The joint effect of temperature and humidity exposure on non-accidental mortality in summer

Objective To explore the joint effect of temperature and humidity exposure on non-accidental mortality in summer.

Methods The time series data on non-accidental mortality in 124 cities (autonomous prefectures) in summer during 2006—2017 were collected from Chinese Center for Disease Control and Prevention and the centers for disease control and prevention of Guangdong, Zhejiang, Hunan, Yunnan, and Jilin provinces in China. The meteorological data and pollutant data of these cities during the same period were obtained from the National Meteorological Information Center and the National Urban Air Quality Real-Time Dissemination Platform, respectively. The distributed lag non-linear model was employed to evaluate the associations of temperature and humidity with mortality. The quantile g-computation model was employed to evaluate the joint effect of temperature and humidity exposure on non-accidental mortality.

Results The exposure-response correlation between summer temperature and cumulative mortality exhibited a "J" shape, while that between relative humidity and cumulative mortality showed an inversed "J" shape. The correlation of summer temperature and humidity exposure with non-accidental mortality was non-linear. Compared to the first quantile of the joint exposure, the relative risks for the second, third, and fourth quantiles were 1.01 (95% confidence interval CI: 1.00-1.02), 1.04 (95% CI: 1.02-1.05), and 1.08 (95% CI: 1.05-1.10), respectively. Males, the elderly (≥65 years), and patients with cerebrovascular diseases showed relatively high sensitivity, while those in central China showed relatively low risk. The contributions of temperature and humidity to the joint effect in entire population were 84.92% and 15.08%, respectively. The contributions of temperature in males and the elderly (≥65 years) were 86.98% and 86.16%, respectively, which were higher than those of humidity in the same subgroups. The contributions of temperature in northern, central, and southern China were about 80%. The contributions of temperature in cardiovascular, cerebrovascular, and respiratory diseases were 68.01%, 72.44%, and 71.46%, respectively.

Conclusion The risk of non-accidental mortality increases as temperature and humidity increase in summer, and the contribution of temperature to the joint effect is relatively higher. The associations of temperature and humidity with non-accidental mortality might be modified by sex, age, region, and diseases.