In the current study, GAM was used to investigate the link of short-term exposure to DTR and the daily number of outpatient visits of college students for URTI at Hospital of Wuhan University in Wuhan, China. After adjusting for multiple covariates in this model, we noticed that with the increase of DTR, the number of outpatients was increasing accordingly. We also found the strongest effects occurred at lag 0 day in single lags and lag 0-6 days at cumulative lags whatever for all, males and females. Females were more vulnerable to extremely acute exposure to DTR than males. In generally, the estimates of the associations between DTR and URTI were larger in warm season than cold season, especially in the cumulative lags. This study might have great significance to the prevention of upper respiratory tract infection in college students and provide scientific epidemiological evidence and reference for relevant administration department to take measures to control temperature change.
Our results showed that per 1 °C increased in DTR would elevate 0.89% (95% CI: 0.40, 1.38) and 1.62% (95% CI: 0.76, 2.50) in the daily number of outpatient visits of all college students for URTI at lag 0 and 0-6 days, respectively. These findings were consistent with some previous studies (Ma et al. 2018; Phosri, Sihabut, and Jaikanlaya 2020). For instant, Ge et al. found per 1 °C increment of DTR could increase 0.94% (95% CI: 0.34, 1.55) and 1.60% (95% CI: 0.62, 2.58) in Emergency-room Visits for respiratory tract infection in Shanghai at lag 0 day and at lag 0-5 days, separately (Ge et al. 2013). Li et al. observed that a 1 °C increase in DTR corresponded to 2.55% (95% CI: 1.97, 4.01) increase in the number of daily Emergency-room Visits for respiratory tract infection in children aged ≤5 years old at lag 1 day (Li, Yang, and Wang 2014).
A tremendous amount of previous studies pointed out that DTR might be an independent risk factor for human health (Kim et al. 2016; Ding et al. 2015). The potential mechanism of action of DTR was not well understood yet, but one research had been suggested that rapid changes in temperature can increase respiratory load and thus induce respiratory health events (Imai et al. 1998). Air with suddenly altered temperature was inhaled into the human body related to up-regulate the release of mast cell-associated inflammatory mediators (Togias et al. 1985). In addition, the rapid decrease of temperature in respiratory epithelial would lead to the decrease of the effectiveness of local respiratory defense (for example, mucociliary clearance and leukocyte phagocytosis) (Diesel, Lebel, and Tucker 1991; Bang 1961). DTR might be an additional independent environmental influence, and its variation can result in the increment of respiratory stress (Ge et al. 2013). The high value of DTR can promote the spread and reproduction of bacteria and viruses (Cheng et al. 2014; Onozuka 2015). Besides, when the temperature of respiratory epithelial fluctuated violently, it might affect the host defense function of the respiratory system, nasal response and airway mucosal cilia clearance, thus increasing the incidence of respiratory infections (Graudenz et al. 2006; Diesel, Lebel, and Tucker 1991). Based on the complexity and diversity of DTR impacts, we called for more researches to explore its exact mechanism.
It was worth noting that in our results the effect values of per 1 °C increment of DTR in females [0.76%, 95%CI: (0.18,1.34)] for URTI at current day was higher than males [1.01%, 95%CI: (0.48,1.54)]. However, in a longer lag period, the males emerged greater effect values than females.
For instant, a 1 °C increased in DTR associated with 1.83% (95% CI: 0.81, 2.86) elevated in the daily number of outpatient visits of male college students for URTI, but only 1.44% (95% CI: 0.51, 2.38) in female college students. One study observed the relative risks of DTR and hospital admission for cardiovascular in males from lower than females at lag 0 day [1.044 (0.991, 1.101) for males vs 1.058 (1.004, 1.116) for females] to higher than females at lag 0-7 days [1.151 (1.022, 1.296) vs 1.115 (0.988, 1.257)], which were similar to our results (Phosri, Sihabut, and Jaikanlaya 2020). Nevertheless, researchers also found females were more likely to be vulnerable to DTR than males at all lag days (Zhang, Yu, Yang, et al. 2017; Zhou et al. 2014). The possible reason for such inconsistent results was that the study area (climate conditions) and population (economy and education) were different (Basu 2009; Ding et al. 2015).
Among all the results with statistically significant in this study, we observed that the effect values in the warm season were higher than those in the cold season. Some previous studies found similar outcomes (Zhang, Yu, Bao, et al. 2017; Zheng et al. 2016). A positive and stronger relationship was revealed between DTR and childhood asthma exacerbations in the warm season (May to September) (Hu et al. 2020). However, researchers in Shanghai pointed out results which were inconsistent with our study, they only found the associations of DTR and respiratory mortality in the cold season (Kan et al. 2007). In addition, a study in Changchun found that for a 1 °C increased in DTR increased the risk of hospital admissions for COPD by 11.5% (95%CI: 10.0,12.9) in the cold season, which was higher than 9.0% (95%CI: 7.7, 10.3) in the warm season (Ma et al. 2018). The results that appeared in the current study can be partially explained as follows: Due to the relationship dictated by the frequency of cooler air masses in the warm season, these days with lower minimum temperatures usually had higher maximum temperatures, which led to the values of DTR were higher than that in the cold season (Davis, Hondula, and Sharif 2020).
In sensitivity analysis, the associations of DTR and the daily outpatient visits of all college students for URTI remained statistically significant despite the effect values were slightly reduced after adjusting for air pollutants (e.g. PM2.5, PM10, SO2, NO2, and O3). Similar results were also found in male and female college students, which meant DTR was a robust and trustworthy predictor for outpatient visits of college students for URTI. Many previous studies pointed out that DTR was still associated with mortalities and emergency room admissions for a variety of cardiovascular and respiratory diseases when adding air pollutants to model (Liang, Liu, and Kuo 2009; Yang et al. 2013; Tam et al. 2009), which were consistent with our findings.
In this study, the exposure-response curves between DTR and the daily outpatient visits of URTI in all, male and female college students were close to linear upward trends at lag 0-6 days. These linear-like trends were also found in previous studies (Wang et al. 2020; Zheng et al. 2016). Therefore, the generalized additive models were used by us to investigate the associations of DTR and the outpatient visits for URTI. However, in some other studies, U- or J-shaped exposure-response curves between DTR and the occurrence of adverse outcomes were revealed (Deng et al. 2020; Hu et al. 2018; Zhao et al. 2017). A study conducted in Hefei even showed slightly M-shaped exposure-response curve between DTR and the risk of admissions for tuberculosis (Huang et al. 2020). The reasons for such inconsistent results might be that the types of population and disease in these studies were very different.
The strongest effect values of DTR were found at the current day in the models of single-day lags, whether for male, female, or all college students. Likewise, a time-series study in Beijing indicated that the adverse effects of DTR peaked at lag 0 day [per 1 °C increased in DTR corresponded to 0.58% (95%CI:0.02, 1.15) increase in emergency room admissions for respiratory disease] (Wang et al. 2013). A study conducted in Changchun, one northeast city of China, reported that the largest relative risk of 1.023 (95%CI:1.004, 1.042) about hospital admissions for chronic obstructive pulmonary disease was observed at lag 7 day in the males when 1 °C increased in DTR (Ma et al. 2018), which was inconsistent with our results.
In summary, our study has several advantages: 1) According to the information I have, this is the first study exploring the associations between DTR and upper respiratory tract infection among college students in China. 2) University is one of the most important stages of learning. College students with upper respiratory tract infection will seriously affect the efficiency of learning. Therefore, it is necessary to investigate the relationship between DTR and upper respiratory tract infection. 3) The population of this study all live in Wuhan University, an area of about 3.5 square kilometers, and they will stay in school most of the year because of their studies. So, it is reasonable to suppose that they have similar exposure backgrounds. 4) The hospital of Wuhan University had set up four branches in four academic departments to facilitate college students with different majors and faculties to seek basic medical treatment. In addition, benefit by the medical insurance policy of Wuhan University, the medical expenses of our college students in the Hospital of Wuhan University are all reduced 90%, which greatly avoids the bias of not choosing to seek medical treatment due to the cost (Zhang et al. 2021), and makes the exposure-response relationship of DTR and the daily outpatient visits of URTI in this study more genuine and believable.
Nevertheless, there are several limitations of this study that must be mentioned: 1) data about meteorological factors and air pollutants were collected from monitoring stations with a certain distance from Wuhan University, and individuals’ indoor and outdoor activity duration cannot be measured, which would inevitably lead to misclassification in exposure to DTR to a certain degree. 2) Due to the unavailability of data about more detailed personal information, for example, body mass index (BMI), eating habits, smoking status, history of diseases and so on, which would result in unavoidable errors. 3) The outpatient visit data provided from the hospital of Wuhan University do not contain patients’ name because of medical ethics, therefore, we cannot distinguish the cases of repeated visits and regard each visit record as a new case. More comprehensive researches were warrant to reveal the associations between DTR and URTI.