Pakistani doctors wear face masks as they leave the Aga Khan hospital where a patient suspected of having contracted coronavirus was admitted, in Karachi, Pakistan, Thursday, Feb. 27, 2020. (AP Photo/Fareed Khan) — Copyright 2020 The Associated Press. All rights reserved.

Aerosols are a bigger coronavirus threat than WHO guidelines suggest — here’s what you need to know

Staying 6 feet from others reduces exposure but might not be sufficient in all situations, such as in poorly ventilated rooms.

Updated 13 Jul, 2020 08:51pm

When someone coughs, talks or even breathes, they send tiny respiratory droplets into the surrounding air. The smallest of these droplets can float for hours, and there is strong evidence that they can carry live coronavirus if the person is infected.

Until this week, however, the risk from these aerosols wasn’t incorporated into the World Health Organisation’s formal guidance for nations. The WHO instead suggested that the coronavirus was primarily transmitted by coughing or sneezing large droplets into someone’s face, rather than being a longer-term threat that can float in the air.

It took pressure from scientists to start to change that.

More than 200 scientists published an open letter to the WHO on July 6 warning about airborne transmission of Covid-19 via aerosols and urging the organisation to recognise the risks. The WHO responded Thursday afternoon with an update in which it acknowledged the growing evidence of airborne spread of the disease, but it did so with hesitation.

As professors who study fluid dynamics and aerosols, we believe it is important for people to understand the risks and what they can do to protect themselves.

What is an aerosol and how does it spread?

Aerosols are particles that are suspended in the air. When humans breathe, talk, sing, cough or sneeze, the emitted respiratory droplets mix in the surrounding air and form an aerosol. Because larger droplets quickly fall to the ground, respiratory aerosols are often described as being made up of smaller droplets that are less than 5 microns, or about one tenth the width of a human hair.

In general, droplets form as a sheet of liquid breaks apart. You’ve probably experienced this phenomenon by blowing soap bubbles. Sometimes the bubble doesn’t fully form, but instead breaks apart into many droplets.

Similarly, in humans, small sheets and strands of liquid — mucus — often stretch across portions of the airway. This most often occurs in locations where the airway opens and closes again and again. That happens deep within the lungs as the bronchioles and alveolar sacs expand and contract during breathing, within the larynx as the vocal folds vibrate during speech, or at the mouth, as the tongue and lips move while talking. The airflow produced by breathing, speaking and coughing breaks apart these sheets of mucus, just like blowing the soap bubble.

This slow-motion view of a sneeze shows suspended droplets. — Credit: Lydia Bourouiba via JAMA Network.

The size of the droplets varies based on how and where they are produced within the airway. While coughing generates the largest quantity of droplets, research has shown that just two to three minutes of talking can produce as many droplets as one cough.

Droplets that are smaller than 5 microns can remain suspended in the air for many minutes to hours because the effect of air drag relative to gravity is large. In addition, the water content of virus-carrying droplets evaporates while they are airborne, decreasing their size. Even if most of the fluid evaporates from a virus-laden droplet, the droplet does not disappear; it just becomes smaller, and the smaller the droplet, the longer it will stay suspended in the air. Because smaller diameter droplets are more efficient at penetrating deep into the pulmonary system, they also pose a much greater infection risk.

The WHO guidelines suggested that the virus RNA found in small droplets wasn’t viable in most circumstances. However, early research on the Sars-Cov-2 virus has shown that it is viable as an aerosol for up to 3 hours.

Do masks protect from aerosol transmission?

Face coverings and masks are absolutely necessary for protection against aerosol transmission. They serve a twofold purpose.

First, they filter the air expelled by an individual, capturing respiratory droplets and thereby reducing the exposure risk for others. This is particularly important as they are most effective at capturing larger droplets that are more likely to have larger quantities of viruses encapsulated within them. This prevents the larger droplets from directly affecting someone, or evaporating down to a smaller size and circulating in the air.

They also reduce the speed of the puff of air that is produced when sneezing, coughing or talking. Decreasing the velocity of the expelled air reduces the distance that droplets are initially transported into the person’s surroundings.

It is important to realize, however, that the protection provided by masks and face coverings varies depending on the material they are constructed from and how well they fit. Nevertheless, wearing face coverings to decrease airborne exposure risk is critical.

Is staying 6 feet away enough to stay safe?

The recommendation to maintain a 6-foot separation is based on a study by W. F. Wells in 1934 that showed an expelled water droplet either falls to the ground, or evaporates, within a distance of roughly 2 meters, or 6 feet. The study did not, however, account for the fact that following evaporation of the water in a virus-laden droplet, the droplet nuclei remains, thereby still posing a risk of airborne infection.

Consequently, while staying 6 feet from other people reduces exposure, it might not be sufficient in all situations, such as in enclosed, poorly ventilated rooms.

How can I protect myself from aerosols indoors?

Strategies to mitigate airborne exposure are similar to strategies for staying dry when it’s raining. The longer you stay in the rain, and the harder it’s raining, the wetter you will get. Similarly, the more droplets you are exposed to, and the longer you stay in that environment, the higher the exposure risk. Mitigating risk is therefore based on decreasing both aerosol concentration levels and exposure time.

Aerosol concentrations can be reduced with increased ventilation, although recirculating the same air should be avoided unless the air can be effectively filtered prior to reuse. When possible, open doors and windows to increase fresh air flow.

Decreasing the number of emission sources — people — within a space, and ensuring that face coverings are worn at all times can further decrease concentration levels.

Methods of deactivating the virus, such as germicidal ultraviolet light, can also be used.

Finally, reducing the amount of time you spend in poorly ventilated, crowded areas is a good way to reduce airborne exposure risk.

Amir Mofakham, a research associate in mechanical engineering at Clarkson University, contributed to this article.

This article originally appeared in The Conversation and has been reproduced with permission.


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Dr Byron Erath is Associate Professor of Mechanical Engineering, Clarkson University. His research interests encompass the field of fluid mechanics, with a particular focus on the laryngeal aerodynamics of voiced speech.

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Dr Andrea Ferro is professor of civil & environmental engineering at Clarkson University. Her technical expertise is focused on indoor air quality and human exposure to particulate pollutants.

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Dr Goodarz Ahmadi is a professor of mechanical engineering at Clarkson University. He is currently working on projects for developing a new technique for modeling pollutant transport, dispersion and turbulent flows of dense and dilute, solid-gas or liquid mixtures with application to coal transport and processing, spray formation, and hot-gas filtration. Other projects are to analyze aerosol particles transport, deposition and removal in turbulent flows with application to micro-contamination control in microelectronic, xerographic and glass industries.

The views expressed by this writer and commenters below do not necessarily reflect the views and policies of the Dawn Media Group.

Comments (14) Closed

Jul 11, 2020 04:52pm
Stop scaring people. One must have immunity first.
Recommend 0
Jul 11, 2020 05:59pm
Finally, reducing the amount of time you spend in poorly ventilated, crowded areas is a good way to reduce airborne exposure risk.
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Jamil Ahmed
Jul 11, 2020 06:16pm
Half knowledge is dangerous
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Syed Raheel Mahmood
Jul 11, 2020 06:24pm
Covid-19 is emerging more and more as a divine punishment. It has been mishandled at a global scale.
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Gordon D. Walker
Jul 11, 2020 08:38pm
Roll the dice... Gordon D. Walker Canada
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Dr. Doctor
Jul 11, 2020 09:46pm
Coming from experts in the field of Mechanical engineering and not actual infectious disease people, premise is still correct.Unfortunately WHO guidelines were based on studies from 30's. Advances in technologies help detect micro aerosol remnants at farther distances. Research shows flu VIRUS RNA can be detected in air at 6 feet; minimal distancing of MORE than 6 feet should've been recommended for when not using masks. In 2014, BACTERIA laden cough aerosols were shown to travel 13 feet! Consequently, a minimal distancing guideline for CORONAVIRUS in closed spaces should've been around 8 or above to be on the safer side. Considering this virus is related to FLU virus, they should've advised distancing of MORE THAN than 6 feet IF NOT USING MASKS. So, for less than 8 feet encounters use masks, social distancing of preferably more than 10 feet if no masks inside, limiting indoor time in closed places & only touching face if hands washed recently for a minimum of 20 seconds with soap.
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Jul 11, 2020 11:38pm
@Dr. Doctor, They "are experts in the field of Mechanical engineering" but you have generalised the term 'Mechanical engineering'. It has different areas. Read their full profile and and field of expertise!
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Jul 12, 2020 12:02am
The recognition by the WHO of the potential aerosol route to viral infection was long overdue. There are many aspects of virus/aerosol generation, transport, and evolution that are not well understood, and much more research is needed, both short term and long term. We know little about how the Covid-19 transports in different environments. Usually very small or very large particles do not penetrate that deeply in the lungs, and deposit in the the upper respiratory tract. Particles around 0.3 micron diameter go deeper. But the physics with the virus could be different. Public scare will not serve anyone, the sensible things are to avoid crowded and poorly ventilated places, and keep safe distances if one can.
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Haris farooqi
Jul 12, 2020 01:00am
The research is limited thats why WHO hesitant on this issue. Some points further to be noted 1.In middle east and south asia where ambient conditions are very hot nowadays, recirculation air with normal filtration is used and there is no complain of any such case of covid 19 spread with Recirculation air. 2.similar case is with shopping/grocery stores with central recirculation air and normal filters. No substance for believing that virus spread thru these environment. 3.use of mask need to be review , if particle size is less than 5 micrometer and is in the air than surgical mask is of no use and everyone need to go for n95 mask. Prolong use of mask must be avoided. 4. Use of ultravoilet germicid not necessary remove all virus in air since installation need lot of other parameters which are difficult to achieved.
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Jul 12, 2020 11:42am
If this is true then there is absolutely no point in having a full lockdown. Forget the vaccine. Take some basic precautions. Avoid closed spaces, use masks, BUILD YOUR IMMUNITY and wrap up work during daylight.
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Imad khan
Jul 12, 2020 12:37pm
Disagree. You have to look at the clinical impact. Just because there are aerosols present, it doesn't necessarily mean the virus is in large enough quantity to be infectious. I am er doc in the us and have been working during the pandemic. Only one of my staff members has gotten covid so far. If this scare about aerosols were so real, every single healthcare worker wouldve been infected by now.
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Jul 12, 2020 12:46pm
@Syed Raheel Mahmood , why bring divine retribution.why place human attribution to the Divine its preposterous.
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A shah
Jul 12, 2020 03:58pm
I do not trust WHO
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Jul 12, 2020 05:33pm
Wearing a mask all the time may also cause respiratory problems.
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