Discussion

Ambient air pollution and meteorological variables can affect viral transmission; however, they have not yet been examined for the presence of SARS-CoV-2. In the current study, we have provided an initial assessment of the potential effects of ambient air pollutants and meteorological variables on the incidence of COVID-19 day by day from January 25 to February 29, 2020. Overall, based on data from China’s 3 worst COVID-19–stricken cities, COVID-19 incidence was negatively associated with PM₁₀ and temperature. Meanwhile, a positive association was observed between COVID-19 and PM_2.5 and relative humidity.

Particulate matter is a mixture of both solid particles and liquid droplets suspended in the air. Although it is not the sole cause of respiratory illness, previous studies have demonstrated that particulate matter is a strong environmental determinant of viral transmissions. For example, a close correlation was discovered between human influenza cases and PM_2.5 concentrations in epidemiological studies based in big cities such as Beijing,^{Reference Liang, Fang and Pan12} Hong Kong,^{Reference Qiu, Yu and Tian25} and Brisbane.^{Reference Xu, Hu, Williams, Clements, Kan and Tong26} Its potential mechanisms are believed to damage bronchial immunity^{Reference Ma, Song and Guo27} and epithelial cell integrity,^{Reference Jaspers, Ciencewicki and Zhang28} which enhances viral attachment to and replication in the bronchus.^{Reference Jaspers, Ciencewicki and Zhang28} In the current study, although our data could not reveal whether particulate matter promotes SARS-CoV-2 transmission, we observed that PM_2.5 is the only air pollutant that is consistently associated with increased COVID-19 incidence in all cities. This finding agrees with the data from a previous influenza study. Contrary to PM_2.5, increased PM₁₀ concentration was associated with decreased COVID-19 incidence. We believe this phenomenon to be a diameter-related effect. As shown by previous study, the receptor for both SARS-CoV-2 and SARS-CoV binding is the angiotensin-converting enzyme 2, which concentrates on type II alveolar cells.^{Reference Zhou, Yang and Wang29} However, type II alveolar cells are located in the alveoli, which can only be reached by particles with a diameter <5 µm.^{Reference Brankston, Gitterman, Hirji, Lemieux and Gardam30} Thus, small airborne pollutants, such as PM_2.5, are particularly harmful because they are more likely to penetrate the respiratory tract all the way to the alveolar region unfiltered.^{Reference Tellier31,Reference Hinds32} For larger particulate matter, penetration ability decreases dramatically when size increases; starting at 20 µm and beyond, particulate matter is not able to penetrate below the trachea.^{Reference Hinds32} Although all particulate matter might provide condensation nuclei for viral attachment,^{Reference Lee, Saravia and You11} PM_2.5 deliver SARS-CoV-2 to their target cells in the alveoli, which is unreachable to PM₁₀. This size factor might also explain our finding that PM_2.5 correlates with COVID-19 incidence in a positive pattern while that of PM₁₀ is negative. Future epidemiological studies in other countries and areas with similar population density but different PM_2.5 and PM₁₀ concentrations could be used to validate this result.

So far, a number of studies have indicated a negative effect of ambient temperature and humidity on the viral viability and transmission. For example, low humidity increases influenza viability in the aerosol^{Reference Brankston, Gitterman, Hirji, Lemieux and Gardam30} and impairs the innate antiviral defense of the host,^{Reference Kudo, Song and Yockey33} which promotes influenza transmission. Meanwhile, temperature has the same effect as humidity on influenza.^{Reference Lowen, Mubareka, Steel and Palese34} Similar to influenza, epidemiological studies have also demonstrated that the transmission of SARS-CoV-2 is more efficient in low temperatures and at low humidity.^{Reference Qi, Xiao and Shi35,Reference Liu, Zhou and Yao36} This hypothesis was supported by the results from a laboratory study; SARS-CoV was more likely to be inactivated in higher temperature and humidity.^{Reference Casanova, Jeon, Rutala, Weber and Sobsey37,Reference Chan, Peiris, Lam, Poon, Yuen and Seto38} In the current study, our data agree with previous findings that temperature is negatively associated with COVID-19 incidence. However, our results regarding humidity do not agree with those from previous studies. Recently, an outbreak of COVID-19 was reported in a public bath center in China.^{Reference Luo, Yao and Zhang39} Since the bath center was extremely humid, SARS-CoV-2 should have been inactivated quickly; however, this was not the case. Thus, our results support this finding and suggest that the study of differences between SARS-CoV-2 and other viruses is warranted in future studies.

Whether SARS-CoV-2 can be transmitted via airborne droplets remains controversial.⁴⁰ Recently, Luo et al^{Reference Luo, Yao and Zhang39} reported a COVID-19 outbreak in a public bath center in China, which suggests the role of ambient droplet in SARS-CoV-2 transmission in conditions of high relative humidity. Later, van Doremalen et al^{Reference van Doremalen, Bushmaker and Morris41} also demonstrated that, under laboratory conditions, SARS-CoV-2 is viable in aerosol droplets for at least 3 hours, which provides theoretical evidence for how SARS-CoV-2 transmission in that environment. However, some researchers dispute ambient droplet transmission because SARS-CoV-2 is not present in air samples obtained from the rooms of hospitalized COVID-19 patients.^{Reference van Doremalen, Bushmaker and Morris41} Most studies thus far have also reported that healthy people are infected by touching contaminated objects⁴⁰ and that SARS-CoV-2 is stable on copper, cardboard, plastic, and stainless-steel surfaces for up to 72 hours.^{Reference van Doremalen, Bushmaker and Morris41} In the current study, we have shown that ambient air pollutants, especially PM_2.5, are closely associated with COVID-19 incidence, which supports the hypothesis regarding airborne droplet transmission of SARS-CoV-2. However, the viability of SARS-CoV-2 attaching to PM_2.5 remains largely unknown. Further studies are required to investigate the extent of SARS-CoV-2 transmission via airborne droplets.

Winter features overlap multiple viral seasons at various degrees. Previous studies have suggested that epidemics of cocirculating viral infections can interfere with each other.^{Reference van Asten, Bijkerk and Fanoy42,Reference Zheng, Song, Li, Zhang and Wang43} For example, influenza A could overlap coronavirus outbreaks.^{Reference van Asten, Bijkerk and Fanoy42} Thus, it is reasonable to hypothesize that COVID-19 incidence might also be significantly affected by other cocirculating viruses. Additionally, 1 study concluded that 2 SARS-CoV-2 strains are cocirculating in Italy.^{Reference Stefanelli, Faggioni and Lo Presti44} Whether these 2 strains affect the infectious ability of the other remains unknown. Further study is required to fully elucidate this issue.

The current study has several limitations. First, because the ambient PM_2.5 is a mixture of solid particles and liquid droplets, the exact components of PM_2.5 that may promote coronavirus transmission remain unknown. Second, due to the relatively short study period and imperfect daily reporting practices, our results are vulnerable to changes resulting from the emergence of more detailed data. Third, we studied only 9 variables in the current study. A number of other variables could potentially have affected COVID-19 transmission; thus, our study can only provide preliminary information on the association between COVID-19 and air pollutants or other meteorological variables. In a future study, we plan to gather data from more countries and regions to remedy these data gaps. Fourth, because detailed information from confirmed COVID-19 patients was not available, we were unable to determine how underlying health problems might promote COVID-19 infection. Thus, a longer study period might have affected our results. Moreover, winter overlapped the entire study period, which limited all meteorological variables (especially temperature) to a narrow band. Thus, further study is urgently needed further elucidate these issues.

In conclusion, our findings consistently suggest that increased temperature and airborne PM_2.5 concentration are associated with increased daily COVID-19 incidence. Meanwhile, a negative association was observed between COVID-19 incidence and relative humidity and airborne PM₁₀ concentration. In the context of the worldwide COVID-19 pandemic, our findings might provide some information for COVID-19 personal protection and transmission reduction.