Communication with the CDC re: COVIDisAirborne

This space is intended to help COVIDisAirborne scientists effectively communicate with the CDC.

Below is an email I received from the CDC’s Office of Science in response to my query. You can see that they point out many changes that have already been made.

I’ve received a lot of excellent input, mainly from Greta Fox and Yaneer Bar-Yam of the COVID Action Group, and also in tweets from Don Milton and Linsey Marr.

Here is the response I sent back to the CDC Office of Science late on March 2:



Thank you for the thoughtful response to my inquiry. It is great to see that the WebInfoQuality office can be used to engage with the CDC on published guidance, I’m very glad to learn that this mechanism of public interaction with CDC exists and works well.

As pointed out in your response, CDC guidance continues to be updated to reflect latest knowledge about the importance of indoor air quality in reducing COVID transmission, particularly the pages listed in your response and also the Feb 26 update on Ventilation in Schools and Child Care Programs. These changes look excellent, they are in line with the suggestions made by COVIDisAirborne scientists and as these updates reach the public, they will help reduce COVID transmission and improve our public health.

Some important issues have not been resolved.

The single most important issue is the current CDC guidance that COVID-19 is mostly spread by respiratory droplets in close contact. However there is little evidence for transmission by close contact respiratory droplets that is not also consistent with transmission via the inhalation of airborne particles, which is described as something that “sometimes” happens in the current CDC guidance. In particular, there is no evidence that transmission is largely restricted to short distances. Also, the expectation would be that transmission by aerosols would decrease as a power law rather than exponentially, this means that the probability per unit time would decrease but would not vanish with distance, consistent with observations.

Why does the CDC guidance not make it clear that the virus can be transmitted via inhalation at all distances from an infected person, as opposed to the current statement that “Droplet transmission consists of exposure to larger droplets, smaller droplets, and particles when a person is close to an infected person” (Marr, 2021)?

The CDC Office of Science email response to my original query stated that further research was needed into the proportion of COVID-19 infections acquired through airborne transmission. This is reminiscent of Chapin (1910, Sources and Modes of Infection), a strong believer in washing hands, who presented evidence that measles and tuberculosis were transmitted by close contact and claimed that the burden of proof lay with anyone assuming they were transmitted through the air (Milton, 2020). And yet, there is very little direct evidence of COVID-19 transmission via fomite or droplet (none as of April, 2020: Morawska et al, 2020). Quoting from Morawska et al. (2020) “it is scientifically incongruous that the level of evidence required to demonstrate airborne transmission is so much higher than for these other transmission modes” (Morawska and Cao, 2020).

Your email response includes a reference to a CDC scientific brief on potential airborne transmission of COVID-19. In this brief, the paragraph describing epidemiological evidence indicating that COVID-19 is most likely spread through close contact, as opposed to airborne transmission basically states that the attack rate is too low for COVID-19 to be mainly spread via airborne transmission.

There are multiple lines of evidence refuting the argument that COVID-19 is unlikely to be an airborne disease because it did not infect as many people as expected. Many of the arguments are provided by Morgenstern (2020). Quoting from Morgenstern:

“The argument is often framed as, “this disease doesn’t look like measles, and therefore cannot possibly be airborne.” This is bad logic, as infectivity and mechanism of transmission are separate concepts. Some pathogens require higher numbers to reliably cause infections, which will result in a lower Ro no matter how the infection is transmitted. “While many airborne infections are highly contagious, this is not, strictly speaking, part of the definition.” (Tellier 2019)

The logic here is clearly faulty. The argument being used has the basic format: “X is a Y. Z is not like X. Therefore Z cannot be a Y.” This is somewhat like saying “a horse is a mammal, therefore that dog cannot be a mammal because they don’t look the same.”

This logic is especially problematic in the context of aerosol spread because there are other diseases with good evidence of aerosol spread, such as influenza, that look nothing like measles, but a lot like COVID-19. In fact, the prototypical airborne pathogen is tuberculosis, and tuberculosis has an Ro between 1–3 (exactly like COVID-19). (Ma 2018)

Furthermore, the statement that ‘COVID looks nothing like measles’ is probably untrue. On average, disease transmission is low, but if you look at super-spreaders, COVID-19 starts to look a lot like measles.

Therefore, the arguments based on Ro are both illogical and inconsistent with science. If anything, the Ro of COVID-19 looks exactly like other known airborne diseases (such as tuberculosis), and so this would be an argument in favour of airborne spread.”

More evidence refuting the notion that COVID-19 is unlikely to be airborne because of low infectivity comes from specific characteristics of COVID-19. It is known that only some individuals who are sick are infectious (Endo et. al, 2020; Adam et al., 2020). While we don’t know the reason for the distinction between individuals that are highly infectious and those who are not, the effective R of those who are infectious is much higher than the average over all of the people who are infected. This is also in part the reason for super spreader events, though the conditions are also part of that reason (i.e. large events). So the straightforward explanation that the average attack rate is lower is that there is averaging being done over high attack and low attack rates.

The primary time of viral transmission is proximate to the time of initial symptoms and since breathing more than coughing may be the source for that transmission, the underlying density of viral particles being emitted may be low and the attack rate can be low even if aerosol transmission dominates.

A reduced transmission compared to measles may result from both the rate of emission of viral particles and the probability of binding to receptors in the lung. This is a different mechanism than for measles.

Out in the last couple days, the statement of the Lancet COVID-19 Safe Work, Safe School, and Safe Travel Task Force supports inhalation of airborne particles as the predominant mode of transmission with evidence that super-spreading events are best explained by far-field aerosol transmission as opposed to close contact, that ventilation and filtration influence disease transmission, and that nearly all outbreaks of three or more people happen indoors.

Out last week, a paper in PNAS finds that “airborne transmission likely accounted for >50% of disease transmission on the Diamond Princess cruise ship, which includes inhalation of aerosols during close contact as well as longer range.” (Azimi et al., 2021)

Also very recently, inquiries into outbreaks of COVID-19 in Australian quarantine hotels have strongly implicated aerosol transmission, causing Australia’s public health bodies to update their guidance.

The arguments made in this email make a case for the Office of Science re-considering the scientific brief describing evidence that COVIDisAirborne, and the guidance on how COVID-19 spreads. This guidance is particularly important because of its bearing on potential air quality standards from OSHA, and implications for PPE requirements for healthcare workers. For those reasons I would appreciate the Office of Science reviewing these arguments quickly.

In preparing this email response, I received assistance from Dr. Yaneer Bar-Yam, Dr. Linsey Marr and Dr. Don Milton, and from Greta Fox, RN

Thank you again for your consideration of this material. We greatly appreciate your continued efforts to improve our public health during this pandemic.

Brian Colder


Here is the email:


Thank you for your interest in CDC’s guidance. We have reviewed your inquiry related to the current guidance on SARS-CoV-2 transmission and the mitigations needed to protect yourself and others.

CDC is currently conducting a comprehensive review of all existing guidance related to COVID-19 to ensure that guidance remains evidence-based, which will include an assessment of the guidance that was highlighted in your inquiry. In addition, we wanted to address some of your specific comments:

Comment 1: CDC guidance should emphasize airborne or aerosol transmission compared to fomite transmission for SARS-CoV-2.

Current CDC Guidance indicates that COVID-19 spreads very easily from person to person and describes possible transmission events from most to least likely. COVID-19 is most commonly spread through exposure to respiratory droplets when a person is in close contact with someone who has COVID-19. In addition, epidemiological evidence supports that COVID-19 can sometimes be spread by airborne transmission under certain conditions (e.g. enclosed spaces that had inadequate ventilation), COVID-19 spreads less commonly through contact with contaminated surfaces; spread from touching surfaces is not thought to be a common way that COVID-19 spreads. Finally, COVID-19 rarely spreads between people and animals.

A more detailed description of the scientific evidence supporting this guidance is available as a Scientific brief. In brief, pathogens that are mainly transmitted through close contact 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 in an enclosed space. Circumstances under which airborne transmission of SARS-CoV-2 appears to have occurred include: enclosed spaces, prolonged exposure to respiratory particles, and inadequate ventilation or air handling. Critical topics that still need to be further researched to refine existing guidance include the proportion of SARS-CoV-2 infections acquired through airborne transmission, and the conditions that facilitate airborne transmission.

CDC’s continued commitment to educating the public about the importance of respiratory droplets and airborne transmission for COVID-19 was recently highlighted by the February 10, 2021 MMWR publication about improving mask fit to reduce transmission. This study demonstrated that improving the fit of medical procedure masks by two different methods (knotting the ear loops and tucking the mask edges or layering a second cloth mask on top) both resulted in improved source control (reduction in the spread of simulated cough particles) and reduced wearer exposure. These important findings have already been incorporated into the CDC’s public guidance on Effective mask use and how to Improve the fit and filtration of your mask to reduce the spread of COVID-19. The findings of this study were also reported by multiple news outlets, which contributed to public dissemination.

CDC is currently conducting a comprehensive review of all existing guidance related to COVID-19 to ensure that it is evidence-based, and will update guidance based on the best available scientific evidence.

Comment 2: CDC guidance should emphasize that mitigations to reduce transmission via respiratory droplets or aerosols (masking and ventilation) are more important than mitigations to reduce fomite transmission (washing hands and disinfection of surfaces).

Current CDC guidance on How to protect yourself & others emphasizes the importance of mitigations that reduce contact with respiratory droplets and possible airborne transmission. Respiratory-related mitigations (wearing a mask, staying 6 feet away from others, avoiding crowds, and avoiding poorly ventilated spaces) are listed before any mitigations that would reduce fomite transmission (washing hands or cleaning and disinfecting). This emphasis is also reflected in the Things to know about the COVID-19 pandemic, which summarizes the respiratory-related mitigations in the “Three Important Ways to Slow the Spread” and “How to Protect Yourself When Going Out.” The information that fomite transmission is not thought to be the main way the virus spreads is also emphasized in the rationale for Why [individuals] should practice social distancing.

There is also CDC guidance on how to prevent fomite transmission by cleaning or disinfecting your home, a facility, and transport vehicles. In each case, there is explicit language on the need to have proper ventilation in place before cleaning: “Ensure proper ventilation during and after application (for example, open windows),”Open outside doors and windows and use fans or other engineering controls to increase air circulation in the area. Wait 24 hours before you clean or disinfect,” and “Door and windows should remain open when cleaning the vehicle.”

Revisions to the CDC’s guidance, including modifications to emphasize the importance of ensuring proper ventilation and the potential for respiratory transmission while cleaning are underway.

Comment 3: CDC does not adequately describe or emphasize mitigations related to cleaning air or how to improve ventilation.

Official CDC guidance on Ventilation in buildings indicates that a layered strategy is needed to reduce exposure to SARS-CoV-2: improving ventilation, social distancing, wearing masks and hand hygiene. It explicitly communicates that “SARS-CoV-2 viral particles spread between people more readily indoors than outdoors” and that “protective ventilation practices and interventions can reduce the airborne concentration, which reduces the overall viral dose to occupants.” This guidance also includes a list of specific actions that can be considered to improve ventilation and as well as a list of FAQs.

In addition to this centralized guidance on ventilation, CDC has ventilation-related guidance specifically targeted to both the general public and businesses. CDC guidance for Improving ventilation in your home is a sub-directory of How to protect yourself & others, which is the mitigations summary webpage for the general public. This ventilation guidance includes instructions on how to bring fresh air into your home, filter the air in your home (including considering use of a portable air cleaner), and multiple options to improve airflow. CDC also provides comprehensive guidance on Reducing COVID-19 transmission in workplace environments, which includes a section for Engineering controls and ventilation that provides specific actions businesses can take to improve their ventilation to maintain a healthy work environment.

In response to this request, we identified that the section titled “Avoid poorly ventilated spaces” on the How to protect yourself & others website could be enhanced. This section will be updated to include links to improving ventilation in buildings, homes, and businesses, which will ensure that this mitigation is perceived to have similar importance and emphasis as wearing masks and staying 6 feet away from others.


Office of Science Quality

Office of Science

US Centers for Disease Control and Prevention

Atlanta, GA 30333


Caltech engineer, UCLA neuroscientist and long-time government consultant