Mortality risk and burden attributable to compound cold extreme in China: A national time series study

With global warming, several previous studies have examined the effect of compound hot extreme on mortality. However, there were little evidence on the associations of compound cold extreme (CCE) with mortality.

Daily meteorological factors, air pollutants and non-accidental mortality data during cold seasons (Nov. to Mar.) of 2006-2017 were collected from 364 counties/districts in China. CCE was defined as a cold night with a following cold day. Distributed lag non-linear model (DLNM) was first used to examine the association between CCE and mortality at county/district level. Then, meta-analysis was employed to pool the county/district-specific exposure-response associations. Finally, the mortality burden attributable to CCE was estimated.

A total of 3,086,602 deaths were included in the study. Compared with normal days, we found that CCE had much greater effect (RR = 1.63, 95 %CI: 1.56-1.71) than exposure to cold days (RR = 1.23, 95 %CI: 1.17-1.30) and cold nights (RR = 1.14, 95 %CI: 1.07-1.21). We observed a significant synergistic interaction effect of CCE. The RRs of CCE for the elderly ≥ 75 years old (1.81, 95%CI: 1.72-1.91), southern China (1.71, 95 %CI: 1.63-1.80), rural area (1.75, 95 %CI: 1.63-1.87) were higher than their counterparts. Moreover, mortality risk increased by 7.94 % (95 % CI: 7.31-8.57 %) for each 1 °C decrease in CCE intensity and by 24.93 % (95 % CI: 22.90-26.97 %) for each additional day of CCE duration. Overall, 46.03 % (95 %CI: 42.44-49.62 %) of death was attributed to CCE, with higher attributable fractions for males (47.30 %, 95 %CI: 42.48-52.12 %), people ≥ 75 years old (56.87 %, 95 %CI: 52.54-61.20 %), respiratory disease (54.74 %, 95 %CI: 47.30-62.18 %), central China (51.22 %, 95 %CI: 44.51-57.93 %) and rural area (48.41 %, 95 %CI: 42.32-54.51 %).

We found CCE significantly increased mortality risk and burden in China with higher risk for the elderly, people in central and southern China and rural areas. These findings suggest that it is necessary to develop clinical and public health policy to alleviate the mortality burden associated with CCEs.