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The COVID-19 pandemic has brought an unprecedented emergency to the health sector worldwide. The novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was declared a pandemic by the World Health Organization on March 11, 2020. The speedy worldwide spread of COVID-19 represents possibly the most significant public health emergency in a century. 

As the pandemic proceeded, a continuous paucity of indication on routes of SARS-CoV-2 transmission has led to shifting of infection prevention and control strategies between airborne and droplet precautions. As public health teams respond to the pandemic of COVID-19, understanding of the modes of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission is of extreme importance for policy making. 

Here we summarized various research studies on the evidence of aerosol transmission of SARS-CoV-2 in various hospital settings around the globe. Mostly all research studies detected viral contamination in air samples of hospitals when caring for COVID-19 patients. Based on these studies, precautionary control plans that are important for public health protection can be designed which can avoid aerosol transmission of SARS-CoV-2. 


The world has nearly been brought to a stop by the COVID-19 pandemic, and hospitals across the world face unusual challenges (1). High infectivity mixed with high case fatality rate during the COVID-19 pandemic has placed an importance on healthcare worker (HCW) protection both from a personal as well as a social viewpoint. Infections of medical staff by SARS-CoV-2 have been seen in hospitals globally; some of them unfortunately lost their lives (2,3).  

Nosocomial outbreaks in different hospitals have been seen in several parts of the world (4-6). Numerous studies indicate the vulnerability of healthcare and non-healthcare workers to be infected, particularly after being exposed to patients firstly not suspected of COVID-19 and who are probably transmitting the virus at the pre symptomatic or asymptomatic stages (7-9). Sometimes COVID-19 is present almost without symptoms could mean a bigger risk in non-COVID-19 units. This study reveals that the hospital areas designated to non-COVID-19 patients can also be a risky environment for a SARS-CoV-2 outbreak. Inside Hospitals the air, frequently touched object surfaces, and floors have been found to be contaminated by SARS-CoV-2 (11-13). 

Healthcare worker protection and effective public health measures for emerging infectious diseases require guidance based upon a solid understanding of modes of transmission. Limited evidence describing SARS-CoV-2 mode of transmission has directed to shifting isolation strategies from the WHO, U.S. CDC and other public health authorities. Though nosocomial transmission of SARS-CoV-2 is described, the role of aerosol transmission and environmental contamination require more study to inform appropriate practices (14-15).

In risky transmission surroundings, such as hospitals, knowledge of the modes of transmission and implementation of the appropriate respiratory precautions are important factors in infection control. 

Studies of the environmental contamination related with COVID-19 patients are required to improve our knowledge of the modes of transmission of SARS-CoV-2(16).

Though, some case reports are available (17,18). Epidemiological studies have directed the Centers for Disease Control and Prevention (CDC) to suggest that person-to-person transmission of SARS-CoV-2 is mediated mainly by respiratory droplets or contact with contaminated surfaces and aerosol transmission. There are some studies on mode of transmission of COVID-19 in hospitals provide information on aerosol transmission of COVID-19 in hospital settings (19).

Review of Literature

Zhou and colleagues aimed to examine the possible presence of SARS-CoV-2 in exhaled breath of patients recovering from COVID-19 who have continually tested negative using throat swabs and to study environmental contamination by SARS-CoV-2 within four hospitals in Wuhan, China (20).

They detected SARS-CoV-2 in exhaled breath (2 of 9, 22.2%), air samples (3 of 44, 6.8%), and surface swabs (10 of 318, 3.1%) collected from hospitals using both RT-PCR and digital PCR.  They discovered that two recovering COVID-19 patients, who are ready for hospital discharge were emitting SARS-CoV-2 RNA, about 104viruses per hour estimated by the method described (21), via breathing.

In deviation to the belief that direct surface contact signifies a main route for COVID-19 transmission, they detected a very low positive rate (3.1%) for surface swabs (N = 318) from various surfaces in Wuhan hospitals. This result concluded that direct surface contact, even in high-risk areas may not represent a major route of COVID-19 transmission (22).

Using conventional RT-PCR, SARS-CoV-2 levels were positive for three air samples out of 44 samples from various hospital environments of Wuhan. The detected low positive rates for both the air and surfaces were a joint significance of a number of factors. First of all, virus emission dynamics from COVID-19 patients are still mainly unidentified. It may be dependent on the patient’s activities like coughing, sneezing, talking, or lung self-cleaning during the day. Though, such activities, which are problematic to document, may have happened before sample collection (23). 

Moreover, the hospitals applied disinfectants three times a day which may perhaps inactivate the virus and its RNA segments. Additionally, all COVID-19 patients must wear a mask which reduces the release of the virus into the air or onto surfaces in the hospital environment.

Last of all, natural air ventilation via open windows (every room had at least one window with an outside wind speed of up to 1.6–3.3 m/s) diluted viruses. However, these data recommend that certain surfaces frequently touched by the medical staff and hospital air should be regularly disinfected to further lower related infection risks and ventilation played an important role in aerosol transmission of COVID-19 (23).

Study by Hamilton and colleagues has shown that the risk of COVID-19 aerosol transmission to hospital staff outside of intensive care is more than indoor staff. 


The risk of SARSCoV-2 aerosolization was high in emergency departments and general wards where covid-19 patients are coughing. These are spaces where hospital staff usually wear face masks only. In contrast, the hazards of SARSCoV-2 aerosolization was lower in conditions where patients receive continuous positive airways pressure and high flow nasal oxygen. These two respiratory support procedures which have been presumed to be high risk for aerosol generation. These procedures are delivered in intensive care settings where hospital staff wear protective FFP3 respirators. 

They further confirmed that contact with patients who were not on respiratory support and coughing have a higher viral load and potentially be more infectious. This is the reason why staff in the emergency department are more likely to be seropositive. Another factor may be Lower-level PPE, particularly face masks. Additional attention must be paid to ventilation inside these hospital departments, along with supporting the essential for patients to wear face masks (24).

Santarpia and colleagues collected air and surface samples of 13 individuals with COVID-19 to examine viral shedding at the University of Nebraska Medical Centre. Patients needing hospital care were managed in the Nebraska Biocontainment Unit (NBU), and mildly ill persons were isolated in the National Quarantine Unit (NQU), both situated in the medical center campus.

They found 63.2% of in-room air samples to be positive by RT-PCR (mean concentration 2.42 copies/L of air). The highest airborne concentrations were documented by personal samplers in NBU though a patient was getting oxygen through a nasal cannula (19.17 and 48.22 copies/L).

Both individual air samplers from sampling personnel in the NQU showed positive PCR results after 122 min of sampling activity and both air samplers from NBU sampling indicated the presence of viral RNA after only 20 min of sampling activity. These results have shown substantial environmental contamination in rooms where COVID-19 patients are kept and cared for, irrespective of the degree of infection. 

Higher percentage of positive samples (81.4% over 3 rooms) was noticed in the NBU later in the progression of illness (tested on Days 10 and 18), signifying that patients with higher acuity of disease may be related with increased levels of environmental contamination.

In the NBU, where patients were usually less moved, dispersal of positive air samples recommends a strong effect of airflow. Personal and high-touch stuff were not collectively positive, yet researchers spotted viral RNA in 100% samples from the floor below the bed and at one window ledge in the NBU. 

Airflow in NBU initiates from a register at the top center of the room and leaves from grills near the head of the patient’s bed. Airflow modelling (25) has recommended that some fraction of the airflow is directed below the patient’s bed, which might the reason of the observed contamination below the bed, whereas the main airflow possibly transports particles away from the patient’s bed, in the direction of the edges of the room, probably passing by the windows resulting in some deposition there (26).


First, in the limited examples where the distance between patients in isolation and air sampling could be surely maintained at greater than 6 ft., 2 of the3 air samples were positive for viral RNA. Second, 58.3% of hallway air samples showed virus-containing particles. It is likely that the positive air samples in the hallway were triggered by viral aerosol particles transported by staff exiting the room (27,28). Finally, personal air samplers worn by staff were all positive in spite of the absence of cough by most patients.

According to Recent literature, human-expired aerosol is produced in all types of activity (e.g., breathing, talking, and coughing) (28). The data of this study suggest that viral aerosol particles are produced by COVID-19 patients even in the absence of cough. 

In hospitals treating COVID-19 patients’ toilets may promote faucal-derived aerosol transmission if used inappropriately (29). A fluid dynamics simulation recommends that through toilet flushing, immense upward transport of virus aerosol particles was observed. 40–60% of particles rising overhead the toilet seat leads to large-scale virus spread indoors (30). 

Various studies confirmed that SARS-CoV-2 genetic material was found in the air in hospital nurses’ stations, in air handling grates, on multiple air outlets, and in the air of patient rooms. (31-34).

What did another study find out?

Study conducted in Singapore hospital found that SARS-CoV-2 particles with sizes>4 μm and 1–4 μm contained 1.8 — 1.8–3.4 viral RNA copies/m3 in two hospital rooms, in spite of these rooms having 12 air changes per hour (35). Swabs taken from air exhaust outlets in the hospital room of a symptomatic patient were positive which indicate that small virus-laden aerosols have been displaced by air flow and dumped on vents (36).

Furthermore, research conducted in Various hospitals and surrounding public areas in Wuhan, China found SARS-CoV-2 RNA in the air inside the patient toilet room (19 copies/m3) and in medical staff areas (18–42 copies/m3 in protective apparel removal rooms) (37). The concentrations of SARS-CoV-2 RNA in air are in two distinct size ranges of 0.25–1.0 μm and>2.5 μm aerodynamic diameter, which indicate that the virus-containing aerosols are sufficient small to remain suspended in air for an extended period of time and can be inhaled (38). This study also showed that SARS-CoV-2 virus on PPE or floor surface, which can be resuspended as a source of aerosols by the activities of medical staff. 

Research study in Huoshenshan Hospital in Wuhan

Another research study in Huoshenshan Hospital in Wuhan has a same result showing that contamination of SARS CoV- 2 was higher in intensive care units (ICU) than General Ward by an extensive distribution on surfaces of floors, trash cans, as well as in air about 4 m (13 feet) from patients. Floor swab samples of ICU indicate relatively high positive rates of 70%, signifying virus droplets from the aerosol falling due to gravity and air flow (39). 

A constraint of research studies measuring SARSCoV- 2 virus in the environment is the dependence on polymerase chain reaction (PCR) to identify and quantify the virus. Though, recent study firstly pointed out that SARS-CoV-2 has been detected in the air in hospital wards with COVID-19 patients by means of cell culture method (40). Santarpia et al. have shown measuring viable SARS-CoV-2 in air of hospital wards with COVID-19 patients (40), which is consistent with detection of airborneSARS-CoV-2 RNA in hospital.

In contrast to many studies, two other studies reported by Guo ZD et al. and Chia PY et al., have shown that quantifiable aerosol concentrations of SARS-CoV-2 RNA in isolation or ICU wards of hospitals was seen but the viral load was low. The researchers did not report whether the samples were collected before or after disinfection (50). 

Further research is clearly needed to determine whether inconsistent findings between studies are related to the different air sampler used, the flow rate and the duration of aerosol sampling. 

However, Li and colleagues’ results recommend that when strict disinfection actions are applied and room ventilation is maintained, the probability of aerosol-borne SARS-CoV-2 in the hospital is low (51). Altogether, these results indicate that SARS-CoV-2 could survive in aerosols for a relatively long time under favourable conditions and potentially spread through aerosols.

In the hospital and healthcare settings, ventilation is a primary control strategy for infectious diseases, which promotes the air dilution around a source and the removal of respiratory viruses (52).  In an optimally ventilated room, the number of droplets could halve after 30 s, whereas with poorly ventilated and no ventilation rooms this could take 1–4 min and 5 min, respectively (53). 

Diagnosis and subsequent isolation measures should be arranged rapidly using single rooms with negative pressure and ventilation capacity (e.g., AIIR). Infected patients should ideally be placed in single rooms, but it is acceptable to co-locate with infected patients. Unless necessary, patients should be restricted to their room and keep windows and doors closed. If not in AIIR (e.g., transport), infected or suspected patients should wear facial masks as a physical barrier to droplets or aerosols (54). Education of patients is necessary to encourage adherence to guidelines. HCWs should be provided respirators and airborne precautions, and the precautionary principle should be followed to protect their occupational health and safety (55)

As long duration of viral shedding was reported in asymptomatic cases

(55-56), with high infectivity relative to symptomatic cases (57), the virus could spread via aerosols during breathing and talking before awareness is triggered by symptoms. 

This poses risks, particularly in confined and poor ventilated environments with prolonged person to person contact. Settings with a large proportion of infected people or contaminated samples, such as hospitals, healthcare are the highest risk, especially to HCWs, who should be provided airborne precautions. The general public and vulnerable populations should be made aware that confined, crowded and poor ventilation environments may pose a risk when an infected person is present. Prevention and control countermeasures are proposed to reduce the potential aerosol transmission under different occasions.


The data available so far showed that indoor transmission of the virus far surpasses outdoor transmission, probably due to lengthier contact times and the decreased turbulence levels found indoors. Research on mode of transmission of SARS-CoV-2 in a hospital environment leads to the conclusion that it can transfer through the air. the concentrations of SARS-CoV-2 in the air could be highest during the first week of COVID- 19 illness.

This indicates that room ventilation, sanitization of protective wear, and correct use and disinfection of patient`s room and toilet areas can effectively reduce the concentration of SARS-CoV-2 RNA in aerosols. hospital Infection and prevention control procedures should be revised as per the risk of potentially airborne transmission of the virus.

Precautionary control strategies should consider aerosol transmission for effective extenuation of SARS-CoV-2. HCWs should be provided airborne precautions, and the preventive principle should be followed to protect their health and safety.

COVID-19 is prevalent in spite of vaccination may be because of the two reasons: Preventive measure is not 100% effective to save from aerosolized SARS-CoV-2 virus and second mutant strains are more virulent and vaccine may not prevent it 100%. Therefore, Research should be done to prevent aerosolized viruses   and explore the infectivity of aerosolized viruses.

MVS Pharma is working on aerosol transmission of Viral diseases and this article is written by Dr Disha Trivedi who is expert in the field of biotechnology and has published several research and review articles in international journals. 

Dr. Disha Trivedi

Dr. Disha Trivedi is PhD in Molecular Genetics and Biotechnology. She is working as a medical writer and researcher at MVS Pharma GmbH.

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