Quantifying the transmission risk of airborne viruses indoors and outdoors

Quantifying the transmission risk of airborne viruses indoors and outdoors
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In light of the COVID-19 pandemic, researchers have developed a tool that can help to quantify the dynamics of how aerosols travel from one person to another to help inform understanding of transmission risk.

The coronavirus is spread through respiratory droplets which fall to the ground after travelling a certain distance. However, mounting evidence points to the ability of smaller floating particles, called aerosols, to carry the virus over longer distances. A team at Colorado State University, led by engineering researcher Tami Bond, a professor in the Department of Mechanical Engineering and Walter Scott, Jr. Presidential Chair in Energy, Environment and Health, have now created a new tool for defining how infectious pathogens, including the coronavirus, transport in the air.

The research has been published in Environmental Science and Technology.

Airborne transmission of viruses

The team has developed a tool which is a metric they have called ‘Effective Rebreathed Volume’. This is the amount of exhaled air from one person that, by the time it travels to the next person, contains the same number of particles.

Treating virus-carrying particles like any other aerosol enabled the team to make physics-based comparisons between different modes of transmission, and account for how sizes of particles would affect the number of particles that travel from one person to another.

For the study, the team investigated three different sized categories of particles that cover a biologically relevant range: 1 micron, 10 microns, and 100 microns, each of which have different air-travel characteristics. The scientists highlight that, depending on the size of the particles, different infection control measures would apply to reduce risk, from opening a window to increasing fresh air delivery through an HVAC system.

Bond said: “It quickly became clear there was some airborne component of transmission. A virus is an aerosol. Health-wise, they are different than other aerosols like pollution, but physically, they are not. They float in the air, and their movement depends on their size.”

Indoors versus outdoors

To understand the risk, the team compiled a set of models to compare different scenarios both indoors and outdoors, and found that distancing indoors, even six feet apart, is not enough to limit potentially harmful exposures. This is because confinement indoors allows particle volumes to build up in the air. Bond highlights that the paper shows the effect of this confinement indoors, and that particle transport can be quantified and compared to other risks that people find acceptable.

Researchers Jeff Pierce in atmospheric science, and Jay Ham in soil and crop sciences, both co-authors of the paper, helped the team understand atmospheric turbulence in ways that could be compared in indoor and outdoor environments. They found that outdoor interactions at distances greater than six-foot were safer than indoor interactions of the same time length and of the same distancing, as particles filled the room rather than being carried away by wind.

Pierce said: “We started fairly early on in the pandemic, and we were all filled with questions about: ‘Which situations are safer than others?’ Our pooled expertise allowed us to find answers to this question, and I learned a lot about air filtration and air exchange in my home and in my CSU classroom.”

Angela Bosco-Lauth, paper co-author and assistant professor in biomedical sciences, added: “The problem we face is that we still don’t know what the infectious dose is for people. Certainly, the more virus present, the higher the risk of infection, but we don’t have a good model to determine the dose for people. And quantifying infectious virus in the air is tremendously difficult.”

The team is now pursuing follow-up investigations, such as comparing different mitigation measures for reducing exposures to viruses indoors, and hope that the work can lay a foundation for more up-front quantification of transmission dynamics in the event of another pandemic.


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