Long-term variations in the association between ambient temperature and daily cardiovascular mortality in Shanghai, China

Objective: The objective of this study was to explore the long-term variation in the association between ambient temperature and daily cardiovascular (CVD) mortality in Shanghai, China.

Materials and methods: We collected daily data on ambient temperature and CVD mortality from a central urban district in Shanghai, China, during the period from 1981 to 2012. We used overdispersed generalized additive Poisson regression together with a distributed lag nonlinear model to estimate potentially lagged and nonlinear effects of temperature on CVD mortality after controlling for the seasonality, relative humidity, day of the week, holidays and population size. To allow for the evaluation of long-term variations in the effects, we divided the entire study period into six sub-periods (1981-1985, 1986-1990, 1991-1995, 1996-2000, 2001-2005, and 2006-2012) and analyzed the effect estimates in each sub-period separately.

Results: The association between temperature and daily CVD mortality was J-shaped with both low and high temperatures increasing the risk of CVD deaths. The effects of extremely low temperatures were delayed and persisted for two weeks, while extreme hot effects were limited to the first five days followed by a significant mortality displacement (9days). The relative risks (RRs) of extremely low, moderately low, moderately high, and extremely high temperatures comparing the 1st, 10th, 90th, and 99th percentile with the reference temperature (26°C) over the cumulative lags of 0-14days were 1.95 [95% confidence interval (CI): 1.84,2.07], 1.61 (95% CI: 1.57,1.66), 1.03 (95% CI: 1.01,1.05), and 1.14 (95% CI: 1.07,1.21). The RRs for extremely low and moderately low temperature attenuated substantially from 9.78 and 5.52 in 1981-1985 to 1.42 and 1.18 in 2006-2012, respectively, but the RRs remained almost stable for extremely high and moderately high temperatures.

Conclusions: This time-series study suggested that there might have been some human adaptation to low ambient temperature in Shanghai, China, over the last 3 decades.