To our knowledge, this study is the first to compare and analyze the effects of DTR and AT on the onset of CVD in rural areas. Both AT and DTR had significant nonlinear, delayed effects on CVD morbidity. Whether cumulative or single lag, the effect of DTR on CVD incidence was greater and more persistent than AT in all groups. Women were more affected by AT above 20°C than men, but this was reversed at AT below 0°C. However, men appeared to be more vulnerable to high DTR (19°C) than women, while this was reversed at low DTR (6°C). There was no difference in cumulative lag impact on age subgroups in both the DTR and AT models, but the single lag impact of AT on adult men was greater than on the elderly.
We found that both DTR and AT had significant impacts on cardiovascular morbidity, while the relationship between the thermal variables and risk of CVD was nonlinear with M-shaped curves. However, this was inconsistent with many prior studies that reported relationships between AT and RR of CVD having reverse J-shaped curves (Mohammad et al. 2018; Wichmann et al. 2011; Liu et al. 2011). This was also the case for DTR. Youn-Hee demonstrated a positive linear relation between DTR and Emergency Room admissions in Korea (Youn et al. 2012; Lim et al. 2012). J-, V- and U-shaped correlations between DTR and CVD have been demonstrated in USA (K.L. et al. 2004), China (Kan et al. 2007; Xiao et al. 2014), and Korea (Youn et al. 2013). These differences may be due to geographical effects, climatic characteristics, differences in use of cooling and heating equipment, and housing types (Xiao et al. 2014).
Overall, DTR was associated with a greater risk and disease burden of CVD (cumulative lag effect: DTR 2.37, AT 1.62). Few studies have compared the extent of the effects of AT and DTR. Increases of 33% and 3% in cumulative DTR-related risk of CVD were associated with 8.2°C and 1°C ranges in southwest China (Ye et al. 2012) and Korea (Youn et al. 2012), respectively. The risk of CVD mortality increased by 7% and 14% with 7°C and 8°C increases in AT in Iran (Mohammad et al. 2018) and Copenhagen (Wichmann et al. 2011), respectively. A study conducted in Virginia, USA, showed that DTR was associated with a higher incidence of Emergency Department visits than were AT and mean temperature (Jian et al. 2017). These studies support the current finding that the RR of CVD was stronger and longer under the influence of DTR than AT. The differences in the impact of AT and DTR on CVD can be attributed to their differing pathogenesis. DTR affects cardiovascular mortality through alterations in autonomic nervous functions. A large DTR can induce disturbances of the autonomic nervous system, resulting in elevated heart rate and decreased heart rate variability, which indicate impacts on the sympathetic and parasympathetic nerves and on sinoatrial node frequency (i.e., regulation of neurohumoral factors of the cardiovascular system). (Youn et al. 2015). Furthermore, heart rate, cardiac workload and blood pressure changes triggered by wider temperature changes have been shown to contribute to the onset of CVD (Shan et al. 2020; Wang et al. 2020). AT-related impacts on CVD can be explained by that body’s circulatory system responding to high or low temperatures by releasing platelets into circulation and the increasing red and white cell counts, blood viscosity and plasma cholesterol resulting from water loss and reduced plasma volume under hot conditions (Yang et al. 2015). The direct effects of DTR on the autonomic nervous system and the indirect effects of AT on the circulatory system explain the differing impact and lag periods of these temperature parameters.
Danet et al. demonstrated that a 1.3% increase in fatal coronary events was associated with a 1°C decrease in DTR in USA (Danet et al. 1999), while a 1°C increase in mean AT was associated with a 2.4% increase in cardiovascular events in Portugal (Sofia et al. 2010). These studies suggest that AT is associated with a higher risk of CVD than DTR, which is not consistent with the current study. Possible reasons for this inconsistency are differences in population demographics, exposure conditions, climatic characteristics, and climate-related behaviors (Wichmann et al. 2011; Song et al. 2017; Zan et al. 2016). The current study area, Qingyang located in the Loess Plateau, has a continental climate which creates a large DTR but a relatively moderate AT. People in Qingyang suffer a greater risk of CVD due to DTR rather than AT because they are exposed to large temperature differences rather than extreme temperatures. Furthermore, farmers, especially those most susceptible to cardiovascular disease, tend to be more vulnerable to DTR because of their lack of awareness of how to protect against extreme DTR. Moreover, the weather forecast broadcasts temperature rather than DTR. People suffer more adverse effects from DTR than AT because it can be hard to respond quickly to sudden temperature changes, while people can deal with high and low temperatures by adding or removing clothing in advance, using air conditioning, and so on. It is proposed that relevant departments should broadcast DTR forecasts for the benefit of CVD-susceptible populations.
Another interesting finding in this study was that women were susceptible to a slightly higher AT than men, whereas, when AT was below 10°C men were at higher risk. Previous studies are consistent this finding, confirming that women have a higher risk of CVD in hot conditions, while men are at greater risk in cold conditions (Mohammad et al. 2018; K.L. et al. 2004; Zhao et al. 2018). This may be due to women being more susceptible to arrhythmia, ischemia, and high blood pressure, all of which are exacerbated by extreme hot and cold temperatures (Yang et al. 2015; Ji et al. 2014). Conversely, hot conditions increased coronary events in men in San Paulo while low temperatures with up to two lag days increased women’s mortality risk in a subtropical climate zone in China (Ji et al. 2014). Meta-analysis performed by Moghadamnia et al. on data from Mohamamad et al. confirmed their finding of no difference in the impact of hot and cold temperatures on CVD in the two genders (Wang et al. 2020; Zan et al. 2016). Basu suggested that different gender temperature effects were the result of location and population (Basu et al. 2005). A European study demonstrated that clothing plays a significant role in such differences and that there are also biological gender differences in thermoregulation (Keatinge et al. 1997). Thus, women should pay more attention than men to protective measures when AT > 20 °C. A further issue is that farmers, whether women or men, have increased exposure to high AT during the harvest and sowing seasons in mid-April and mid-September (Dian et al. 2016; Tao et al. 2018; Yi et al. 2015) when temperatures range from 20 to 25 °C (Fig S3, see additional file 4). Both men and women work during these periods but women bear the greater risk of CVD (Hong et al. 2003). Again, clothing was shown to play a significant role in this gender difference (Guo et al. 2011).
DTR-related RR of CVD in men was higher than in women at high DTR (20°C), while this was reversed at low DTR (5°C). Epidemiological studies have proven that CVD mortality in females was more strongly associated with DTR than in males in China (Xiao et al. 2013; Breitner et al. 2014), Korea (Mohammad et al. 2018), and Japan (Jayeun et al. 2016). This is in line with the present results at low DTR, but conflicts with the results at high DTR. This sex-related difference may be attributable to demography-related behavioral differences and geographical effect modifiers (Mohammad et al. 2018). The present data were obtained from the NRCMI in order to examine a population of farmers. In rural Qingyang more than 30% of farmers are migrant workers (Ting et al. 2011; Zhi et al. 2011; Chun et al. 2014) and most of them are men whose work is usually outdoors, while the women generally work at home. The woman could, therefore, be more flexible at work compared with the men in the face of adverse weather conditions, possibly decreasing or avoiding exposure during high-DTR days, while the men remained exposed to the adverse temperatures. Consequently, on suitable days with low DTR, women may go out to work along with the men but they also bear a greater CVD risk due to sex-related physical differences (Hong et al. 2003). However, the men had longer exposure on the high-DTR and low-AT days, leading to a higher risk of CVD compared with women. A previous study has shown that greater DTR exposure or weaker intrinsic susceptibility factors may result in increased risk (Youn et al. 2015). Different subgroups and mortality categories were sensitive to different temperature indicators (Yu et al. 2011), which also illustrates why women were sensitive to high AT and men were sensitive to high DTR.
To our knowledge, this is the first study to compare and analyze the impact of DTR and AT on CVD. The relationship between thermal variations and morbidity was studied in a developing rural area of Northwest China (Qingyang). This study used validated and credible data originating from NRCMI of Gansu Province which recorded all outpatient visits by farmers. The results provide information for local government to enhance the protection of citizens of rural and less-developed areas. This analysis has both strengths and limitations. Firstly, only one area was selected as representative of typically poor rural regions. Further studies need to be conducted to evaluate DTR impact in other rural regions and confirm these findings. Secondly, the effects of characteristics such as personal behavior, medical history and living conditions were not considered due to their absence from the available data. Thirdly, the assumption that all individuals were exposed to the same thermal environment was a generalization. Finally, the retrospective data collection method may allow biases from diagnostic and coding inaccuracies.