Health experts say central government’s latest advisory mentioning that aerosols, the key mode of transmission of Coronavirus, can travel up to 10 metres is not based on science or published or experimental studies
Dr Jacob John, noted virologist from Vellore
Health experts have debunked the Centre's May 13 advisory stating that aerosols, one of the key modes of transmission of the novel Coronavirus, can travel up to 10 meters. They said there is no scientific base to prove such a claim.
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Dr Wiqar Shaikh, professor of Medicine at Grant Medical College and Sir JJ Group of Hospitals, said the advisory stated about aerosols in general and did not specify that SARS-CoV2, which causes Covid-19, itself travels for 10 metres.
He said that an extensive survey of which? literature revealed no published or experimental studies to prove that the Coronavirus aerosol indeed travels 10 metres or more. Although there have been review articles, comments, etc, in various journals, there is no actual research publications to prove as to how far away Coronavirus aerosols can travel in the air, he added.
Also Read: Aerosols can travel 10 mts, physical distancing critical
The journal, Physics of Fluid, published a review article on May 19, 2020, stating that Coronavirus aerosols can travel more than 2 metres, said Dr Shaikh. The International Journal of Environment Research in a review article in April, 2020, made similar comments. The Lancet published a 'comment' on April 15, 2021, stating that aerosolised SARS-CoV2 can travel more than 2 metres (6 feet). However, none of these publications provided evidence of any published, experimental study that proves the virus-laced aerosol can travel up to 10 metres or more, he added.
Dr Shaikh said that on the contrary, the US Centres for Disease Control (CDC) in an update on May 7, 2021, stated that 'Covid-19 infections through inhalation at distances greater 2 metres from an infectious source are less likely than at closer distances'. The CDC quoted 12 different research publications to prove its point.
“It is rather surprising that the central government has put out this advisory without support from any research publication on Covid-19. Undoubtedly, appropriate behaviour is of paramount importance in a pandemic," he said.
Dr Ketan Vagholkar, professor of Surgery at D Y Patil Medical College in Nerul, said, “Aerosols and droplets are both responsible for transmission of airborne viral diseases. The distances may vary from a few inches to 10 feet. However, in a densely populated country it may be difficult to follow a suggested norm of physical distance, especially in public places like buses and trains.”
“A three-feet distance as originally advised should be followed strictly. Citizens should avoid crowded areas as much as possible. Therefore, meticulous use of a proper mask continues to be the best way in preventing infection,” Dr Vagholkar said.
Aerosol vs droplet spread
“We hear terms like ‘aerosol’ and ‘droplets’, but their precise meaning remains unclear, and it is hard to know how to adjust our daily practice so as to avoid generating or getting exposed to Covid-19. These are often divided into small invisible aerosols (1-10 µm size) and large, visible droplets (more than 10 µm). Generally, during breathing, talking, coughing, sneezing, etc, aerosol and droplets are generated simultaneously in different proportions. Large droplets then drop to the ground before they evaporate, causing surface or floor contamination. Disease transmission through these large droplets is what we often refer to as “droplet/contact spread”, where disease transmission occurs because one touches a surface contaminated by these droplets, or gets caught within the spray zone when the patient is coughing. Aerosols are so small that buoyant forces overcome gravity, allowing them to stay suspended in the air for 15-20 minutes, or they evaporate before they hit the floor, leaving solid particulate (aerosol nuclei) free to float very long distances for up to 8 hours," says Dr Subhash Hira, professor of Global Health at the University of Washington and health adviser to several UN and Indian health agencies.
Dr Hira added, “A 1,000 µm droplet will fall 1 meter in 0.3 seconds. A 100 µm droplet will fall 1 meter in 3 seconds. A 10 µm droplet will take 300 seconds, and a 1 µm droplet will take 30,000 seconds. How long a droplet remains in the air is clearly determined by the ventilation. This ventilation becomes a big factor in how far it is able to travel, the number of particles or droplets in the inoculum, and how many people will get exposed to SARS-COV-2 along the pathway. Most published experiments have varied results based on multifactorial determinants of airborne transmission. Hence, public health recommendations are based on meta-analyses of these studies and come up with average distances that the two varieties of airborne transmissions are likely to occur. The Indian health ministry recommends that the average distance that large droplets travel is 2 meters (surfaces that are contaminated) and 10 meters (airspaces that are contaminated), but does not specify ventilation speed and time that viruses remain afloat.”
Expert view
Dr Jacob John, a noted virologist from Vellore, said, “Droplets fall within one meter, not two metres, and aerosol travels to two meters, not 10 metres. If the person shedding virus has unusually heavy virus load (as is likely with B.1.617.2), then the distance must be considered longer than 2 meters, theoretically. In closed spaces the meter distances are not important but the virus concentration is. In ventilated places there is hardly any risk at 10 meters. If all are masked, a 2 meters distance is sufficient.”
“Viruses do not carry tape measures. this one meter, two meters and 10 meters are not 'all or none' but dependent on circumstances. When everyone is masked one meter is adequate as inter-individual distance. If the mask is 'tight fitting' then risk of infection is virtually minimal,” Dr John added.
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