The dynamics of how aerosols travel from one person to another, under different circumstances
Colorado State University
The issue of whether our real-life pandemic virus, SARS-CoV-2, is "airborne" is predictably more complex.
The current body of evidence suggests that COVID-19 primarily spreads through respiratory droplets -- the small, liquid particles you sneeze or cough, that travel some distance, and fall to the floor.
But
consensus is mounting that, under the right circumstances, smaller floating
particles called aerosols can carry the virus over longer distances and remain
suspended in air for longer periods. Scientists are still determining
SARS-CoV-2's favorite way to travel.
That the science was lacking on how COVID-19 spreads seemed apparent a year ago to Tami Bond, professor in the Department of Mechanical Engineering and Walter Scott, Jr. Presidential Chair in Energy, Environment and Health.
As an engineering
researcher, Bond spends time thinking about the movement and dispersion of
aerosols, a blanket term for particles light and small enough to float through
air - whether cigarette smoke, sea spray, or hair spray.
"It quickly
became clear there was some airborne component of transmission," Bond
said. "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."
The rush for scientific understanding of the novel coronavirus has focused -- understandably -- on biological mechanisms: how people get infected, the response of the human body, and the fastest path to a vaccine.
As an aerosol scientist, Bond went a different route, convening a team at Colorado State University that would treat the virus like any other aerosol. This team, now published in Environmental Science and Technology, set out to quantify the dynamics of how aerosols like viruses travel from one person to another, under different circumstances.
The
cross-section of expertise to answer this question existed in droves at CSU,
Bond found. The team she assembled includes epidemiologists, aerosol
scientists, and atmospheric chemists, and together they created a new tool for
defining how infectious pathogens, including SARS-CoV-2, transport in the air.
Effectively rebreathed air
Their tool is a metric they're calling Effective Rebreathed Volume, or simply, 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
agnostically like any other aerosol allowed the team to make objective,
physics-based comparisons between different modes of transmission, accounting
for how sizes of particles would affect the number of particles that traveled
from one person to another.
They looked at three size categories of particles that cover a biologically relevant range: 1 micron, 10 microns, and 100 microns -- about the width of a human hair. Larger droplets expelled by sneezing would be closer to the 100-micron region.
Particles closer to the size of a single virion would be in the 1-micron
region. Each have very different air-travel characteristics, and depending on
the size of the particles, different mitigation measures would apply, from
opening a window, to increasing fresh air delivery with through an HVAC system.
They compiled a
set of models to compare different scenarios. For example, the team compared
the effective rebreathed volume of someone standing outdoors 6 feet away, to
how long it would take someone to rebreathe the same amount of air indoors but
standing farther away.
Confinement matters
The team found that distancing indoors, even 6 feet apart, isn't enough to limit potentially harmful exposures, because confinement indoors allows particle volumes to build up in the air. Such insights aren't revelatory, in that most people avoid confinement in indoor spaces and generally feel safer outdoors.
What the paper
shows, though, is that the effect of confinement indoors and subsequent
particle transport can be quantified, and it can be compared to other risks
that people find acceptable, Bond said.
Co-authors Jeff
Pierce in atmospheric science and Jay Ham in soil and crop sciences helped the
team understand atmospheric turbulence in ways that could be compared in indoor
and outdoor environments.
Pierce said he sought to constrain how the virus-containing particles disperse as a function of distance from the emitting person. When the pandemic hit last year, the public had many questions about whether it was safe to run or bike on trails, Pierce said.
The researchers found that longer-duration interactions outdoors at
greater than 6-foot distances appeared safer than similar-duration indoor
interactions, even if people were further apart indoors, due to particles
filling the room rather than being carried away by wind.
"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," Pierce said.
More to learn
What remains
unclear is which size particles are most likely to cause COVID-19 infection.
Viruses can be
carried on droplets large and small, but there is likely a "sweet
spot" between droplet size; ability to disperse and remain airborne; and
desiccation time, all of which factor into infective potential, explained
Angela Bosco-Lauth, paper co-author and assistant professor in biomedical
sciences.
The paper
includes an analysis of the relative infection risk of different indoor and
outdoor scenarios and mitigation measures, depending on the numbers of
particles being inhaled.
"The
problem we face is that we still don't know what the infectious dose is for
people," Bosco-Lauth said. "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."
Follow-up pursuits
The team is now pursuing follow-up questions, like comparing different mitigation measures for reducing exposures to viruses indoors. Some of these inquiries fall into the category of "stuff you already know, but with numbers," Bond said.
"People are now thinking, OK, more ventilation is better, or remaining
outside is better, but there is not a lot of quantification and numbers behind
those recommendations," Bond said.
Bond hopes the
team's work can lay a foundation for more up-front quantification of
transmission dynamics in the unfortunate event of another pandemic. "This
time, there was a lot of guessing at the beginning, because the science of
transmission wasn't fully developed," she said. "There shouldn't be a
next time."