To the best of our knowledge, this is the first study to compare the short-term effects of daily and hourly TV metrics related to cardiorespiratory mortality. Based on 15.4 million deaths, our multi-city analyses showed that both TVdaily and TVhourly were significantly associated with the increase of all-cause and cardiorespiratory mortality, with higher risks observed in the elderly and larger attributable fractions were estimated using TVdaily.
Given no unified definition of TV up to date, DTR and TCN are widely used to represent TV to assess the associations with health outcomes (Li et al. 2021; Lim et al. 2015; Zhan et al. 2017). By contrast, TVdaily and TVhourly can better capture the variation of temperature over a short period of time. Relying on very few temperature records (maximum and minimum), the assessed results based on TVdaily are only an inaccurate assessment of the relationship between TV and mortality. In other words, the standard deviations calculated by the small-scale temperature (i.e., hourly temperatures) provide sufficient data to better capture the fluctuation of temperature within a day. Despite similar TV-mortality patterns between TVdaily and TVhourly were observed in our studies, the potential overestimates were notable in attributable fractions when using TVdaily. These findings were in line with two comparative analyses (Zhan et al. 2020; Zhang et al. 2019a), both of which argued the priority of TVhourly in studies investigating TV-mortality associations.
In this study, a 1°C rise in TV at lag 0–7 days was associated with an increase of 0.55% (TVdaily, 95% CI: 0.41–0.70%) and 0.51% (TVhourly, 0.35–0.66%) in all-cause mortality. A generally greater risk of 0.9% (0.82–0.98%) for all-cause deaths was derived from a nationwide study in Japan using TVdaily (Ma et al. 2019), while a study in England and Wales experienced twice higher risk (1.13% [0.88–1.39]) (Zhang et al. 2018). Focused on urban areas, TVhourly-associated increase in this study was comparable to 0.5% (0.3–0.7%) increase from the five most populous Australian cities (Cheng et al. 2019b). We also estimated an increase of 0.53% (TVdaily, 0.31–0.76%) and 0.52% (TVhourly, 0.26–0.79%) in cardiovascular mortality and 0.62% (TVdaily, 0.26–0.98%) and 0.53% (TVhourly, 0.13–0.94%) in respiratory mortality with per 1°C rise in TV at lag 0–7 days. No study has ever estimated TVhourly-related cardiorespiratory mortality, whilst stronger TVdaily-mortality effect of 0.65% (0.24–1.05%) and 0.98% (0.57–1.39%) for cardiovascular and respiratory deaths have been reported from a previous study in 31 major Chinese cities (Yang et al. 2018). Yet the increase of respiratory mortality risk was larger than that of other causes of death, which was similar to our findings. The effect magnitude conducted in various countries and regions was quite different, and this spatial and temporal inequality was also documented in preceding multi-country studies (Guo et al. 2016).
Though plenty of epidemiological studies focus on TV-mortality relationships, mechanisms behind health effects of TV-related cardiorespiratory mortality have not yet been understood. Numerous pathophysiological responses and mild inflammation associated with temperatures are postulated to be responsible for the increase in cardiovascular and respiratory diseases (Gasparrini et al. 2015; Graudenz et al. 2006). Sudden variations of temperature within several days exceed the capacity of thermoregulation, causing inflammatory nasal responses and immune system depression (Lim et al. 2015) and the changes in blood pressure, blood viscosity and coagulability (Martinez-Nicolas et al. 2015). These alterations may trigger cardiovascular and respiratory events. In accordance with previous publications, our stratified analysis observed people over 75 years old were at greater risks related to large TV, suggesting higher vulnerability to temperature changes (Hu et al. 2019; Zhang et al. 2019b). In the context of global population ageing (Lee et al. 2019; Yang et al. 2021), cause- and group-specific prevention strategies should be considered to enhance populations’ temperature adaptability, especially in elderly people.
Our study had some notable strengths. Compared to daily TV, hourly TV had a superior description of temperature variations. Hourly temperature records can better capture the temporal changes in temperature, which may decline the potential bias. Moreover, we investigated cardiovascular and respiratory mortality related to TV, which could further the understanding of cause-specific TV-mortality associations. There are also some limitations of the present study. First, ambient air pollutants were not taken into account due to data unavailability for some cities, and the previous analyses did not find a significant contribution of air pollutants when assessing the relationship between temperature and health (Lim et al. 2015). Second, this study could not be considered nationally representative because we selected 45 megacities in urban areas to analysis and did not cover rural regions. Third, the small sample size of respiratory deaths might introduce uncertainty when assessing TV-mortality association, with respiratory results exhibiting unstable linear relationships and relatively large errors. Moreover, we did not classify cardiovascular and respiratory disease into a finer scale, which further investigation should concentrate on.