Aerosols, Droplets, and Airborne Spread: Everything you could possibly want to know
by Justin Morgenstern|source www.first10em.com Published -Updated |The rapid emergence of COVID-19 has created tremendous uncertainty in medicine. We don’t know where this pandemic is headed. We don’t know the ideal management strategy. Every day brings conflicting information. Emergency medicine is a field that embraces (or at least tolerates) uncertainty, but knowledge is an important pillar of our sense of control in medicine, and COVID-19 is doing a good job highlighting massive gaps in our knowledge. One of those gaps is the precise mechanisms through which infectious diseases spread and how best to protect ourselves. We hear terms like “aerosol generating” and “droplets”, but their precise meaning can be unclear, and so it is hard to know how to adjust our practice. In this post, I will review everything I have been able to learn about aerosols and droplets, how they spread, and how they should impact our practice.
I will start with a major caveat: despite reading hundreds of papers on this topic, I still have a lot of uncertainty. I think that uncertainty is born from uncertainty in the literature. There was debate and conflicting information with every new paper I found. However, it is also important to recognize that I am an emergency physician attempting to distill in a few weeks topics that people have dedicated entire careers to. If you think I missed something, or want to add to the discussion, please do so below.
I also want to acknowledge that these are incredibly trying times. We are all anxious, and that anxiety is made worse by the conflicting information that we are receiving. There is a risk that by adding even more potentially conflicting information I might add to that anxiety. I think science is fundamentally important. I think this information is important. How we act on this information is equally important. Remember that nothing here is definitive. In already trying times, we don’t want to create conflict with our colleagues. Try to use any information available to work collaboratively, focusing not on the negatives of uncertainty and disagreement, but on the positives of growth and a common goal of safety for all healthcare workers and our patients. For the most part, I am reassured by what I read, and will continue to work hard to use this information to keep my entire team safe.
What exactly is an aerosol?
I have to say, I didn’t expect this to be such a complicated question to answer. There is actually a pretty heated academic debate, centering around desiccation rates and the formulas for turbulent flow, such that it seems that no one really agrees on an exact definition. You will see some pretty definitive definitions in some sources, but that definition will invariably be refuted in the next paper you encounter. In general, aerosols are liquid or solid particles suspended in air. (Tellier 2009; Judson 2019) They can be visible, like fog, but are most often invisible, like dust or pollen.
They are often divided into small droplets (and many, but not all, people reserve the term “aerosol” to refer only to these small droplets) and large droplets. Large droplets drop to the ground before they evaporate, causing local contamination. Disease transmission through these large droplets is what we often refer to as “droplet/contact spread”, where disease transmission occurs because you touch a surface contaminated by these droplets, or get caught within the spray zone when the patient is coughing. Aerosols are so small that buoyant forces overcome gravity, allowing them to say suspended in the air for long periods, or they evaporate before they hit the floor, leaving the solid particulate (“droplet nuclei”) free to float very long distances, causing what we often refer to as “airborne” transmission. (Nicas 2005; Judson 2019)
Respiratory aerosols are created when air passes over a layer of fluid. (Fiegel 2006; Morawska 2006) There are a large number of factors that can alter this process. The viscosity of the fluid layer is an important determinant of aerosol generation, and could be a very important practical consideration in medicine. Increases in surfactant increase overall droplet formation, and produce smaller droplets (which will travel farther). (Fiegel 2006) This could be an important consideration, as some people are discussing the use of surfactant to manage COVID-19 lung disease. Conversely, nebulized saline has been shown to decrease the number of bio-aerosols produced, and has been suggested as a possible (but unproven) infection control strategy. (Fiegel 2006)
In the world of aerosols, there seems to be two main points of contention. The first is the size cutoff between large and small droplets. Various sources will put the cutoff at 2 µm, 5 µm, 10 µm, 20 µm, or even 100 µm. (Judson 2019; Morawska 2006; Fiegel 2006; Xie 2007; Chen 2010; Nicas 2005; Tellier 2009) This is a key distinction, because it is the difference between airborne and droplet precautions. Many papers make definitive statements based on one of the cutoffs that would be incorrect if a different cutoff was used. (For example, Morawska 2006 states that droplets smaller than 100 µm, which is almost all droplets, will evaporate before hitting the floor, meaning that they can transmit disease through the airborne route, while other documents will use 5 µm as the cutoff.) There is probably a grey area in which droplets can behave either way, depending on how quickly they evaporate compared to how quickly they fall to the ground based on the atmospheric conditions of the room.
The second main point of contention is exactly how clean the distinction between airborne and droplet transmission is. Some sources treat this as black and white, but others point out that large droplets evaporate and become smaller, and most activities create a very large variety of sizes, so it is more like a spectrum than a dichotomous distinction. A lot of epidemiologic studies will make strong claims that a disease is only spread by close contact, but we have to remember, those studies cannot possibly distinguish between short distance aerosol transmission (I caught it while breathing a few feet away from you) and contact transmission (I touched the door handle and then rubbed my eye.) Too often, if you were close together, studies will just assume it was contact instead of aerosol spread, biasing the literature in that direction.
What are aerosol generating procedures?
An aerosol generating procedure is a medical procedure that creates aerosols in addition to those that the patient creates regularly from breathing, coughing, sneezing, and talking. (Judson 2019) In other words, it is important to remember that patients will create their own aerosols even when we are not performing these procedures. Aerosol generating procedures can produce both large and small droplets. Each procedure will be unique, so they really need to be considered independently. (Judson 2019) Importantly, aerosol generating procedures can cause transmission through pathways that microbes don’t usually use (a virus normally spread through contact or droplets can become airborne). Procedures can either generate aerosols directly or by inducing the patient to cough or sneeze, a distinction that may be important when trying to mitigate risk. (Judson 2019)
Although respiratory infections are the primary source of aerosols, they are created in other ways as well. Surgery can aerosolize pathogens found in the blood or tissues. (For example, HIV was found in aerosols created by surgical power tools.) (Judson 2019) Aerosols can also be produced by seemingly mundane things, such as fast running tap water and flushing toilets. (Morawska 2006)
Because the individual risks (and benefits) of each procedure is likely unique, I will consider them each independently. For the sake of space, I have done so in a second post that accompanies this one.
Aerosols and normal activities
Throughout our preparations for COVID-19, we have all been incredibly focused on aerosol generating procedures, but it is important to understand that aerosols are also produced through normal human activities, including simply breathing. (Tellier 2009; Asadi 2019) Essentially any air passing through the respiratory tract will create droplets. The clinical significance will depend on the number of droplets produced, their size, the concentration of infectious agents, the frequency with which the activity is performed, and the PPE used by staff. (Morawska 2006) For example, although a single cough produces far more droplets (of all sizes) than a single breath, breathing occurs much more frequently, and so may be responsible for more droplet production overall. (Morawska 2006; Fiegel 2006) It is also important to understand that although the majority of the droplets produced by a cough may be small enough to stay airborne, their small size means that collectively they add up to only a tiny fraction of the volume produced (perhaps less than 0.1%), and therefore only a tiny fraction of the total virus spread. (Nicas 2005) However, despite carrying smaller numbers of microorganisms, there is evidence that smaller droplets don’t need to contain as many microorganisms as larger droplets to cause a clinical infection (by several orders of magnitude). (Nicas 2005; Tellier 2009) Furthermore, we must remember that not every droplet will contain virus, and even if it does, it may not be enough to effectively transmit disease.
Table adapted from Morawska 2006, with similar numbers reported in the Fiegel 2006 review:
|Activity||Number of droplets produced||Small (1-2 um) aerosols?|
|Normal breathing (5 min)||A few||Some|
|Single strong nasal exhalation||Few to a few hundred||Some|
|Counting out loud (talking)||Few dozen to few hundred. Some sources say a few thousand (Xie 2007)||Mostly|
|Cough||Few hundred to many thousand||Mostly|
|Sneeze||Few hundred thousand to a few million||Mostly|
If you want a more specific breakdown, you can look at table 2 from Nicas 2005, but these numbers are estimates, and you will see different numbers even in this same paper:
Older studies concluded that humans primarily produce large droplets, but they were significantly limited because their instruments were insensitive to smaller sizes. (Morawska 2006) Recent research has indicated that as many as 80-90% of the particles generated by human exhalation are smaller than 1 µm in size. (Papineni 1997) Although the exact size of droplets produced is still debated, most sources agree that speaking, coughing, and sneezing produce droplets that are sufficiently small to remain airborne. (Fiegel 2006; Chen 2010)
Interestingly, the total amount of bioaerosols produced varies tremendously among individuals, with some people creating very few, and others acting as “super producers”. (Fiegel 2006) I wonder whether this explains why we have observed super-spreaders of SARS and COVID-19, as “it appears that a minor percentage of the population will be responsible for disseminating the majority of exhaled bioaerosol”. (Fiegel 2006)
Vomiting, in which humans can shed up to a million virus particles per milliliter of vomit, can also produce aerosols. (Morawska 2006) A vomiting SARs patient was associated with nosocomial spread in a hospital in Hong Kong, although it isn’t clear by what route (contact, droplet, or airborne) the transmission occurred. (Morawska 2006) Similarly, there can be as many as a hundred million virus particles in every gram of feces, and flush toilets are known to result in aerosolization. As is discussed below, this form of aerosolization is thought to have spread SARS in the Amoy Garden apartment complex in Hong Kong. (Morawska 2006)
However, whether these aerosols are capable of transmitting disease still depends heavily on the number produced, the concentration of the infectious agent, the virulence of the microbe, environmental factors (the virus needs to be able to survive, whether in the air or on a surface, until it enters a host), and the health and immunity of the host. (Morawska 2006) Although it is clear that aerosols are commonly produced, it is also clear that the vast majority of disease transmission occurs among people who are in very close contact and therefore exposed to the largest of the droplets.
The fact that humans constantly produce aerosols is really important when assessing studies of aerosol generating procedures. The result sections of these papers will often only present a change in aerosols from baseline, and frequently our procedures won’t produce more droplets. However, if you look closely, we are already producing a ton of aerosols, and even if the procedures don’t produce more, their ability to spread those aerosols further is a big concern. (Simmonds 2010; Rule 2018)
Update: In one of the more entertaining and yet still scientific tweetorials of all time, Dr. Andy Tagg asks the question, “Is farting an aerosol-generating procedure?”: