts of South Korea from 2011 to 2019. The temperature was fitted using a distributed lag non-linear model (DLNM) with 10 lag days. A linear interaction between the cross-basis of temperature and humidity was included in each model to examine the different patterns of association between heat and mortality by humidity level (low and high humidity defined by 5th and 95th percentile of each humidity distribution). RESULTS A total of 780,102 deaths were recorded in the summer from 2011 to 2019, in South Korea. The association between extreme heat (temperature approximately above the 99th percentile of the temperature distribution) and mortality was modified more by absolute humidity than by relative humidity, although the effect modification of both humidity indicators was not statistically significant. The relative risks (95% confidence interval) at the 99.9th percentile temperature compared to the minimum mortality temperature were 1.21 (1.11, 1.31) and 1.22 (1.03, 1.44) for low and high relative humidity, respectively, and 1.11 (0.89, 1.37) and 1.25 (1.15, 1.34) for low and high absolute humidity, respectively. DISCUSSION Our findings provide epidemiological evidence on the role of relative and absolute humidity in heat-related mortality and suggest that absolute humidity may be more appropriate metric than relative humidity when assessing health impact.. https://doi.org/10.1289/EHP15827.