Ambient air pollution and meteorological variables can impact viral transmission (e.g., influenza) but have not yet been examined for the SARS-CoV-2. In the current study, we provided an initial assessment of ambient air pollutants and meteorological variables on the incidence of COVID-19 day by day from Jan 25th to Feb 29th in 2020. Overall, the results, based on data from China’s two worst-hit cities, we showed that the COVID-19 incidence was tightly correlated with PM2.5, NO2 and local temperature, indicating their potential role in promoting viral transmission.
The PM is a mixture of both solid particles and liquid droplets suspended in the air. PM2.5 and PM10 is identified as the PM diameter cut off at 2.5 and 10 µm, respectively. Although it is not the sole cause of respiratory illness, previous studies had provided some evidence to support the hypothesis that ambient air pollutant is a strong environmental determinant of viral transmissions. For example, tight correlation was discovered between human influenza cases and PM2.5 concentrations in epidemical studies based in big cities, such as Beijing[23], Hong Kong[24] and Brisbane[25]. Its potential mechanisms are believed to the increased oxidative stress caused by ambient air pollutants that damage bronchial immunity[26] and epithelial cells integrity [27] and thus enhance viral attachment. However, it remains particularly unclear whether the ambient air pollutant could assist SARS-CoV-2 transmission. In the current study, we observed that among six ambient air pollutants, the PM2.5 concentration is constantly and positively correlated with the COVID-19 incidence, which agreed with the data from previous influenza study. Moreover, although not statistically significant in Wuhan, the correlation between PM10 concentration and COVID-19 incidence is prominent in XiaoGan. We tend to believe that PM2.5 and PM10 could both promote coronavirus transmission based on following reason: The receptor for both SARS-CoV-2 and SARS-CoV binding is the angiotensin-converting enzyme 2, which concentrates on the type II alveolar cells [28]. However, the type II alveolar cells locates in the alveoli, which is only reachable to particles with a diameter less than 5 micrometers[29]. Thus, small airborne pollutants, such as PM2.5, are particularly harmful as they are more likely to penetrate the respiratory tract all the way to the alveolar region unfiltered[30, 31]. For larger PM, penetration ability decreased dramatically when its size rise. We believe PM10 is also harmful respiratory tract to some extent as starts at 20 µm and beyond, PM is not able to penetrate below the trachea[31]. This might also explain our finding that PM2.5 is more tightly correlated with the COVID-19 incidence than that of the PM10. The safety guideline concentration of PM2.5 of PM10 are 15 and 50 µg/m3 by US EPA[32], respectively. Meanwhile, both Wuhan and XiaoGan were constantly suffered from particularly higher than safe PM2.5 and PM10 levels during our study period. Future epidemiological study in other world regions with similar population density but different PM2.5 and PM10 concentration could be used to verify this result.
There are a number of studies demonstrate the adverse health effects of NO2 exposure. For example, short-term increase of outdoor NO2 concentration can significantly increase the risk of upper respiratory tract infection[33]. This phenomenon is was particularly notable in children, as this subpopulation is highly susceptible to NO2 induced lung injury [34–36]. Viral infection was common after NO2 exposure. According to Chauhan et al.[37], four viruses were frequently detected in NO2-related respiratory tract infection and coronavirus was one of them. Previous study indicated that preceding NO2 exposure can decrease host immunity and thus significantly increase infection risk of cytomegalovirus in mice[38]. Moreover, recovered mice tended to be re-infected after re-exposing to NO2[39]. In the current study, although the NO2 level was constantly lower than the US EPA standards[40], our data revealed that COVID-19 incidence were highly correlated with the ambient NO2 concentration. This finding agreed to epidemiological studies from other regions of the world [35, 37].
So far, epidemiological studies had identified at least nine viruses categories that are capable of infecting respiratory tract [41, 42]. Although all feature seasonal oscillation of outbreaks, only three viruses show peak incidences in the winter months, which are the Influenza, human coronavirus, and human respiratory syncytial virus[43, 44]. Although the epidemiological characteristics of SARS-CoV-2 is not clear, recent study predicted the SARS-CoV-2 transmits more efficiently in winter than summer[45], indicating the importance of temperature. In current study, our data agreed with previous findings as we demonstrated that temperature is an important metrological variable that is negatively correlated with COVID-19 incidence, indicating lower temperature promote the SARS-CoV-2 transmission. This phenomenon might be related to life-style as people tend to huddle indoors together during winter season. Future study needs to investigate the direct effect of temperature on viral activity as well.
There also exist some discrepancies between the results current and previous studies. For example, it had been shown that low humidity could increase the viability and thus promote viral transmission [29, 46]. However, we failed to observe the role of humidity on COVID-19 incidence. This issue might be related to the high humidity (constantly > 50%) of Wuhan and XiaoGan during the entire period. Without proper shifting of humidity for comparison, we believe our data could not properly reveal the importance of humidity on viral transmission.
The current study has some limitations. First, as the ambient PM2.5 is a mixture of solid particles and liquid droplets, the exact components of PM2.5 that could promote coronavirus transmission remain unknown. Second, due to the relatively short study period since the current outbreak and imperfect daily reporting practices, our results are vulnerable to changes as the emerging of more detailed data.