Using uniform and the most up-to-date methodology, we characterised the exposure-response associations between ambient temperature and mortality from all natural causes, overall cardiovascular disease, coronary heart disease, overall stroke, haemorrhagic stroke, ischaemic stroke, overall respiratory diseases, and chronic obstructive pulmonary disease in 272 main Chinese cities. We further estimated the consequent disease burden of non-optimum cold and hot temperatures in five different climatic zones and for different subgroups. In addition, we examined a variety of socioeconomic, geographical, and climatic characteristics as potential effect modifiers. Our results showed that both cold and hot temperatures were associated with increased mortality risks from various cardiopulmonary diseases, and non-optimum temperatures (especially moderate cold) were responsible for a considerable fraction of premature deaths in China. Finally, we identified some effect modifications by climatic zones, demographic characteristics, and socioeconomic characteristics.

Principal findings and interpretations

We found associations between temperature and mortality to be consistently inversely J shaped, which have been shown in the majority of previous multicity studies.123917 Also consistent with previous studies, the effects of cold temperature could last longer than two weeks, whereas the effects of hot temperature appeared immediately, persisted only two or three days, and were followed by a mortality displacement.92223 Accordingly, we estimated much higher relative risks for cold temperatures than for hot temperatures. The findings on the lag patterns implied that prompt and transient preventive measures could help address heat related health risks, while prolonged protection would be need to address cold related risks. In the present study, the minimum mortality centile for the association between temperature and total mortality was centred at 84th centile (median) of temperature, which was similar to China’s estimate (15 cities) reported in a previous global analysis.6

We also evaluated the mortality burden attributable to non-optimum temperatures and separated the attributable fractions due to different components of non-optimum temperatures. Our findings showed that non-optimum temperature could account for an overall attributable fraction of 14.33% in total mortality in China, which was comparable to China’s estimate of 11.00% reported in the global analysis.6 Consistent with this study, the present analysis showed that cold temperature was mainly responsible for the mortality burden, which was caused by the right shifted minimum mortality temperature (leading to more cold days) in the temperature distribution as well as the higher and more delayed effects of cold temperature than of hot temperature (fig 1, fig 2, and fig 3). The prolonged mortality effects of cold temperature might be explained by the indirect pathway through influenza infection.24 Moderate cold and moderate heat resulted in much larger attributable fractions than extreme cold and extreme heat, merely because they accounted for more days.

The present study provides ample evidence about the mortality risk and burden of various non-optimum temperatures on major cardiorespiratory diseases, which have rarely been investigated. The overall attributable fractions of cardiovascular and stroke mortality in our study were similar to those reported previously in 15 or 16 large Chinese cities.78 We found higher minimum mortality temperatures and cold related relative risks for cardiovascular diseases than for respiratory diseases. Accordingly, the overall attributable fractions of cardiovascular mortality attributable to cold temperature were larger than of respiratory mortality. The stronger and more prominent effects of cold temperature found on the cardiovascular system than on the respiratory system were consistent with previous studies,172526 and were also biologically plausible. For example, the effects from cold temperatures on the cardiovascular system are often due to potential complications associated with increased cardiovascular risks in relation to changes in autonomic nervous system, blood pressure, thermogenesis, inflammatory response, and oxidative stress.2728 The effects from cold temperatures on the respiratory system might be due to increased respiratory infections in cold days.29 Compared with ischaemic stroke mortality, haemorrhagic stroke mortality was more affected by cold temperature and less affected by hot temperatures, which might be explained by the elevated blood pressure in cold weather and reduced blood pressure in hot weather.30

In this nationwide analysis, we also identified potential effect modification in terms of demographic, climatic, and socioeconomic characteristics, which were important to develop evidence based health protection plans against abnormal weather or climate changes.31 We found that the mortality burden attributable to non-optimum temperatures were larger in people who were aged 75 years and older, who were female, and who spent relatively less time in education (up to nine years), which may be explained by potential vulnerabilities in these subgroups.32 Compared with the subtropical monsoon zone, the temperate monsoon zone had a larger mortality burden due to non-optimum temperatures, especially in hot temperatures. This finding could be due to the weak capability of heat adaptation in the temperate monsoon zone, reflected by the much lower minimum mortality centile in this zone than in the subtropical monsoon zone (72.1st centile v 82.5th centile). The increased effects from hot temperatures in colder regions was also consistent with previous studies.1723

The associations between temperature and mortality and the resulting disease burden were weak and statistically non-significant in the temperate continental zone, alpine zone, and tropical monsoon zone, which could be due to high statistical uncertainty in relation to the small populations and the few cities included. We also found some significant modifications in terms of socioeconomic characteristics. In multivariate meta-regression models, GDP per capita and urbanisation rates could modify the temperature-mortality association and its lag structure. Increased urbanisation rates corresponded to stronger effects from hot temperatures, which might be due to so-called heat island effects33; and a longer duration of central heating decreased mortality risk and burden attributable to cold temperature, increasing the adaptive capability against cold exposure.33