Ask Dr. Halverson: Aerosol spread — the ‘smoking gun’ of the novel coronavirus

web 4 17 Halverson photo
By Dr. Jim Halverson
It is estimated that up to 80% of COVID-19 cases are caused by only 10% to 20% of infected individuals. It is critical to have a clear physical description of the ways in which the novel coronavirus is transmitted so that we can visualize it in our mind and understand how to protect ourselves.
There are three possible ways in which the virus is transmitted. The first is through “fomites,” objects that are contaminated with the virus (which could include someone else’s skin). Early in the pandemic, concern over fomite transmission drove some people to bleach groceries and packages. The CDC now says fomites are a much less common means of transmission. For example, an intensive handwashing program in the United Kingdom led to only a 16 percent reduction in transmission. Viruses that have a lipid envelope, like the novel coronavirus, do not survive long on human hands. That means someone would need to touch their eyes, nostrils or mouth a short time after touching a contaminated surface in order to contract the virus.
The second possibility for how COVID-19 spreads is through droplets, small bits of saliva or respiratory fluid that infected individuals expel when they cough, sneeze or talk. These droplets move ballistically like a cannonball from someone’s mouth and travel no more than 3 to 6 feet through the air until they either hit something (worst case, someone else’s eyes, mouth or nostrils) or fall to the ground.  Published research, which has been peer reviewed and replicated, shows that droplets are only important when coughing or sneezing. When it comes to talking in close proximity to another person, which appears to play a major role in COVID-19 transmission, droplets are less important than the third potential pathway: aerosols.
Aerosol (sometimes referred to as “airborne”) transmission is similar to droplet transmission, except that the bits of fluid are so small that they can linger in the air for minutes to hours. To understand the scale of aerosols, the diameter of a human hair is about 80 microns, and aerosols smaller than 50 microns can float in the air long enough to be inhaled. The novel coronavirus is only 0.1 microns in diameter, so there is room for plenty of viruses in aerosols.


Evidence overwhelmingly supports aerosol transmission in COVID-19. To understand why, it is useful to use cigarette or vaping smoke as an analogy for aerosols. When you are near an infected person who is talking (smoking), you inhale lots of smoke. If you are standing on the other side of a room that is well ventilated, or outdoors, you would inhale very little smoke. But, in a very poorly ventilated room, the smoke will accumulate, and people in the room may end up inhaling a lot of smoke over time. 
Talking produces mostly aerosols. Singing and shouting increase aerosol exhalation by factors of 10 and 50 over talking, respectively. With this “picture” of viral transmission in mind,  it is easier to understand why cluster outbreaks often occur when people gather in crowded, insufficiently ventilated indoor spaces, such as singing at karaoke parties, cheering at clubs, having conversations in bars, and exercising in gyms. These superspreading events, where one person infects many, occur almost exclusively in indoor locations and are driving the pandemic. In fact, contact tracing has shown that, when it comes to COVID-19, being outdoors is 20 times safer than being indoors. 
The visual analogy of smoke can help guide our risk assessment and risk reductions strategies. One just needs to imagine that others they encounter are all smoking and the goal is to breathe as little smoke as possible. The CDC says that 15 minutes of close proximity to a COVID-19 infected person often leads to contagion, which provides an estimate of how much “exhaled smoke” one may need to inhale for infection. Inhaling a little whiff of “smoke” here and there is probably okay, but a lot of “smoke” for a sustained period of time and without a mask is risky. (To be clear, actual smoke does not increase the probability of infection.)
We should continue doing what has already been recommended: wash hands and keep 6 feet apart whenever possible. But that is not enough. A new, consistent and logical set of recommendations must emerge to reduce aerosol transmission. Dr. Jose-Luis Jimenez, professor of chemistry and highly cited researcher on aerosols, who published his data on aerosols from which I have written much of this article, proposes the following acronym: “A CIViC DUTY.” (AVOID: CROWDING, especially INDOORS, low VENTILATION, CLOSE proximity, long DURATION, UNMASKED, TALKING closely to people, singing or YELLING. 
A CIViC DUTY suggests that we should do as many activities as possible outdoors, as schools did to avoid the spread of tuberculosis a century ago, despite harsh winters. That said, stepping outdoors is not a magical protection against contagion. A windy day in an open area while keeping our distance is very safe, but an unmasked close outdoor conversation with still air is risky. 
In addition, masks are important, even when we maintain social distance. We should also pay attention to fitting masks snugly, over the mouth and nose, as they are not just a protection against ballistic droplets, but also a means to prevent “smoke” from leaking in through the gaps. We should not remove masks to talk, nor allow someone who is not wearing a mask to talk to us, because we exhale aerosols 10 times as much when talking as when breathing. Everyone should also be careful to not stand behind someone with a poorly fitting mask, as the curvature of an ill-fitting mask can cause aerosols to travel behind the person wearing it.
Whether you remember “A CIViC DUTY” or the three C’s of mitigating COVID-19 (avoid CLOSE contact,  CLOSED spaces with poor ventilation and CROWDED spaces), remember to imagine that people you are with can spread the virus as if they were smoking. Hopefully, with these actions, our Ojai Valley population can avoid superspreader events and clusters of cases as we move into the cooler fall and winter months. Help (in the form of safe and effective vaccines and improving treatment) is on the way in the next several months for all of us who want it.
Stay positive, stay committed, stay properly informed, stay safe and stay well.

— Dr. Jim Halverson is a longtime Ojai physician who writes a weekly column on COVID-19 for the Ojai Valley News.


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