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. 2021 Feb 9;93:243–251. doi: 10.1016/j.gr.2021.02.002

Multiple relationships between aerosol and COVID-19: A framework for global studies

Yaxin Cao a, Longyi Shao a,⁎, Tim Jones b, Marcos LS Oliveira c,d, Shuoyi Ge a, Xiaolei Feng a, Luis FO Silva c, Kelly BéruBé e
PMCID: PMC7871891  PMID: 33584115

Abstract

COVID-19 (Corona Virus Disease 2019) is a severe respiratory syndrome currently causing a human global pandemic. The original virus, along with newer variants, is highly transmissible. Aerosols are a multiphase system consisting of the atmosphere with suspended solid and liquid particles, which can carry toxic and harmful substances; especially the liquid components. The degree to which aerosols can carry the virus and cause COVID-19 disease is of significant research importance. In this study, we have discussed aerosol transmission as the pathway of SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2), and the aerosol pollution reduction as a consequence of the COVID-19 lockdown. The aerosol transmission routes of the SARS-CoV-2 can be further subdivided into proximal human-exhaled aerosol transmission and potentially more distal ambient aerosol transmission. The human-exhaled aerosol transmission is a direct dispersion of the SARS-CoV-2. The ambient aerosol transmission is an indirect dispersion of the SARS-CoV-2 in which the aerosol acts as a carrier to spread the virus. This indirect dispersion can also stimulate the up-regulation of the expression of SARS-CoV-2 receptor ACE-2 (Angiotensin Converting Enzyme 2) and protease TMPRSS2 (Transmembrane Serine Protease 2), thereby increasing the incidence and mortality of COVID-19. From the aerosol quality data around the World, it can be seen that often atmospheric pollution has significantly decreased due to factors such as the reduction of traffic, industry, cooking and coal-burning emissions during the COVID-19 lockdown. The airborne transmission potential of SARS-CoV-2, the infectivity of the virus in ambient aerosols, and the reduction of aerosol pollution levels due to the lockdowns are crucial research subjects.

Keywords: Atmospheric aerosols, COVID-19, PM2.5, SARS-CoV-2, Transmission routes

Graphical abstract

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1. Introduction

COVID-19 (Corona Virus Disease 2019) is another infectious disease caused by coronavirus following MERS (Middle East Respiratory Syndrome) and SARS (Severe Acute Respiratory Syndrome). The number and speed of COVID-19 infections significantly exceed those of MERS and SARS (Gautam et al., 2020; Javed et al., 2020). The virus that causes COVID-19 is named SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) (Gorbalenya et al., 2020). Since the first confirmed COVID-19 patient was identified on December 12th, 2019, the total number of patients diagnosed in the World has reached 91,293,732 cases by January 12, 2021, especially in the USA (23,143,197 cases) and India (10,479,179 cases) (WHO World Health Organisation, 2021), this number is still rapidly rising. In addition to the respiratory disease, SARS-CoV-2 can also cause other clinical symptoms, such as damage to the nervous system (Huang et al., 2020). COVID-19 has high infectivity with treatments being rapidly optimized, and it is typically most dangerous for the elderly (Pagani et al., 2020) or those with underlying health issues. Since SARS-CoV-2 was first identified a number of variants have been found in COVIDs-19 cases around the world (Weisblum et al., 2020), including the UK and South Africa (Koyama et al., 2020; Tang et al., 2021). Currently, the 501Y·V2 variant is considered to be a more highly transmissible strain due to the rapidity with which it became the dominant circulating genotype in South African over a few weeks (Tegally et al., 2020). Thus, the variants of SARS-CoV-2 further challenged the campaign against the COVID-19 pandemic. Studies have shown that close contact and respiratory droplets can't explain all infections (Tabatabaeizadeh, 2021), and the environmental transmissions have become an important potential mechanism of COVID-19 spread, such as water (Sunkari et al., 2021), aerosol (Santarpia et al., 2020, Zhang et al., 2020), and low-temperature enhanced spread ‘cold chain’ (Zhang, 2020). Among these, aerosol transmission is relatively difficult to prevent.

Aerosols are a multiphase system consisting of the atmosphere with suspended solid and liquid particles, which can carry toxic and harmful substances; especially the liquid components (Duan et al., 2021, Mao et al., 2002). According to their aerodynamic diameter, the airborne particles are divided into PM10, PM2.5, PM1 and nanoparticles (Chen et al., 2020; Boongla et al., 2020). The aerosol is considered potentially harmful to human health as it can contain not only hazardous elements and chemicals (Shao et al., 2006), but also pathogens such as bacteria, fungi, and viruses (Han et al., 2021). Airborne fine particles (PM2.5) are considered of greater health significance with their large surface area and strong adsorption capability (Ding and Zhu, 2003).

In such a severe COVID-19 pandemic, it is essential to study the transmission routes of this virus. According to the WHO (2020), the main transmission routes of SARS-CoV-2 are respiratory droplets and contact transmission, and in a relatively closed environment, long-term exposure to a high concentration of aerosol may cause aerosol transmission. Before the 1930s, it was thought that respiratory infectious diseases could be transmitted by airborne substances, but there was no size division of these substances (Brown and Allison, 1937; Kramer et al., 1939). With the development of aerosol detection technology, more in-depth studies have been undertaken, and droplet transmission has been subdivided into large droplets and small droplets, and the small droplets are classified as being an ‘aerosol’ (Bourouiba, 2020). This characterization is now widely used, but the critical diameter discriminating between droplets and ‘aerosol’ is variable, ranging from 5 μm to 10 μm (Bourouiba, 2020). The WHO considers 5 μm as the boundary, with the respiratory droplet having a diameter > 5 μm, and respiratory aerosol having a diameter < 5 μm (Tellier et al., 2019). Studies have shown that the large droplets are more easily dropped out of atmospheric suspension, whereas multiphase turbulent buoyant clouds, i.e., the small droplets or aerosol particles contained in a locally humid and warm atmosphere will stay airborne for a longer time (Bourouiba et al., 2014). These aerosol particles will take much longer to be removed from the atmosphere (Scharfman et al., 2016), and therefore have a greater potential to spread the virus. In addition to direct virus aerosol transmission, some virus-containing substances in the environment can generate aerosols for further transmission. For example, SARS-CoV-2 has been detected in feces and urine (Du et al., 2020; Perchetti et al., 2020), potentially allowing aerosol transmission caused by poor hygiene and practices with human excrement.

The meteorological factors such as temperature and humidity have impacts on transmission of COVID-19. It is generally accepted that a higher temperature would inactivate SARS-CoV-2 (Guo et al., 2021; Notari, 2021) and a higher humidity is associated with spreading SARS-CoV-2 (Ratnesar-Shumate et al., 2020; Crema, 2021; Fernandez-Raga et al., 2021) although there are very few cases which showed the opposite result (Ma et al., 2020). The uncertainty exists about the influence of temperature and humidity on the propagation of COVID-19, which requires more systematic investigation.

In response to the COVID-19 emergency, many countries over all the World, in an attempt to curb the spread of the infection, have introduced a range of social-distancing measures including shutdowns and traffic restrictions. Emission control measures initiated and enforced due to major events can have a significant effect on reducing aerosol pollution. For example, after the 2008 Olympic Games, and the Asia-Pacific Economic Cooperation (APEC) meeting, particulate pollutant levels have been reduced (Guo et al., 2016, Qiao et al., 2017). Air quality has also been significantly improved in Beijing in response to intensified control strategies over 2013–2019 (Chang et al., 2019, Li et al., 2020d, Shao et al., 2019). It is expected that the unprecedented pandemic lockdowns could have a considerable impact on ambient aerosol pollution (Tung et al., 2021).

Although some phenomena demonstrate that there are mutual relationships between SARS-CoV-2 and aerosols, the nature of these complicated relationships remains unclear. In order to provide a framework for future prevention strategies, it is necessary to study the relationship between SARS-CoV-2 and aerosols. In this review, we have considered the multiple pathways and mechanisms of aerosols on the transmission of SARS-CoV-2, as well as the possible changes to aerosol pollution as a consequence of the COVID-19 lockdowns. Along with the airborne transmission potential of SARS-CoV-2, the infectivity of the virus in ambient aerosols requires further research.

The relationship between aerosol and COVID-19 can be divided into two aspects (Fig. 1 ). One association is that SARS-CoV-2 spreads through aerosol. The other is that aerosol pollution decreased during COVID-19 lockdown. The aerosol exhaled by the COVID-19 patients can directly transmit the virus. Ambient aerosols affect the transmission of SARS-CoV-2 in two modes. One mode is that ambient aerosols act as virus vectors indirectly; the other mode is that ambient aerosols can stimulate the expression of SARS-CoV-2 receptor and protease, and increase the binding site of SARS-CoV-2, thus increasing the morbidity and mortality of COVID-19. As a result of the prevention and control measures during the lockdown, coupled with the self-constraint of people, human activities have been greatly reduced, which leads to a great decrease of the mass concentration of ambient aerosols.

Fig. 1.

Fig. 1

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The relationship between aerosol and SARS-CoV-2.

2. Influence of human-exhaled aerosol on the transmission of COVID-19

2.1. Airborne transmission characteristics of the human-exhaled aerosol

The aerosol produced by sneezing and coughing can travel for 7–8 m (Bourouiba, 2020). In a simulation test of a Laryngo-Tracheal Mucosal Atomization Device, which enables clinicians to deliver a fine mist of atomized medication across the mucosa membrane, the upper airways and beyond the vocal cords, the aerosol produced appeared on doctors' necks, face, hands, arms, goggles, masks, and protective clothing, and also around the operating room (Endersby et al., 2020). Studies have shown that when people sneeze or cough, the droplets larger than 10 μm will sediment nearby, pollute that environment, and risk direct and indirect transmission of the virus, whereas the droplets smaller than 10 μm when leaving the airway will become droplet nuclei or aerosols (Bourouiba, 2020). These aerosols can stay airborne in the atmosphere much longer (Bourouiba, 2020), and aerosol particles with an aerodynamic diameter less than 2.5 μm can enter the alveoli directly (Feng et al., 2020). When compared with the nasal cavity and trachea, when the virus accumulates in alveoli, small doses can cause infection (Lindsley et al., 2010). In contrast to sneezing or coughing that can produce a large amount of aerosol, breathing and speaking can produce finer particles (<1.5 μm) (Asadi et al., 2019), and these smaller aerosol particles can travel further in the air (Lindsley et al., 2014).

2.2. Evidence of human-exhaled aerosol containing SARS-CoV-2

The particle sizes of exhaled aerosol produced by COVID-19 patients during speaking and coughing ranged from <0.25 μm (submicron) to about 10 μm, which has been shown to contain SARS-CoV-2 RNA and has the ability to transmit the virus in the air (Santarpia et al., 2020). Both symptomatic and asymptomatic patients have high SARS-CoV-2 viral load in their nasopharynx and trachea (Baggio et al., 2020, Zou et al., 2020), which provides the required conditions for exhaled aerosols to carry the virus. There are conflicting evidences for airborne transmission of SARS-CoV-2 (Falahi and Kenarkoohi, 2020). A study has detected SARS-CoV-2 virus RNA on the surface of an air vent, room air and corridor air in a COVID-19 ward (no patient cough was observed during sampling), and it is found that 63.2% of the samples were positive for SARS-CoV-2, and the concentration level reached 2420 RNA copies / m3 (Santarpia et al., 2020). The presence of SARS-CoV-2 in aerosols was also monitored in the hospital environment, which accounted for 179‐2738 RNA copies/m3 (Ong et al., 2021). The viruses have also been detected in the samples collected on the surface and in the air of buses and subway trains (Moreno et al., 2020), and on the surface of an ICU ward (Orenes-Pinero et al., 2021).

Some medical procedures are more likely to produce human-exhaled aerosols. In March 2020, Public Health England defined AGP (Aerosol Generation Procedure) in the medical processes, such as intubation, dental surgery, high flow nasal oxygen and other related procedures (Simonds, 2020). Transnasal drill and cautery use is associated with the production of the aerosol in the range of 1 to 10 um under endonasal procedures (Workman et al., 2020). The SARS-CoV-2 has been detected in the submicron and ultra-micron aerosol of two hospitals in Wuhan (Liu et al., 2020c).

SARS-CoV-2 RNA appeared inside the air conditioner and the air samplers, or on object surfaces more than 2 m away from patients, within only 20 min after the patients registered into the ward (Santarpia et al., 2020), which shows that the airflow can take the virus aerosol particles from the patient bed to the edge of the room by ventilation. A full-scale test by (Ai et al., 2019) has revealed the transmission characteristics of the exhaled aerosol in the air and they have shown that people near the virus carriers have a relatively high exposure risk, especially those facing the infectious person. All these studies indicate that the SARS-CoV-2 infection may occur within a very short period after exposure to the COVID-19 patients.

In summary, when compared with the large droplets, the human-exhaled aerosol has a stronger diffusion ability, and similarly, the aerosols carrying SARS-CoV-2 produced by COVID-19 patients have a higher transmissibility. In addition to the contact transmission and closed airborne transmission, SARS-CoV-2 may also be transmitted by aerosols in ventilation systems. The possibility of long-distance aerosol transmission needs further and urgent epidemiological and experimental studies. Aerosols carrying SARS-CoV-2 are likely to be produced in the common treatments of cardiopulmonary, oral and airway diseases. Hospitals are densely populated environments, where strict protective measures must be implemented to ensure the safety of medical staff and other personnel.

2.3. Similarity of air transmission of SARS-CoV-2 and other viruses

Phylogenetic analysis revealed that SARS-CoV-2 and SARS-CoV (Severe Acute Respiratory Syndrome Coronavirus) are both in the subgenus Sarbecovirus of the genus Betacoronavirusare (Lu et al., 2020), and therefore SARS-CoV-2 is similar to the SARS-CoV in terms of gene sequence homology (Gorbalenya et al., 2020). On February 11th, 2020, ICTV (International Committee on Taxonomy of Viruses) stated that CSG (Coronaviridae Study Group) has recognized SARS-CoV-2 as a sister clade to SARS-CoV (Gorbalenya et al., 2020). In terms of structure and function, both of them are the coronavirus associated with severe acute respiratory syndrome, and they are homologous RNA viruses that can cause human pneumonia.

Table 1 provides some comparisons between SARS-CoV-2 and SARS-CoV, which illustrates our understanding of the transmission mechanism, prevention, and treatment of COVID-19. As shown in Table 1, SARS-CoV-2 and SARS-CoV belong to the same genera Betacoronavirus. Both have similar diameters, with the size of 65–125 nm (Shereen et al., 2020). The host cell receptors of both SARS-CoV-2 and SARS-CoV are the ACE-2 protein, but the affinity between SARS-CoV-2 and receptor protein is higher which would facilitate a relatively fast transmission of corresponding diseases (Giron et al., 2020). van Doremalen et al. (2020) have established an experimental environment to test the stability of SARS-CoV-2 and SARS-CoV, and they have found that the survival time (aerosol half-life) of the two viruses in the air after artificial aerosolizing was similar, but the retention time of SARS-CoV-2 on the surfaces of objects was relatively longer, which increased the risk of resuspension. SARS-CoV has the known ability for airborne transmission and this virus was found in an air sampler 5 ft (1.52 m) away from the patient (Booth et al., 2005). SARS-CoV can also be transmitted between buildings (Yu et al., 2004) and aircraft passengers (Olsen et al., 2003). A study on a hospital in Beijing suggested that nosocomial, hospital-derived, infection could be the main cause of the early prevalence of SARS in the hospital (He et al., 2003).

Table 1.

Comparison of SARS-CoV-2 and SARS-CoV.

Properties SARS-CoV-2 SARS-CoV References
Genus Betacoronavirus Betacoronavirus Lu et al., 2020
Species Severe acute respiratory syndrome-related coronavirus Severe acute respiratory syndrome-related coronavirus Lu et al., 2020
Size 65–125 nm 65–125 nm Shereen et al., 2020
The receptor of the host cell ACE-2(Higher affinity) ACE-2 Giron et al., 2020
Infection rate Relatively fast Relatively slow Wang et al., 2020a
Half-life period on aerosol 0.64–2.64 h 0.78–2.43 h van Doremalen et al., 2020
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Other coronaviruses and common viruses can also have the ability of aerosol transmission. Studies on the human coronavirus 229E (a common cold virus) have shown that the experimental virus-carrying aerosol can persist at 20 °C and 50% relative humidity for 6 days (Ijaz et al., 1985). Influenza patients emit aerosol particles containing the influenza virus when they are coughing, and most of the virus RNA is incorporated into the particles within the respiratory size range (Lindsley et al., 2010). In seasonal influenza transmission, a large number of virus copies were detected in fine aerosol particles (Milton et al., 2013).

In summary, there are similarities between SARS-CoV-2 and SARS-CoV in gene sequence and stability. The SARS-CoV-2 virus, other coronaviruses and common viruses also have aerosol transmission capacity. SARS-CoV-2 can be directly transmitted through human-exhaled aerosol. In future prevention and control research, the characteristics of SARS-CoV and other viruses, especially their airborne transmission potential needs to be further elucidated.

3. Influence of ambient aerosols on the transmission of COVID-19

3.1. Epidemiological relationship between ambient aerosol and COVID-19

Long-term exposure to poor air quality can cause a range of diseases (Guo et al., 2016). Epidemiological and in-vitro experimental evidence shows that aerosol pollution exposure has a positive correlation with respiratory diseases, such as COPD (Chronic Obstructive Pulmonary Disease), asthma (Kesic et al., 2012), ILI (Influenza Like Ill) (Su et al., 2019), ALI (Acute Lung Injury) (Li et al., 2019) and SARS (Yao et al., 2020).

Since the outbreak of COVID-19, scholars in many parts of the World, including Europe, North America and Asia, have undertaken studies on the epidemiological relationship between air pollution indicators and COVID-19. Konstantinoudis et al. (2020) used high-resolution hierarchical spatial analysis to investigate 38,573 cases of COVID-19 deaths in 32,844 small areas in England as of June 30th, 2020 and used the Bayesian hierarchical model to quantify the impact of air pollution. The results showed that the mortality of COVID-19 would increase by 1% for every 1 μg/m3 increase of NO2 and PM2.5. The COVID-19 cases in Germany from February 24th to July 2nd, 2020 have been examined by correlation analysis and WTC (Wavelet Transform Coherence), and it has been found that the concentrations of PM2.5, O3, and NO2 were significantly associated with the prevalence of COVID-19 (Bilal Bashir et al., 2020). The data from 55 Italian regional samples, as of April 7th, 2020 showed that the rapid spread of COVID-19 in northern Italy was highly correlated with local air pollution (Coccia, 2020). In northern Italy, the geographical factors of the local mountains and the high densities of factories and transportation were the main causes of PM2.5, PM10, NO2 and O3 pollution, which mirrored the higher occurrence frequency and severity of COVID-19 (Daniele and Francesco, 2020). Milan, located in the Po Valley Basin, is a recognized hot spot of aerosol pollution. Through comprehensive time series analysis, Zoran et al. (2020) found that the PM2.5 and PM10 in the metropolitan area of Milan from January 1st to April 30th, 2020 were significantly positively related with the prevalence of COVID-19. The association between air quality indicators and COVID-19 cases in California was analyzed using the Spearman and Kendall correlation test, and the results indicated that ambient pollutants including the mass concentrations of PM10, PM2.5, SO2, and NO2 were negatively correlated with the prevalence of COVID-19 and only the CO concertation showed a positive correlation with the COVID-19 (Bashir et al., 2020). In another study in California, the time-series analysis revealed that, in addition to the CO and O3, the concentration of PM2.5 was also positively correlated with increases in the incidence and mortality of COVID-19 (Meo et al., 2021). A national cross-sectional study was conducted on more than 3000 counties (98% of the population) in the United States, and the results showed that the increase of PM2.5 by only 1 μg/m3 was associated with an 8% increase in the mortality rate of COVID-19 (Wu et al., 2020).

Air pollution and meteorological data have been collected from January 25th to April 7th, 2020 in Wuhan, China, and Pearson and Poisson regression models have been used to study the relationship between COVID-19 mortality and each risk factor; this research concluded that PM2.5 was the only pollutant with positive correlation with COVID-19 mortality (Jiang and Xu, 2020).

A longitudinal cohort study of 6529 patients from 28 urban areas of Japan has been conducted, and the results showed that short-term exposure to the suspended particulates may affect respiratory tract infection caused by SARS-CoV-2 (Azuma et al., 2020). The outpatient data of 21 Japanese cities demonstrated a delayed association between PM2.5 and cardiopulmonary examination (Seposo et al., 2020).

The positive association between aerosol and confirmed cases or deaths of COVID-19 in Zhu et al. (2020) has been questioned for lack of the study of population density (Copiello and Grillenzoni, 2020). Therefore, when we analyze the relationships between the concentrations of airborne particles and the confirmed cases or deaths of COVID-19, we shouldn't ignore the impacts from population density. The data of Bashir et al. (2020) showed the concentration of PM2.5, had a negative correlation with prevalence of COVID-19, while the study by Meo et al. (2021) revealed a positive correlation between these two parameters. Therefore, the use of Spearman and Kendall correlation tests may not give a solid evidence, some of the associations resulted from the correlation analysis may still need to have proof from other parameters.

The results described above have been summarized in Table 2 , and most of these studies have supported the hypothesis that poor air quality increases the prevalence and mortality of COVID-19. In particular, a positive relationship has been observed between PM2.5 and COVID-19 morbidity or mortality. The consistently positive significant correlation provides further evidence that long-term exposure to relatively high concentrations of ambient aerosols is responsible for the increased transmission and pathogenicity of SARS-CoV-2 in the relevant population. Table 2 also showed the positive correlation between most gaseous pollutants and COVID-19. NO2, SO2, O3 and CO may play important roles in two possible ways; one is that exposure to high levels of gaseous pollutants can cause inflammation of the airways to affect lung function and respiratory symptoms (Huang and Brown, 2021), and the other is that the secondary reaction between aerosols and gaseous pollutants will be strengthened at lower temperature and higher relative humidity condition (Ding et al., 2021), which can enhance the harm of aerosols.

Table 2.

Studies showing associations of air quality indicators with the COVID-19 in different regions of the world.

Region PM2.5 PM10 NO2 SO2 CO O3 References
England +(uncertain) + + + + Konstantinoudis et al., 2020
Germany + + + Bilal Bashir et al., 2020
Italy + + + + Daniele and Francesco, 2020
Milan, Italy + + Zoran et al., 2020
California, USA + + + Meo et al., 2021
California, USA – – – – + Bashir et al., 2020
USA + Wu et al., 2020
Wuhan, China + – – – Jiang and Xu, 2020
Japan + Azuma et al., 2020
Japan + Seposo et al., 2020
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Note: ‘+’ stands for promoting effect or positive correlation, ‘-’ stands for negative correlation, and blank space represents no research.

It is important to study the relationship between air pollution and human health in order to help policy-makers to formulate positive strategies to reduce the pollution of ambient aerosols, especially PM2.5, which would help to alleviate the rapid spread of COVID-19 and, potentially, to decrease the spread of epidemic viruses and diseases in the future.

3.2. Mechanism of ambient aerosol affecting COVID-19 diffusion

3.2.1. Ambient aerosols play a role as the carrier of SARS-CoV-2

SARS-CoV-2 may enter the human body through aerosol; not only long-term but also short-term exposure will have a great adverse impact on the human immune system (Zoran et al., 2020). Although the atmospheric processes experienced by the aerosol particles after release from the human body could, to some extent, cause severe damage to biomass (Zhen et al., 2013), SARS-CoV-2 can survive in aerosols for 3 h, and can survive on the surfaces of other contact materials for even longer times, i.e. copper (3.4 h) < cardboard (8.45 h) < stainless steel (13.1 h) < plastic (15.9 h) (van Doremalen et al., 2020). Under certain conditions, viruses on the surface of objects and in water can resuspend into the air and combine with existing aerosols (Ravi et al., 2020).

Ambient aerosols play a carrier or enhancement role for SARS-CoV-2 (Martelletti and Martelletti, 2020). The morbidity and mortality of COVID-19 are related to air pollution emission sources. In addition to humans as the source of ambient aerosol transmission, TSDF (hazardous waste treatment, storage and disposal facilities) and RMP (Risk Management Plan) sites are potential air pollution sources (Hendryx and Luo, 2020; Tung et al., 2021). Under certain conditions, ambient aerosols, such as water-borne aerosols, can provide favorable surfaces for the adsorption of organic molecules and viruses, and facilitate higher transmission rate under certain ambient conditions (Manoj et al., 2020). Polluted water can be a source of viruses, and aerosols from this source can carry a variety of viruses, leading to a higher exposure rate for residents living around the contaminated area (Rocha-Melogno et al., 2020). In Australia, SARS-CoV-2 was detected in a wastewater treatment plant (Ahmed et al., 2020).

Fecal-oral transmission could be an additional route for SARS-CoV-2 spread. After the virus enters the body, the virus-specific RNA and protein are synthesized to assemble new viruses, which are then released into the gastrointestinal tract, and finally expelled from the body (Xiao et al., 2020b), so the feces of COVID-19 patients have a high viral load (Xiao et al., 2020a). Aerosols in sanitary pipeline systems can carry viruses, resulting in a higher-risk of infection (Gormley et al., 2017). It is postulated that there is a risk of SARS-CoV-2 infection through aerosol when using contaminated toilets (Wang et al., 2020b). Since fecal aerosol transmission may have caused the community outbreak of COVID-19 in high-rise buildings (Kang et al., 2020), understanding the transmission routes of aerosol-related sewage and fecal sources may be important for reducing the spread of COVID-19, especially in developing countries.

The described evidence above shows that SARS-CoV-2 can combine with ambient aerosols and enter the human body, but there is little experimental evidence about the combination of the SARS-CoV-2 and aerosols. Whether virus aerosol detected around patients are human-exhaled aerosol or ambient aerosol is worth further experimental verification.

3.2.2. Ambient aerosols can up regulate SARS-CoV-2 receptor and related protease

Aerosol pollution exposure is associated with various respiratory and cardiovascular diseases (Pun et al., 2017), and one of the mechanisms is the up-regulation of ACE-2 (Angiotensin Converting Enzyme 2) and TMPRSS2 (Transmembrane Serine Protease 2) (Lin et al., 2018). ACE-2 is the main receptor protein of SARS-CoV-2, and the synaptic glycoprotein of the virus has a high affinity for ACE-2 in host cell targets (Vankadari and Wilce, 2020). TMPRSS2 is a protease that can cleave viral spike protein and make it combine with target cells to promote infection (Kaur et al., 2021). The up-regulation of ACE-2 is a protective mechanism when the respiratory system is exposed to aerosol, which can maintain the dynamic balance of RAS (Renin Angiotensin System) and reduce inflammatory reaction (Ye and Liu, 2020). ACE-2 is abundantly expressed not only in the lung, but also in the glandular cells of gastric, duodenal, and rectal epithelia of the patients with COVID-19 (Xiao et al., 2020b).

When PM2.5 invades the human body, ACE-2, as the receptor for SARS-CoV-2 to enter cells, will protect against renin–angiotensin system-induced lung injuries by cleaving Angiotensin II to limit substrate availability in the adverse AEC-Ang II-Ang II receptor 1 axis (Parajuli et al., 2014). Therefore, PM2.5 can increase the SARS-CoV-2 susceptibility for human body by enhancing the expression of AEC-2 and its cofactor TMPRSS2 (Kim et al., 2020). An in vivo experimental study has confirmed that the expression level of ACE-2 in the lung of experimental mice was significantly increased after being induced by PM2.5 (Lin et al., 2018). Statistical analysis suggests that PM2.5 may be the cause of the overexpression of ACE-2 in human epithelial cell surfaces of the respiratory tract (Paital and Agrawal, 2020). For smokers, a large number of aerosols with a particle size of less than 1 μm will be generated by the process of smoking. After smoking, these aerosols can be suspended in the indoor atmosphere for a long time (Cao et al., 2018), and this would impact the secondhand (passive) smokers who would also show an increase in the expression of ACE-2 in their bronchi (Aliee et al., 2020). Compared with non-smokers, the expression of ACE-2 and TMPRSS2 in smokers and patients with COPD were significantly up-regulated (Sharif-Askari et al., 2020).

Overall, there was a significant correlation between aerosol concentration level, ACE-2 expression, and severity of COVID-19 infection (Paital and Agrawal, 2020). Therefore, the decision-makers should pay particular attention to the air pollution in areas where COVID-19 is prevalent, and appropriate measures should be implemented in order to reduce this air pollution. Smoking promotes the expression of ACE-2 and TMPRSS2 in the airway. Therefore, during the pandemic, the control of smoking in public places needs more strict legislation, and non-smoking individuals should be advised to avoid proximity to smokers. It may be worthwhile to explore the therapeutic effects of recombinant ACE-2 protein in the early stage of COVID-19 infection (Li et al., 2020b).

4. Changes in aerosol pollution during COVID-19

Since Wuhan announced the implementation of COVID-19 lockdown on January 23rd, 2020, China and other regions in the world have taken measures to restrict travel and shut down industry and commerce to avoid crowd gathering and reduce the spread of COVID-19. The prevention and control measures taken by these countries have greatly reduced the transmission rates of COVID-19 (Kerimray et al., 2020). With the large-scale shutdowns and traffic restrictions, the aerosol pollution had a general corresponding decrease in levels (Zhou et al., 2012; Liu et al., 2020a, Liu et al., 2021, Shi et al., 2021, Zhou et al., 2012; Shi et al., 2021). The results of aerosol concentration changes in different regions during the COVID-19 lockdown are summarized in Fig. 2 , which demonstrates the impact of prevention and control measures on aerosol pollution.

Fig. 2.

Fig. 2

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The decline in concentrations of PM2.5 and PM10 in various regions during COVID-19 lockdown.

As shown in Fig. 2, compared with the preceding period of COVID-19 lockdown, the average concentration of PM2.5 has decreased by 31% in California (Liu et al., 2020b), 21.8% in Hat Yai, Thailand (Stratoulias and Nuthammachot, 2020), 52% in Pearl River Delta (Wang et al., 2021) and 41.2% in Wuhan (Sulaymon et al., 2021) during COVID-19 lockdown, and the average concentration of PM10 has decreased by 31% in Barcelona, Spain (Tobias et al., 2020), 22.9% in Hat Yai, Thailand (Stratoulias and Nuthammachot, 2020) and 33.1% in Wuhan (Sulaymon et al., 2021). Compared with the preceding years, PM2.5 and PM10 mass concentrations of 22 cities in India decreased by about 43% and 31% during the lockdown (Sharma et al., 2020). The PM2.5 concentration in Almaty, Kazakhstan, during the lockdown, is 21% lower than the average level in the same period of 2018–2019 (Kerimray et al., 2020). Compared with the same period in the previous four years, the pollutants PM10 and PM2.5 in Singapore decreased by 23% and 29%, respectively (Li and Tartarini, 2020). A survey was conducted in 19 countries in the South and Southeast Asian region, compared to the same period of 2019, the PM2.5 level decreased by an average of 20.25% (Roy et al., 2021). Compared with the same period last year, the PM2.5 concentration in the regional Level I and Level II response periods in the Yangtze River Delta region decreased by 31.8% and 33.2% respectively (Li et al., 2020c).

The large reduction of human activities has significantly improved air quality in many areas during the control of COVID-19. Compared with the pre-lockdown period, the concentrations of PM10 (14–20%) and PM2.5 (7–16%) in 597 major cities in the world have decreased significantly (Liu et al., 2021). For the areas with more serious air pollution problems, the improvement of the aerosol is greater, and the decrease of PM2.5 is greater than that of PM10 (Fig. 2). Therefore, the decrease of air pollutants in areas with high pre-lockdown levels is more obvious, and PM2.5 is more sensitive to emission reduction (Wang et al., 2021).

Some studies have investigated the reasons for the decrease of atmospheric aerosol concentrations. Measures such as city closure and vehicle restrictions greatly reduced the types of primary aerosols related to traffic and reduced the levels of air pollution (Liu et al., 2021; Chen et al., 2021), and at the same time, many factories shut down and stop production, and the emission of the secondary industry decreased (Wang et al., 2020c). In Beijing, the vast majority of restaurants were closed, and the aerosol emissions of cooking and gas burning decreased by 30–50% on average (Sun et al., 2020). The reduction of secondary aerosol species was very small (5–12%) (Sun et al., 2020). These results indicate that the control of anthropogenic emissions will greatly improve air quality, but they may not be able to effectively suppress secondary aerosols under stagnant weather conditions.

The studies discussed above are mostly focused on local small-scale cases. Future research should consider expanding time and space domains, combining satellite data and monitoring station data to better characterize the change of aerosol pollution. Also, meteorological factors need to be considered when studying the impact of ambient aerosols (Guo et al., 2021). The improvement of air quality caused by the epidemic prevention measures provide reference for policy-makers to formulate measures to reduce aerosol pollution.

5. Concluding remarks

  • (1)

    The relationship between aerosols and COVID-19 can be subdivided into three types; human-exhaled aerosols directly transmitting COVID-19; COVID-19 transmitted by ambient aerosols; ambient aerosol concentrations decrease as a result of the COVID-19 lockdowns.

  • (2)

    The human-exhaled aerosol produced by breathing, speaking and sneezing can survive for a significant time (3 h), and the airborne transportation distance can reach 7–8 m. The airborne transmission potential of SARS-CoV-2 must be considered in prevention and control work, and the transmission of virus aerosol should be effectively decreased by ventilation, disinfection and wearing protective devices.

  • (3)

    Overexposure to ambient aerosols can cause respiratory diseases, and ambient aerosols are associated with increased morbidity and mortality by COVID-19. Two mechanisms have been discussed in this process. Firstly, SARS-CoV-2 may combine with ambient aerosols from contaminated sites (such as medical waste treatment sites, polluted water bodies and toilet pipes) to enter the human body. Secondly, the ambient aerosol can stimulate the expression of ACE-2 and TMPRSS2, leading to the increase of SARS-CoV-2 binding sites and the acceleration of infection efficiency. The binding mechanism, survival time and residual activity of SARS-CoV-2 in ambient aerosols need to be further studied. The infectivity of the virus in ambient aerosols should be further researched.

  • (4)

    Due to the epidemic minimizing measures during COVID-19 in numerous locations worldwide, traffic emissions and factory emissions were reduced. This has been an opportunity to observe the relationship between human factors and air quality. Compared with the same period in previous years before the epidemic, aerosol pollution, especially PM2.5, decreased significantly. The reduction of aerosols in areas with high air pollution is more obvious, and the levels of PM2.5 are more sensitive to emission reduction than PM10.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study is supported by the National Natural Science Foundation of China (Grant No. 42075107), the Projects of International Cooperation and Exchanges NSFC (Grant No. 41571130031) and the Yueqi Scholar fund of China University of Mining and Technology (Beijing). The authors are indebted to editor-in-chief Professor M. Santosh for his constructive discussion and comments and also for his patience in editing the paper. Two anonymous reviewers are particularly acknowledged for their critical evaluations of the manuscript.

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