COVID-19 & airborne transmission: Is it time to update social distancing guidelines?
M3 India Newsdesk Jul 09, 2020
The way, the new coronavirus, causing COVID-19, spreads in the bio-environment and reaches a potential patient remained controversial for some time. However, now it is official. This article reviews recent studies which highlight the importance of airborne virus load and the role it plays in the transmission of the virus.
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Some specialists feel that there is infection possibly due to an airborne component as well. The need for everyone to wear a face-covering is accepted universally. This development gave incentives to scientists to carry out research of great practical relevance in the field.
“While the use of face masks in public has been widely recommended by public health officials during the current COVID-19 pandemic, there are relatively few specific guidelines pertaining to mask materials and designs,” a press release from American Institute of Physics said.
A paper by researchers from Florida Atlantic University's College of Engineering and Computer Science, published in the journal Physics of Fluids, demonstrated how to visualise emulated coughs and sneezes and to assess the effectiveness of facemasks in obstructing droplets.
Dr. Siddhartha Verma, lead author and an assistant professor who co-authored the paper with Professor Manhar Dhanak, department chair, professor, and director of SeaTech; and John Frakenfeld, technical paraprofessional, all within FAU's Department of Ocean and Mechanical Engineering reviewed many studies which have investigated respiratory droplets produced by both healthy and infected individuals when performing various activities. They observed that transport characteristics of these droplets can vary significantly depending on their diameter.
The size of aerosols
“The reported droplet diameters vary widely among studies available in the literature and usually lie within the range 1 µm to 500 µm, with a mean diameter of ∼10 µm. The larger droplets (diameter >100 µm) are observed to follow ballistic trajectories under the effects of gravity and aerodynamic drag. Intermediate-sized droplets may get carried over considerable distances within a multiphase turbulent cloud. The smallest droplets and particles (diameter <5 µm - 10 µm) may remain suspended in the air indefinitely, until they are carried away by a light breeze or ventilation airflow,” they clarified based on extensive review of literature (not listed here).
The researchers vividly described how aerosols behave after being expelled into the ambient environment; they experience varying degrees of evaporation depending on their size, ambient humidity, and temperature.
“The smallest droplets may undergo complete evaporation, leaving behind a dried-out spherical mass consisting of the particulate contents (e.g. pathogens), which are referred to as “droplet nuclei”. These desiccated nuclei, in combination with the smallest droplets, are potent transmission sources on account of two factors,” they observed.
The researchers described two possibilities (verbatim):
- The droplet nuclei can remain suspended in the air for hours after the infected individual has left the area, potentially infecting unsuspecting individuals who come into contact with them and
- They can penetrate deep into the airways of individuals who breathe them in, which increase the likelihood of infection even for low pathogen loads.
The researchers clarified that presently, the role of droplet nuclei in the transmission of COVID-19 is not known with certainty and the matter is the subject of ongoing studies. Besides generating microscopic droplets, the action of sneezing can expel sheet-like layers of respiratory fluids, which may break apart into smaller droplets through a series of instabilities. Most of the fluid contained within the sheet falls to the ground quickly within a short distance.
Regardless of their size, all droplets and nuclei expelled by infected individuals are potential carriers of pathogens.
A COVID-19 exposure is defined as face-to-face contact within 6 feet with an infected person (with symptoms or not) that is sustained for at least a few minutes -- at least 10 and maybe more.
Various studies have investigated how effective medical-grade face masks and other personal protective equipment (PPE) are in reducing the possibility of cross-infection via these droplets. The researchers found that while there have been prior studies on how medical-grade masks perform, data on cloth-based coverings used by the vast majority of the general public are sparse.
"The main challenge is to represent a cough and sneeze faithfully,” Verma said.
"The setup we have used a simplified cough, which, in reality, is complex and dynamic," he added.
The researchers developed it in a laboratory setting using a laser light sheet and a mixture of distilled water and glycerin to generate the synthetic fog that made up the content of a cough-jet. They visualised droplets expelled from a mannequin's mouth while simulating coughing and sneezing. They collected masks that are readily available to the general public, which do not draw away from the supply of medical-grade masks and respirators for healthcare worker and tested them systematically.
They mounted the mannequin at a height of ∼5 ft and 8 in. to emulate respiratory jets expelled by an average human male. The circular opening representing the mouth is 0.75 in. in diameter. The pressure impulse that emulates a cough or a sneeze may be delivered via a manual pump, or via other sources such as an air compressor or a pressurised air canister. The air capacity of the pump is 500 ml, which is comparable to the lower end of the total volume expelled during a cough. The setup here emulates a simplified representation of an actual cough, which is an extremely complex and dynamic problem.
A news release from the University published in phys.org stated that the researchers tested a single-layer bandana-style covering, a homemade mask that was stitched using two-layers of cotton quilting fabric consisting of 70 threads per inch, and a non-sterile cone-style mask that is available in most pharmacies.
“By placing these various masks on the mannequin, they could map out the paths of droplets and demonstrate how differently they perform,” they revealed.
The news release summarised the researchers’ findings:
- Loosely-folded facemasks and bandana-style coverings provide minimal stopping-capability for the smallest aerosolised respiratory droplets
- Well-fitted homemade masks with multiple layers of quilting fabric, and off-the-shelf cone style masks, proved to be the most effective in reducing droplet dispersal
- They curtailed the speed and range of the respiratory jets significantly, though there were some leakages through the mask material and from small gaps along the edges
- Without a mask, droplets traveled more than 8 feet; with a bandana, they traveled 3 feet, 7 inches; with a folded cotton handkerchief, they traveled 1 foot, 3 inches; with the stitched quilted cotton mask, they traveled 2.5 inches; and with the cone-style mask, droplets traveled about 8 inches
We may recall that a COVID-19 exposure is defined as face-to-face contact within 6 feet with an infected person (with symptoms or not) that is sustained for at least a few minutes -- at least 10 and maybe more.
"In addition to providing an initial indication of the effectiveness of protective equipment, the visuals used in our study can help convey to the general public the rationale behind social-distancing guidelines and recommendations for using facemasks," said Dr. Siddhartha Verma.
"Promoting widespread awareness of effective preventive measures is crucial at this time as we are observing significant spikes in cases of COVID-19 infections in many states, especially Florida," he added.
“When the mannequin was not fitted with a mask, they projected droplets much farther than the 6-foot distancing guidelines currently recommended by the United States Centers for Disease Control and Prevention.” the researchers cautioned.
“They observed droplets traveling up to 12 feet within approximately 50 seconds. Moreover, the tracer droplets remained suspended midair for up to three minutes in the quiescent environment. These observations, in combination with other recent studies, suggest that current social-distancing guidelines may need to be updated to account for the aerosol-based transmission of pathogens.” the phys.org news release clarified.
"We found that although the unobstructed turbulent jets were observed to travel up to 12 feet, a large majority of the ejected droplets fell to the ground by this point," said Dhanak. "Importantly, both the number and concentration of the droplets will decrease with increasing distance, which is the fundamental rationale behind social-distancing."
“The pathogen responsible for COVID-19 is found primarily in respiratory droplets that are expelled by infected individuals during coughing, sneezing, or even talking and breathing. Apart from COVID-19, respiratory droplets also are the primary means of transmission for various other viral and bacterial illnesses, such as the common cold, influenza, tuberculosis, SARS (Severe Acute Respiratory Syndrome), and MERS (Middle East Respiratory Syndrome), to name a few. These pathogens are enveloped within respiratory droplets, which may land on healthy individuals and result in direct transmission, or on inanimate objects, which can lead to infection when a healthy individual comes in contact with them.” the news release cautioned.
On June 2, 2020, a paper in the Proceedings of the National Academy of Sciences stated that droplet clouds emitted during 1 minute of loud speech by an individual infected with the SARS-CoV-2 virus could contain more than 1000 virus particles. The researchers stated that these speech-generated particles which may persist in a confined space for 8 to 14 minutes are eminently capable of transmitting disease. Every passing day brings the sound, scientifically supported evidence highlighting the need to wear appropriate masks and keep recommended physical distancing between individuals.
Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.
Dr. K S Parthasarathy is a freelance science journalist and a former Secretary of the Atomic Energy Regulatory Board. He is available at email@example.com
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