COVID-19 Transmission: Is the Air Contaminated?

This post was reviewed by Dr. Carolina Camargo, one of our subject matter experts.

For the original article, please follow this link.

For the biological definitions of aerosols and respiratory droplets, please visit this page.

Brief Summary 

This case study1 at two Wuhan Hospitals and public areas nearby detected the genetic material of SARS-CoV-2 in the air. They found that negative air pressure, strict sanitization protocols, and low crowding of active cases are associated with low detection of SARS-CoV-2 genetic material. Even though the genetic material of SARS-CoV-2 is detectable in the air of crowded areas with active cases, it is still unknown whether aerosolized SARS-CoV-2 can cause infection. 


The scientific community understands a moderate amount regarding COVID-19 transmission. For starters, SARS-CoV-2  (the virus causing Coronavirus Disease 19 (COVID-19)  is known to spread through people inhaling human respiratory droplets. It can also spread through contact with contaminated surfaces, or direct contact with other infected people.  However, we still don’t know if and how long SARS-CoV-2 can contaminate the air we breathe. 

Respiratory  droplets and aerosols are made  by actions like breathing, talking, sneezing, and coughing. Respiratory droplets are water droplets bigger than 5 micrometers (a millionth of a meter) in size. They can’t travel very far (less than 1 meter) due to gravity2. Aerosols remain airborne for longer thanks to their smaller size and environmental factors such as airflow, temperature, and relative humidity. Many pathogens (viruses and bacteria) such as tuberculosis, measles, and chickenpox are known to hang out in the air as aerosols, which can cause an infected person to give the disease to just through the air. During the 2003 SARS epidemic, a study based in Hong Kong3 showed that aerosol routes could spread SARS-CoV in closed spaces. Based on this there is some concern that SARS-CoV-2 could do the same. However, we haven’t confirmed whether SARS-CoV-2 can spread through the air yet. This is mainly because  it’s hard  to locate virus-containing aerosols and identify them if they are at low concentrations in the air. 

Research Re-hashed

Today’s Demystified article reviews the work of director Ke Lan and his associates at the State Key Laboratory of Virology at Wuhan University. In this study, the air was sampled in 30 areas across two Wuhan hospitals (Fangcang and Renmin Hospital). It was also sampled at  many  public locations such as two department stores, a supermarket, and a residential building.

Three different methods were used to collect air samples:
(1) The total suspended particles (TSP) were collected by a pump that sucked air through a filter, gathering all the particles on top of the filter. 
(2) The size-segregated samples were collected by separating the aerosols with a cascade impactor, a device that contains multiple filters with decreasing pore sizes. As the air flows through, larger particles are trapped on earlier filters, letting smaller ones pass onto the next until all the aerosols are separated based on the size range they fall into.
(3) The deposition samples were collected by allowing the aerosols to fall onto a filter placed on the floor.

A total of 34 samples were taken across 30 sites. 25 total suspended particle (TSP) samples were taken from February 17th to February 24th, 2020, during the peak of the SARS-CoV-2 epidemic in Wuhan. Additionally, three size-segregated samples were taken across three medical staff areas. Also, two deposition samples were taken in two patient areas. Another round of four TSP samples were taken on March 2nd, 2020, as the number of new cases per day in Wuhan were beginning to drop.

SARS-CoV-2 carries its genetic information in a molecule called RNA, in contrast to human cells whose genetic material is encoded by DNA. For more information on the difference between these two types of molecules, check out Lasya’s Sidebar, “On Mutation”. Since SARS-CoV-2 RNA has a unique sequence, it can be detected in aerosol samples by a highly sensitive molecular biology technique called Droplet Digital Polymerase Chain Reaction (ddPCR), even if the virus is present in small amounts. ddPCR measures the number of SARS-CoV-2 RNA copies within a sample. Knowing the volume of air taken in each sample, the researchers calculated the RNA copies per cubic meter of air (copies m-3) for each sample. The concentration of RNA copies in the air suggests the presence of the virus; however, this method is limited in that fact that it is not accurate in measuring the number of virus particles in the air. 

In the hospital wards where COVID-19 patients were physically present, a low amount of SARS-CoV-2 RNA was detected in TSP aerosols. The authors suggested that this may be due to hospital-grade ventilation of COVID-19 patient areas. This ventilation sucked out the airborne virus by creating negative air pressure. However, a lot more viral RNA was detected in a COVID-19 patient toilet room, a non-ventilated area about 1m by 1m in size. The authors suggest that SARS-CoV-2 RNA may have potentially built up  from the patients’ breath, or from aerosolized feces or urine, in this enclosed area. Interestingly, the medical staff areas had higher airborne SARS-CoV-2 RNA levels compared to COVID-19 patient wards, with seven of eight sampling sites having more than 16 copies m-3 during the peak of the pandemic. This included both TSP samples and size-segregated samples. Finally, the samples taken on March 2nd, after the peak of the epidemic, detected no airborne RNA copies at some of the sites previously sampled at Fangcang Hospital. The authors suggest that this big drop in airborne viral detection can be explained by both a lower number of active cases at Fangcang Hospital, and the use of stricter sanitization protocols.

As shown  by the size-segregated samples, SARS-CoV-2 particles were in two size ranges. The smaller size particles were found to be higher in areas where medical staff were removing personal protective equipment (PPE). The smaller particles may be able to move further, so it is important to consider sanitizing PPE before removal to reduce infection risk to medical staff. The deposition samples placed in the Renmin hospital ICU near patient beds detected high amounts of SARS-CoV-2 RNA (113 and 31 copies m-3). This suggests that aerosol deposition by infected people can easily result in contamination of close-by surfaces. Notably, the TSP sample in the same area detected no SARS-CoV-2. This suggests that SARS-CoV-2 containing water droplets fall to the ground quickly.  In public areas outside the hospital, very little airborne SARS-CoV-2 was detected (below 3 copies m-3). The exception to this was an entrance area to a department store that frequently gets overcrowded. 

The key takeaways? Using negative pressure ventilation and strict sanitation protocols (especially when taking off PPE) seems to help reduce the amount of  SARS-CoV-2 in the air, as seen in both Wuhan hospitals. In public areas, overcrowding seemed to be the biggest risk factor for producing airborne SARS-CoV-2. 

Limitations: What is Still Unclear? 

Since this was a case study, there are limits to how these results can be applied.  For example, the aerodynamics of aerosols is very dependent on environmental factors such as temperature and humidity. Considering that all of these sampling sites were within Wuhan’s hospitals and public areas, the results of similar studies across the globe may yield different results. Beyond that, although the samples collected on March 2nd, 2020 detected lower quantities of airborne virus, there were a small number of secondary sampling sites. If the secondary sample size had been larger, this study could have provided more insight into the aerodynamics of SARS-CoV-2 in hospitals during versus after the peak of the Wuhan epidemic. Lastly, even though SARS-CoV-2 RNA was detectable in the air, its ability to actually infect people through this route is unknown. SARS-CoV-2 may be too structurally unstable in aerosols to cause infection. Therefore, more laboratory studies on the aerodynamics and infectivity of aerosolized SARS-CoV-2 must be conducted to understand its transmission better.


  1. Liu, Y. et al. Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals. Nature 2271-3 (2020).
  2. Atkinson J, Chartier Y, Pessoa-Silva CL, et al., editors. Natural Ventilation for Infection Control in Health-Care Settings. Geneva: World Health Organization; 2009. Annex C, Respiratory droplets. Available from:
  3. Yu, I. T., Qiu, H., Tse, L. A. & Wong, T. W. Severe acute respiratory syndrome beyond Amoy Gardens: completing the incomplete legacy. Clin. Infect. Dis. 58, 683–686 (2014).

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