COVID-19 opened the doors for very rewarding multidisciplinary research. A recent paper in 'Physics of Fluids' highlighted the importance of emission of the virus- loaded particles by infected persons. The paper showed that emission can occur even when the infected person is speaking normally. The results of aerosol emission will motivate the readers to follow COVID norms strictly, particularly wearing masks. 

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Advanced research on the fluid dynamics of exhaled flows explains how the virus infection may spread rapidly.

COVID-19 news these past weeks was quite disquieting. Six states see an uptick in new COVID infections; Maharashtra, Kerala account for 74% of active cases followed by Punjab, MP, Chhattisgarh, J&K are the other states. Thoughtless relaxation of COVID-19 norms by large sections of the population together with the virus-friendly fluid dynamics conditions in many situations may be one of the reasons. The weird ways of the virus need closer study and the disease needs constant and eternal vigilance.

So far, most studies on how exhaled air flows away from persons, considered coughing or sneezing which can send aerosols flying long distances. However, a paper published in Physics of Fluids on 23 February 2021 clearly showed that speaking while near one another can also be a cause for potential virus infection. A few other studies showed how the number of particles emitted from the mouth depended on the intensity of sound!

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Research from Japan

Scientists used artificial smoke from electronic cigarettes and laser light to study the flow of exhaled breath near and around two people conversing in various relative postures commonly found in the service industry, such as in hair salons, medical exam rooms, or long-term care facilities.

The artificial smoke consisted of droplets about a one-tenth micron in diameter, similar to the size of a virus particle emerging from electronic cigarettes

The liquid used in the vaping devices, a mixture of glycerin and propylene glycol, produces a cloud of tiny droplets that scatter light from a laser, assisting visualisation of airflow patterns.

The author Keiko Ishii in a press release issued by the American Institute of Physics said:

"We analysed the characteristics of exhalation diffusion with and without a mask when a person was standing, sitting, facing down, or lying face up."

To study the effect of speech on exhalation, researchers uttered the word "onegaishimasu," a typical Japanese greeting in a business setting, repeatedly while filming the resulting vapour cloud. They carried out the experiments in a hair salon at the Yamano College of Aesthetics in Tokyo, with postures chosen to simulate typical customer service scenarios, including shampooing where a customer is lying back and the technician is standing and leaning over the customer.

Ishii added:

"A significant amount of similar face-to-face contact would occur not only in cosmetology but also in long-term and medical care."

The experiments revealed that the exhaled air from an unmasked person who is speaking tends to move downward under the influence of gravity. If a customer or patient is lying below, they could be infected.

If one wears a mask while standing or sitting, the vapour cloud tends to attach to that person's body, which is warmer than the surrounding air and flows upward along the body. If the technician is leaning over, however, the aerosol cloud tends to detach from that person's body and fall onto the client below.

When the investigators experimented with face shields they found that it can prevent any aerosols that leak from around the technician's mask from travelling down to the customer. According to Ishii, the face shield promoted the rise of the exhaled breath. "Hence, it is more effective to wear both a mask and a face shield when providing services to customers,” he concluded.

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Visualising speech-generated oral fluid droplets

In a brief report titled “Visualising Speech-Generated Oral Fluid Droplets with Laser Light Scattering” published in the New England Journal of Medicine, scientists from the National Institutes of Health, Bethesda, Mary Land, USA and another from Perelman School of Medicine at the University of Pennsylvania showed how aerosols and droplets generated during speech have been implicated in the person-to-person transmission of viruses. Obviously, there is current interest in understanding the mechanisms responsible for the spread of COVID-19 by these means.

“The act of speaking generates oral fluid droplets that vary widely in size, and these droplets can harbor infectious virus particles. Whereas large droplets fall quickly to the ground, small droplets can dehydrate and linger as “droplet nuclei” in the air, where they behave like an aerosol and thereby expand the spatial extent of emitted infectious particles.” they clarified.

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Experimental setup

When a person spoke through the open end of a box, droplets generated during speech traversed approximately 50 to 75 mm before they encountered a laser light sheet. An iPhone 11 Pro video camera aimed at the light sheet through a hole (7 cm in diameter) on the opposite side of the box recorded sound and video of the light-scattering events at a rate of 60 frames per second. Researchers estimated the size of the droplets from ultrahigh-resolution recordings.

When the person said “stay healthy,” he generated numerous droplets ranging from 20 to 500 μm. These droplets produced flashes as they passed through the light sheet. The brightness of the flashes reflected the size of the particles and the fraction of time they were present in a single 16.7-msec (1/1000 sec) frame of the video. The number of flashes in a single frame of the video was highest when the “th” sound in the word “healthy” was pronounced. Repetition of the same phrase three times, with short pauses in between the phrases, produced a similar pattern of generated particles. When the same phrase was uttered three times through a slightly damp washcloth over the speaker’s mouth, the flash count remained close to the background level (mean, 0.1 flashes); this showed a decrease in the number of forward-moving droplets

Researchers found that the number of flashes increased with the loudness of speech, a finding consistent with observations by other investigators. In one study, droplets emitted during the speech were smaller than those emitted during coughing or sneezing. Some studies have shown that the number of droplets produced by speaking is similar to the number produced by coughing.

The study provided visual evidence of speech-generated droplets and qualitatively described the effect of a damp cloth cover over the mouth to curb the emission of droplets. “Certain persons called “super spreaders” produce many more aerosol particles than other persons.” Dr. Matthew Meselson, Harvard University, said in an accompanying correspondence in the same issue of the journal. He clarified that aerosols from infected persons may pose an inhalation threat even at considerable distances and in enclosed spaces, particularly if there is poor ventilation.

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Need to wear a mask

Dr. Meselson cautioned:

“The possible contribution of infective aerosols to the current pandemic suggests the advisability of wearing a suitable mask whenever it is thought that infected persons may be nearby and of providing adequate ventilation of enclosed spaces where such persons are known to be or may recently have been.”

Loud speaking during electioneering and agitations with crowds milling round is a super spreader activity. With infinite scope for face to face contact among thousands of people, it is a nightmare with unprecedented consequences not from a gentle wave but from a tsunami of COVID-19 cases.

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Speech can produce jet-like transport relevant to asymptomatic spreading of the virus

The French and US researchers declared in a paper published in the Proceedings of the National Academy of Sciences:

“Medical reports and news sources raise the possibility that flows created during breathing, speaking, laughing, singing, or exercise could be the means by which asymptomatic individuals contribute to spread of the SARS-CoV-2 virus.”

They used “experiments and simulations to quantify how exhaled air is transported in speech.” Interestingly, they found that phonetic characteristics introduce complexity to the airflow dynamics and plosive sounds, such as “P,” produce intense vortical structures that behave like “puffs” and rapidly reach 1 m.

Speech, corresponding to a train of such puffs, creates a conical, turbulent, jet-like flow and easily produces directed transport over 2 m in 30 s of conversation. “This work should inform public health guidance for risk reduction and mitigation strategies of airborne pathogen transmission.” they hoped.

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Take away from these studies, the way forward

The Centers for Disease Control and Prevention noted that following a choir practice attended by 61 persons, including a symptomatic patient, 32 confirmed and 20 probable secondary COVID-19 cases occurred. Three patients were hospitalised, and two died. Transmission was likely facilitated by close proximity (within 6 feet) during practice and augmented by the act of singing.

Last week, in casual discussions with individuals in different parts of Kerala, this writer heard similar anecdotal instances of groups of individuals getting infected after a funeral, a marriage ceremony, a religious gathering etc. With elections around the corner in many states, such instances may multiply at alarming rates. Now the weird virus is raising its head in many regions of the country.

The Centers for Disease Control and Prevention (CDC) said that more serious long-term complications appear to be less common but have been reported. These have been noted to affect different organ systems in the body including (Verbatim):

• Cardiovascular: Inflammation of the heart muscle
• Respiratory: Lung function abnormalities
• Renal: Acute kidney injury
• Dermatologic: Rash, hair loss
• Neurological: Smell and taste problems, sleep issues, difficulty with concentration, memory problems
• Psychiatric: Depression, anxiety, changes in mood

Even small percentages of them may add up to sizable numbers to overwhelm the health delivery system. We must prevent COVID-19. Dr Anthony Fauci, Director, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland, USA and his colleagues quoted evidence that up to 40% to 45% of people infected with SARS-CoV-2 may never be symptomatic but still can transmit the virus.

“Viral spread from people without symptoms may account for more than 50% of transmission events in COVID-19 outbreaks. Since it has now become evident that individuals capable of transmitting SARS-CoV-2 cannot be identified solely by the presence of symptoms, universal mask-wearing in the community for source control is recommended,” they clarified.

They strongly urged people to continue to follow universal masking, hand washing, physical distancing, and avoiding crowds etc to keep away the virus effectively all the time, even when safe and effective vaccines are available.

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 former Secretary of the Atomic Energy Regulatory Board and a former Raja Ramanna Fellow, Department of Atomic Energy. A Ph.D from University of Leeds, UK, he is a medical physicist with specialization in radiation safety and regulatory matters. He was a Research Associate in the University of Virginia Medical Centre, Charlottesville, USA. He served the International Atomic Energy Agency as an expert and member in its Technical and Advisory Committees.