Aerosol study shows how SARS-CoV-2 spreads

The researchers studied air samples from hospital and residential zones and proximity to infected persons

The researchers studied air samples from hospital and residential zones and proximity to infected persons

The transmission of SARS-CoV-2 through the air has been studied at various places around the world. Researchers led by those at CSIR-Centre for Cellular and Molecular Biology (CCMB), Hyderabad, and Tata institute for Genetics and Society (TIGS), Bengaluru carried out a study to get further insight into the transmission characteristics of the novel coronavirus in air in hospital settings and in residential areas and public places. The study, published in Journal of Aerosol Science, gives insights that are quite in keeping with common-sensical notions and underline the importance of masks in comparison with mere physical distancing. 

Gauging virulence

A machine, which sucks in air, similar to a vacuum cleaner, fitted with a filter on which a gel was spread was taken and worked in the target areas. After running the machine for long durations stretching to an hour or two, samples collected from the filter were either used to carry out an RT-PCR test to determine the presence of the virus, or were cultured by infecting vero cells in the lab. The latter was done to find out if the collected virus samples were infectious.

The researchers saw the virus in samples collected from COVID-19 wards. Outside, in the corridors and near the hospitals they did not detect it, implying a dilution or removal of the virus.

“Having separate COVID-19 wards and negative pressure rooms are good, because infection does not spread very far,” says Dr. Rakesh Mishra, Director, Tata institute for Genetics and Society (TIGS), Bengaluru, who led the study.

Negative pressure rooms are isolation wards in hospitals to keep persons with infectious diseases.

Not mere proximity

The researchers detected the virus even 20-25 feet away from infected persons. “This means it is in the air for quite some time. That is why this study confirms that this virus is spreading through air. It is not only with close proximity but also it can hang in the air for quite some time in the droplets,” says Dr. Mishra. 

Samples taken from air from toilets also showed presence of the virus. The group detected the virus when they sampled the air in the toilet even two hours after an infected person had used it. The study implies that when using public toilets, masks should not be removed, even if no one is present.

“The virus can hang in the air for close to two hours after an infected person has used the toilet. It is very risky if someone enters the toilet, sees no one there and removes the mask,” he adds.

Sterilising rooms

The study further observes that in a closed space, physical distancing alone is not helpful, because the virus borne on droplets can travel upto 20-25 feet. “Thus, in a hall where there is poor circulation, and if some people are infected, you have to keep the mask on even if you are 6-10 feet away from that person,” explains Dr. Mishra. 

The study thus established that a lot of the (SARS-CoV-2) infection is due to the movement of the virus particles in droplets through the air. It can hang for several hours, and go up to several metres. Air cleaning and circulation in closed rooms is very important. Sterilisation filters may be employed for air conditioning.

Challenges in sampling

Some of the challenges in conducting this study were to do with the location of the sampling. “The problem was that people were not allowing us to sample in the hospitals… [they were concerned perhaps that] the personnel may get disturbed and leave. We then had to talk to people who were scientific and carry out our sampling there. That is why in the paper, several hospitals are listed in author affiliations,” says Dr. Mishra. 

The second challenge was in estimating the virus in the filters, because it had not been used for this purpose earlier. The researchers had to modify the methods a bit, and they are writing about it in a forthcoming paper.

Thirdly, there are a lot of variations due to the dynamics of the air from which the samples had to be collected, so the researchers had to repeat measurements several times in order to not miss out positives.

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