Skip to Content

Q&A: DU Professor Explains COVID-19 Aerosol Transmission

Back to News Listing


Alyssa Hurst


Alyssa Hurst



Feature  •

Since COVID-19 first took hold of the U.S. early this year, Americans have heard several messages repeatedly:  Wear a mask. Maintain physical distance of at least 6 feet. Wash your hands for 20 seconds. But the science behind COVID-19 transmission indicates that these steps alone may not be enough to stop the spread.

Alex Huffman, associate professor of chemistry in the University of Denver’s College of Natural Sciences and Mathematics, is an expert in aerosols and bioaerosols. Huffman says data suggests that COVID-19 is likely to spread through the air, and that guidance offered earlier in the pandemic simply doesn’t go far enough.

In a recent email interview, Huffman shared the latest information on what it means for a disease to be airborne, how people can protect themselves and where politics come into play.

What does it mean for a disease to travel through aerosols or through the air? How is this different from other methods of transmission?

Viral diseases like COVID-19 are transmitted when the virus travels from an infected individual to someone else. SARS-CoV-2 virus that causes COVID-19 has been found in most bodily fluids, but spreads mostly through the mouth and nose. When you cough, talk or even breathe, you emit thousands of tiny, invisible drops of water, mucous and, if you are infected, virus. The biggest of these drops are heavy and fall down quickly, which is the origin of the now-famous 6-foot rule. Even though you won’t notice, when you stand close to someone, especially if unmasked, your breath will spray their face with little virus-containing cannonballs that are too big to inhale, but can bombard their eyes or fly into their open mouth. These big droplets can also fall down and leave an infectious smear on surfaces that can lead to touch-related infection.

During the same normal activities, however, a bunch of much smaller particles (aerosols) also come out of your mouth. These are so much lighter and slower to fall that they hang around in room air for long periods of time, even hours. This is really important, because aerosols are small enough to be inhaled deep into your lungs. It means that in indoor spaces, we can’t rely on distancing alone. The more time you spend in an indoor space with someone, the more breaths you share, and the higher the risk. To help lessen your risk, wearing masks, especially when talking, reduces the amount of both exhaled and inhaled virus. To visualize how aerosols travel in a room, think about smoke building up as someone smokes indoors. Wherever smoke travels, so can the virus.

Understanding that aerosols can spread COVID-19 means that it is important to focus also on room ventilation and filtration. Not all respiratory viruses have a significant aerosol or room-level airborne component, but there is now a lot of evidence that shows that the SARS-CoV-2 virus can indeed be spread this way, and so we have to treat it that way.

Can we consider aerosol transmission to be more or less dangerous than other forms of transmission? Why?

The fact that SARS-CoV-2 can spread through the air doesn’t mean it has to be any more frightening than other virus. There is long-standing fear that if a disease is announced to be airborne, it will lead to population wide pandemonium. I think this comes largely from a single scene in the 1995 movie Outbreak, when Dustin Hoffman dramatically announces, “It’s airborne!” A virus traveling through the air isn’t necessarily wildly infectious like measles (SARS-CoV-2 isn’t) and doesn’t need to be scary. It simply means that we need to be aware of the simple things we can do to limit spread from both surfaces and through the air.

There has been a lot of talk about “superspreading” events lately. What does that mean, and why is it important?”

In some cases, one infected person (usually when asymptomatic) can infect dozens of other people, especially in crowded indoor spaces. This is called a superspreading event and has happened at a wide variety of events, including choir rehearsals and conferences. It likely even explains the recent virus spread at the White House. These events highlight the danger of certain seemingly innocuous behaviors, such as going to a restaurant, bar, a house of worship or anywhere many people are gathered indoors. Because people emit droplets and aerosols at much higher concentrations and to greater distances when speaking at higher volume, places where people shout (bars or restaurants with loud music) or sing (choir practice or church) can be surprisingly risky places. Even if you wear a mask, the accumulated aerosol in the room makes it so that your overall risk can be really high. If even one person in the room is shedding virus as a superspreader, they can quickly infect many in the room.

In July you joined hundreds of experts in asking the WHO to add indoor aerosol transmission to the list of likely ways COVID-19 spreads. Is this still your professional opinion?

Scientific and medical communities have learned a tremendous amount about all aspects of COVID-19 in the last few months, including about how it is spread. That said, nothing significant has changed in the recommendations we made to the WHO in that article earlier this year, but piles of evidence have mounted in further support.

Much of what dictates aerosol spread of this and other viruses are fairly basic aerosol science concepts that I teach even in first-year undergraduate courses. In many cases, the science discussed about COVID-19 isn’t new; it is just new awareness by public health officials who weren’t educated in aerosol science.

How can we determine whether COVID-19 in particular is spread through aerosols?

You can look, for example, at studies showing aerosol spread from infected patients, in animal studies, and in some cases by using statistical data from community spread. Recent studies in hospital rooms of patients infected with COVID-19 have shown viral aerosol spread. There also are important physical arguments showing the mechanics of how aerosols work; no other known transmission mechanism could explain most of the superspreading events. Fundamental knowledge about how people emit droplets and aerosols is well understood, and a lot is also known about aerosol spread of viruses like measles and influenza. All of what we’re learning about SARS-CoV-2 is adding specificity to this body of knowledge, but basic mechanisms of how aerosols spread infection remain constant. The general scientific consensus is that COVID-19 can be spread by all of the routes I’ve mentioned, but the most likely route in many cases is through inhalation of aerosols in indoor spaces.

Recently, the CDC announced that COVID-19 was spread through aerosols and then quickly backtracked. Now they again say aerosols are at play. Do you have any sense of what’s behind this back and forth?

The quickest summary of my opinion on the CDC decision to retract its updated guidance from Sept. 18, saying that inhalation was “thought to be the main way the virus spreads,” is that the decision was politically motivated by people who aren’t educated in aerosol science. There is also a feeling that data on the dominance of airborne spread must be indisputable before politically unpopular actions can be suggested, but that is not the way physical science works and that kind of evidence could take years or be impossible to gather. I think people want to be empowered with the truth now to help take care of themselves.

Interestingly, just today the CDC released new recommendations that include brief mention of inhalation risk and the need for proper ventilation. The strong statement from Sept. 18 emphasizing inhalation was not used, and the wording of the newest statement is relatively vague. I would like to have seen clearer guidance on the risks of indoor airborne transmission. I also disagree with statements too strongly emphasizing close-contact spread, but the update is a small step in the right direction.

If aerosols are at play here, what should the public be doing to mitigate the spread of COVID-19?

The answers are simple and consistent over the last few months, with only minor additions. Stay outside and distanced at least 6 feet from people not in your household. Wear a mask, especially when you are talking. Wash your hands and limit handshakes and touch. The main addition is to be aware of indoor air and not rely primarily on the 6-foot rule. If you are inside a room with someone (even masked), infectious aerosols can build up and increase your exposure. Be judicious about where you spend time indoors. Especially limit your time inside bars, restaurants, or places where anyone is singing or shouting. If you have control over your room ventilation, upgrade your air filters and increase the airflow from outside using HVAC or by opening a window or door. Even cracking the car window makes a big difference. You can also filter out airborne virus using a portable air filter. There are some nice tools available to help you pick the right filter for your room and even some easy DIY solutions to make your own room filter for around $40. They work well both on viral aerosol and on wildfire smoke. There are also many other important situations that are harder to advise on quickly, so I encourage people to look for answers in the FAQ document that some in my community put together to help.

I understand you were involved in helping DU account for aerosols in its defense against COVID-19. What was your approach?

I’ve done quite a bit with educational outreach recently — for example, by writing modules for the remote “Pathway Back to Campus” course for the DU community. I also curate an openly available Google spreadsheet with resources like: interactive tools and guides, informational infographics, links to webinars, scientific journals, media articles and excellent scientists to follow on Twitter.

Additionally, I’ve been doing experiments with particle emissions and filtration, including collaboration with colleagues at the University of Colorado Boulder. Over the summer, I also applied a mathematical model to calculate the probability of COVID-19 aerosol spread in certain classroom scenarios, primarily in DU’s Lamont School of Music. Using modeled probabilities, we were able to suggest which preventative efforts would bring the most value. In partnership with the Department of Facilities Management, I’ve also been deploying carbon dioxide and particle sensors in certain classrooms to make sure the ventilation rates are well matched to the number of students and activities in those classrooms