Effect of absolute humidity on heat-mortality relation in Japan

Abstract

Background: Previously, we evaluated the confounding effect of relative humidity on temperature-mortality relation in Japan using 1972-1995 dataset, and concluded that humidity did not confound the relation. Since then, we accumulated the data up to 2010. Another finding was that absolute humidity was better variable than relative humidity in evaluating the humidity effect on influenza incidence. Aims: To show the absolute humidity effect on heat-mortality relation using the extended dataset. Methods: We used daily deaths of 65+ years of age and controlled the year trend by setting each year's minimum level mortality to be the reference. As the main weather variable, we used daily maximum temperature. Using distributed lag non-linear model, we set the lag effect of daily maximum temperature up to 15 days. In conducting regression analyses, we used two models, i.e., without and with absolute humidity term along with daily maximum temperature-lag crossbasis. The temperature-mortality relation shows V-shaped pattern, and we used only "heat" part of the temperature range and calculated the excess mortality ratio, i.e.,"without" to "with." Results: The without/with ratio was between 0.90 to 1.10. In this regard, the effect of absolute humidity was not large. However, the ratio tended to exceed one in northern part and southern areas tended to have lower than unity ratios. Conclusions: The absolute humidity effect on heat-mortality relation appeared to be related to climate. If this is generalizable, we need to incorporate humidity variable in the model for future projection of heat-related excess mortality. Acknowledgements: This study was supported by the Environment Research and Technology Development Fund (S-8 & S10) of the Ministry of the Environment, Japan, and the Global Research Laboratory (grant K21004000001-10A0500-00710) through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology, Korea.