How Viruses Spread Indoors and What to Do About It

Nov. 29, 2023

Spiky viral particles floating around inside a building.
Understanding indoor viral transmission is important for preventing disease.
Source: ker_vii/iStock
Humans are indoor creatures; most of their time (roughly 90%) is spent inside, especially when the winter months bring biting cold temperatures. But when people congregate in buildings, they share more than space—they also share microbes, some of which cause disease.

Over the past few years, the way pathogens (especially viruses) move through indoor spaces—from schools and offices to homes and hospitals—has become an important piece of the puzzle for controlling the spread of infectious diseases like COVID-19. Understanding the ins and outs of viral transmission inside buildings can inform how these structures are designed and managed to keep occupants healthy. 

How Do Viruses Spread Indoors?

Viral transmission depends on environmental factors (temperature, humidity, how the space is used), the people occupying the space and their activities (flushing toilets, talking, eating, vacuuming) and characteristics of viruses themselves (surface charge, interactions with other microbes, presence of viral envelope and more). “It’s not a simple matter, but really a complex ecology of how viruses survive in the environment,” said Charles Gerba, Ph.D., a professor of virology at the University of Arizona Water & Energy Sustainable Technology Center.

While all these factors create a unique transmission picture for each indoor space, there are a couple key avenues for pathogen dissemination.

Diagram depicting different routes viruses spread indoors.
Viruses spread indoors through multiple, interconnected routes, including via fomites and aerosols.
Source: Ijaz, M.K., et al./PeerJ, 2023 via a CC BY 4.0 license.

Contaminated Surfaces

One way pathogens spread is through contaminated surfaces (fomites), like door handles, tabletops, keyboards, light switches and drinking fountains, to name a few. Viruses are deposited on surfaces directly (e.g., touched by a person infected with a virus) or settle onto them from the air. If someone touches a surface harboring infectious virus, then touches their face (something adults do every 3-5 minutes depending on situation, and kids do roughly 80 times per hour depending on age), they may be infected.

The importance and duration of fomite transmission depends on the virus (e.g., if it has an envelope, which makes it more sensitive to environmental stressors, like disinfectants) and how much of it is present. For example, norovirus, a non-enveloped virus that infects the gut, can persist on surfaces for up to 2 weeks, and fomites are integral for transmission. SARS-CoV-2, an enveloped virus, can survive on surfaces for several days, and fomite transmission is possible and likely involved in viral dissemination. However, SARS-CoV-2 transmission is highly multimodal, with aerosols and respiratory droplets playing a critical role.  

Aerosols

To that end, aerosols (tiny airborne particles on which viruses can hitch a ride) represent another route through which viruses spread in buildings. Sources of microbe-harboring aerosols include humans (e.g., breathing), pets and more. Inhalation of aerosols containing viruses like SARS-CoV-2, influenza and respiratory syncytial virus (RSV) may lead to sickness. 

Whereas respiratory droplets (which are larger than aerosols) are heavier and more likely to fall from the air before evaporating, aerosols can remain in the air for minutes to hours, thus posing a potential risk for extended periods of time. This risk is related to how the air flows through a space (e.g., how much ventilation there is) and the purpose of the building. Schools, for instance, have potential for more putative pathogens in the air, due to lots of people congregating in a space, for long periods of time, with high turnover.
Toilet with a green light in the bowl illuminating a spray of droplets in the air above the bowl.
Toilets spew aerosols several feet above and around the bowl after flushing.
Source: Crimaldi, J.P., et al./Scientific Reports, 2022 via a CC BY 4.0 DEED license.

"One of the things we’ve learned quickly [is that] viral spread in the indoor environment depends on the scenario and avenue that you’re in—whether you’re in a hotel room or a hospital situation can make a big difference,” Gerba said.

Aerosols from water sources, such as sinks and toilets, can also spread pathogens, and water systems/contaminated water are another route through which microbes traverse buildings. “When you're in the bathroom and you flush the toilet, you have a plume of aerosolization,” said Stephanie Boone, Ph.D., a research scientist in the Gerba Lab. “We've measured [the plume] go as [high] as 3 ft. from the toilet surface, and as far as 2.5 ft. outside the toilet surface. If you have, say [SARS-CoV-2] or influenza, or norovirus, those viruses [are included] in that plume.” These plume-associated pathogens contaminate environmental surfaces, which could pose a potential infection risk for days if not decontaminated. 

Gerba emphasized that all modes of transmission are interrelated. “It’s a fairly dynamic process, and I think one of the challenges that we have is [studying] the dynamics of this and how to characterize [them]. We have to have a better understanding of all these factors and how they interact within the environment.”  

Virus Resuspension: A Key Player in Transmission?

With that in mind, there is another, often overlooked, mode of transmission that bridges surface contamination and aerosol transmission: viral resuspension. Resuspension occurs when airborne particles are deposited onto a surface, then are swept back into the air by activities like walking or opening a door. Could the yo-yoing of virus from the air to surfaces, and back again, lead to infections?

Boone has been exploring this question. She uses a bacteriophage (a virus that only infects bacteria) as a proxy for how human-infecting viruses travel through indoor spaces. In recent experiments, Boone and her colleagues applied phage to carpet, wooden floors, curtains and other surfaces. They quantified the amount of phage that settled onto agar plates scattered throughout the space 1 hour after completing a disruptive activity (e.g., vacuuming). 

The scientists found that activities like vacuuming, walking and opening curtains led to movement of viruses far from the original site of contamination. For example, when someone walked in place 5 times on carpet in an unventilated room, phage was found over 7 feet away from the walking site, and nearly 6 feet above the ground (for wooden floors, the suspension was less dramatic). “We were stunned,” Boone said, noting that if the phage was viable respiratory virus, it would be suspended into breathing range of children and adults occupying the space, especially in the presence of dust.

Diagram depicting the ways viruses can be resuspended.
There are various events that can lead to virus resuspension, such as walking or vacuuming.
Source: Joseph J., et al./Exploration, 2022 via a CC BY 4.0 DEED license.

In fact, in all cases, dust played a key role in how far and high virus spread. This phenomenon has also been shown for viruses that infect people—“aerosolized fomites” (i.e., environmental dust) contributed to spread of influenza A in a guinea pig model. Boone highlighted that particulate matter also increases the expression of ACE2 (the SARS-CoV-2 receptor) in mouse lung tissues, which could promote susceptibility to infection. However, more research is needed to understand if, and how, dust influences infection dynamics. 

Whether virus resuspension broadly poses an infection risk to humans is also still unclear. One study suggested that particle resuspension from surfaces is an important source of SARS-CoV-2 RNA in the air of hospital rooms, though the scientists did not look at infectious virus. Another report found that lab-simulated resuspension of influenza A did send virus into the air, but concentrations were 2 orders of magnitude lower than those generated via simulated direct respiratory emission.

“We’ve demonstrated we can re-aerosolize virus from surfaces into the inhalation range—[but] is there really a risk? Is there enough virus being generated into the air to cause a risk? Would cleaning and disinfecting fomites reduce the risk of aerosol resuspension?” Gerba asked. “Those are questions we haven’t answered yet.” 

Building Solutions

Knowledge surrounding the movement of viruses throughout buildings is a key consideration in how indoor spaces are designed and managed to minimize pathogen transmission. Such solutions can start from the ground up, including by designing buildings to minimize close interactions between occupants and control the flow of people and traffic. Boone noted actions could be as simple as opting for hard floors instead of carpeting in homes to reduce the potential for resuspension and dust build-up.

Disinfection

Disinfecting surfaces can also minimize surface contamination and could reduce the risk of virus resuspension. Boone suggested paying attention to “high touch” areas like refrigerator handles, door handles and light switches that are often overlooked during routine cleaning. She also recommends avoiding microbe-ladened cleaning tools, like sponges, and, instead, opting for paper towels or items that can be washed regularly. Scientists are also developing self-disinfecting materials and/or those with virucidal coatings that can minimize contamination risk, while avoiding potentially negative environmental and health effects of chemical cleaners. 


Yet, even the most contaminated surfaces are minimally problematic if nobody interacts with them. Gerba highlighted the need for risk assessment studies to determine what, and where, infection risks are, and if there are ways to optimize energy and resources to disinfect in a targeted manner. At the beginning of the COVID-19 pandemic, “a lot of effort went into disinfection for SARS-CoV-2,” he said. “Were we overdoing it for SARS-CoV-2? Could we use a better allocation of resources? That’s why it’s important to understand virus transmission in the indoor environment.” 

Air Filtration and Ventilation

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A Corsi-Rosenthal Box.
Source: Festucarubra/Wikimedia Commons via a CC BY-SA 4.0 license.
When it comes to aerosol transmission, architecture that supports adequate air ventilation and avoids possibilities of air stagnation (like closed-end hallways) is ideal. Moreover, indoor air systems are critical for controlling the spread of airborne viruses. Choosing heating, ventilation and air conditioning (HVAC) systems that suit the planned use for a space (e.g., a health care facility vs. a school or home) and promote equipment and energy cost efficiency, while also effectively removing contaminants from the air, is integral for creating and maintaining buildings with microbes in mind.

Portable filters also remove infectious viruses, including SARS-CoV-2, from the air. People can even make their own out of 4 MERV-13 filters and a box fan (known as a Corsi-Rosenthal Box). The U.S. Environmental Protection Agency (EPA) conducted a study with phages to show that running one of these do-it-yourself filters for 60 minutes could reduce the presence of airborne virus by 99%. Emerging technologies for capturing aerosols, or those that detect and quickly alert room occupants to the presence of viruses in the air, may further inform actions to prevent indoor transmission.


Research in this article was presented at ASM Microbe, the annual meeting of the American Society for Microbiology, held June 15-19, 2023, in Houston. Ready to share your science at ASM Microbe 2024?


Author: Madeline Barron, Ph.D.

Madeline Barron, Ph.D.
Madeline Barron, Ph.D. is the Science Communications Specialist at ASM. She obtained her Ph.D. from the University of Michigan in the Department of Microbiology and Immunology.