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New CDC guidance on Aerosol Transmission of Coronavirus Still Falls Short

On October 5, 2020, the CDC revised its guidance on coronavirus transmission, recognizing that the virus can be transmitted by aerosols.

While welcome, the updated guidance remains muddled. It appears to be more of a negotiated bureaucratic outcome than a statement based purely on the current science.


Centers for Disease Control, “How COVID-19 Spreads, Updated Oct. 5, 2020

The guidance states, in part

COVID-19 can sometimes be spread by airborne transmission

“”Some infections can be spread by exposure to virus in small droplets and particles that can linger in the air for minutes to hours. These viruses may be able to infect people who are further than 6 feet away from the person who is infected or after that person has left the space.
“This kind of spread is referred to as airborne transmission and is an important way that infections like tuberculosis, measles, and chicken pox are spread.
“There is evidence that under certain conditions, people with COVID-19 seem to have infected others who were more than 6 feet away. These transmissions occurred within enclosed spaces that had inadequate ventilation. Sometimes the infected person was breathing heavily, for example while singing or exercising.
“Under these circumstances, scientists believe that the amount of infectious smaller droplet and particles produced by the people with COVID-19 became concentrated enough to spread the virus to other people. The people who were infected were in the same space during the same time or shortly after the person with COVID-19 had left.
Available data indicate that it is much more common for the virus that causes COVID-19 to spread through close contact with a person who has COVID-19 than through airborne transmission.

The last statement is highly misleading, because aerosol transmission occurs at distances of less than 6 feet. By failing to explain the semantic wrinkles, e.g., the different usages of “aerosol” v. airborne” transmission, the guidance fails to underline the fact that, according to one leading expert on aerosol transmission, Jose-Luis Jimenez, some 75% of coronovirus transmission is through aerosols.

More specific details are to be found in the Scientific Brief.


Centers for Disease Control, “Scientific Brief: SARS-CoV-2 and Potential Airborne Transmission,” October 5, 2020.

“The principal mode by which people are infected with SARS-CoV-2 (the virus that causes COVID-19) is through exposure to respiratory droplets carrying infectious virus.

“Respiratory droplets are produced during exhalation (e.g., breathing, speaking, singing, coughing, sneezing) and span a wide spectrum of sizes that may be divided into two basic categories based on how long they can remain suspended in the air:

* Larger droplets some of which are visible and that fall out of the air rapidly within seconds to minutes while close to the source.
* Smaller droplets and particles (formed when small droplets dry very quickly in the airstream) that can remain suspended for many minutes to hours and travel far from the source on air currents.

“Once respiratory droplets are exhaled and as they move outward from the source, their concentration decreases through fallout from the air (largest droplets first, smaller later) combined with dilution of the remaining smaller droplets and particles into the growing volume of air they encounter.

“Infections with respiratory viruses are principally transmitted through three modes: contact, droplet, and airborne.

*Contact transmission is infection spread through direct contact with an infectious person (e.g., touching during a handshake) or with an article or surface that has become contaminated. The latter is sometimes referred to as “fomite transmission.”
*Droplet transmission is infection spread through exposure to virus-containing respiratory droplets (i.e., larger and smaller droplets and particles) exhaled by an infectious person. Transmission is most likely to occur when someone is close to the infectious person, generally within about 6 feet.
*Airborne transmission is infection spread through exposure to those virus-containing respiratory droplets comprised of smaller droplets and particles that can remain suspended in the air over long distances (usually greater than 6 feet) and time (typically hours).

“The epidemiology of SARS-CoV-2 indicates that most infections are spread through close contact, not airborne transmission

“Diseases that are spread efficiently through airborne transmission tend to have high attack rates because they can quickly reach and infect many people in a short period of time. We know that a significant proportion of SARS-CoV-2 infections (estimated 40-45%) occur without symptoms and that infection can be spread by people showing no symptoms. Thus, were SARS-CoV-2 spread primarily through airborne transmission like measles, experts would expect to have observed considerably more rapid global spread of infection in early 2020 and higher percentages of prior infection measured by serosurveys. Available data indicate that SARS-CoV-2 has spread more like most other common respiratory viruses, primarily through respiratory droplet transmission within a short range (e.g., less than six feet). There is no evidence of efficient spread (i.e., routine, rapid spread) to people far away or who enter a space hours after an infectious person was there.

Airborne transmission of SARS-CoV-2 can occur under special circumstances

“Pathogens that are mainly transmitted through close contact (i.e., contact transmission and droplet transmission) can sometimes also be spread via airborne transmission under special circumstances. There are several well-documented examples in which SARS-CoV-2 appears to have been transmitted over long distances or times. These transmission events appear uncommon and have typically involved the presence of an infectious person producing respiratory droplets for an extended time (>30 minutes to multiple hours) in an enclosed space. Enough virus was present in the space to cause infections in people who were more than 6 feet away or who passed through that space soon after the infectious person had left. Circumstances under which airborne transmission of SARS-CoV-2 appears to have occurred include:

*Enclosed spaces within which an infectious person either exposed susceptible people at the same time or to which susceptible people were exposed shortly after the infectious person had left the space.
*Prolonged exposure to respiratory particles, often generated with expiratory exertion (e.g., shouting, singing, exercising) that increased the concentration of suspended respiratory droplets in the air space.
*Inadequate ventilation or air handling that allowed a build-up of suspended small respiratory droplets and particles.

Prevention of COVID-19 by airborne transmission

“Existing interventions to prevent the spread of SARS-CoV-2 appear sufficient to address transmission both through close contact and under the special circumstances favorable to potential airborne transmission. Among these interventions, which include social distancing, use of masks in the community, hand hygiene, and surface cleaning and disinfection, ventilation and avoidance of crowded indoor spaces are especially relevant for enclosed spaces, where circumstances can increase the concentration of suspended small droplets and particles carrying infectious virus. At this time, there is no indication of a general community need to use special engineering controls, such as those required to protect against airborne transmission of infections, like measles or tuberculosis, in the healthcare setting.

The tortured language in both documents reflects the bureaucratic battles that have been and are being fought. It is all muddled, mixing the categories on “droplet transmission” (including aerosols) and äirborne transmission” (aerosols) into overlapping categories, which is a fundamental scientific error. With such meddled thinking, it is not surprising that the statements contain clearly erroneous or misleading assertions, such as

1) “Existing interventions to prevent the spread of SARS-CoV-2 appear sufficient to address transmission both through close contact and under the special circumstances favorable to potential airborne transmission…”; and

2) “At this time, there is no indication of a general community need to use special engineering controls, such as those required to protect against airborne transmission of infections, like measles or tuberculosis, in the healthcare setting.”

With respect to (1), the language refers to “ventilation and avoidance of crowded indoor spaces as among “existing interventions” to avoid the spread of Covid-19. Ventilation is critical. It is not widely understood as an existing intervention.

Avoidance of crowded indoor spaces is important, but without a full understanding of aerosol transmission one might adopt measures that are not really protective. For example, one might stagger occupation of closed, inadequately ventilated spaces (e.g.,, in restaurants or communal dining rooms in nursing homes or assisted care facilities), creating the false illusion of protecting against coronavirus transmission, when the opposite might actually be the case.

If diners eat in shifts (e.g., observing a 25%occupancy rule), but the air is not properly filtered or ventilated (replaced with filtered or outside air), the aerosols left by the first group might be left in the enclosed space, and only added to the aerosols emitted by diners in the later shifts. Thus, the 25% occupancy standard would be of no avail. A new argument for getting an early reservation might exist, but even an early reservation would not protect diners from aerosol transmission in the first shift.

The carefully worded assertion that there is no need at present “to use special engineering controls, such as those required to protect against airborne transmission of infections, like measles or tuberculosis, in the healthcare setting,” is disingenuous, and deeply misleading. There is a huge need to update ventilation systems to effectively filter out the coronavirus (e.g., with NMERV 13 or HEPA filters, changing the mixture of outside air, etc.).

This need for updated air filtering and conditioning systems cuts to the heart of the matter. Such changes may be required to make closed spaces in schools, colleges, and other indoor spaces secure from coronavirus contamination). It could be very expensive.

Recognition of the need to update ventilation systems as we move into the colder months of autumn and winter could strengthen resistance to reopening schools, for example. This would not mesh well with the political objectives of some leaders. This may have been an important factor in the the CDC’s flip-flopping on updating its guidance on aerosol transmission, posting updated guidance on Friday, September 18 and taking it down on Monday, September 21, 2020.

It will take time for this muddled CDC guidance on aerosol transmission of the coronavirus to work its way down through the bureaucracies of state and local health authorities, and even when it does the muddled nature of the guidance is likely to lead to muddled implementation.

Bureaucracies take time to get things right. The intervention of politics can muddle and slow things down even further.

Unfortunately, the virus works much more swiftly.

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