Many ventilation systems may increase risk of COVID-19 exposure, study suggests

Date:
September 29, 2020
Source:
University of Cambridge
Summary:
Ventilation systems in many modern office buildings, which are
designed to keep temperatures comfortable and increase energy
efficiency, may increase the risk of exposure to the coronavirus,
particularly during the coming winter, according to new research.



FULL STORY ========================================================================== Ventilation systems in many modern office buildings, which are designed to
keep temperatures comfortable and increase energy efficiency, may increase
the risk of exposure to the coronavirus, particularly during the coming
winter, according to research published in the Journal of Fluid Mechanics.


==========================================================================
A team from the University of Cambridge found that widely-used 'mixing ventilation' systems, which are designed to keep conditions uniform in
all parts of the room, disperse airborne contaminants evenly throughout
the space.

These contaminants may include droplets and aerosols, potentially
containing viruses.

The research has highlighted the importance of good ventilation and mask- wearing in keeping the contaminant concentration to a minimum level and
hence mitigating the risk of transmission of SARS-CoV-2, the virus that
causes COVID- 19.

The evidence increasingly indicates that the virus is spread primarily
through larger droplets and smaller aerosols, which are expelled
when we cough, sneeze, laugh, talk or breathe. In addition, the data
available so far indicate that indoor transmission is far more common
than outdoor transmission, which is likely due to increased exposure
times and decreased dispersion rates for droplets and aerosols.

"As winter approaches in the northern hemisphere and people start
spending more time inside, understanding the role of ventilation is
critical to estimating the risk of contracting the virus and helping slow
its spread," said Professor Paul Linden from Cambridge's Department of
Applied Mathematics and Theoretical Physics (DAMTP), who led the research.

"While direct monitoring of droplets and aerosols in indoor spaces is difficult, we exhale carbon dioxide that can easily be measured and used
as an indicator of the risk of infection. Small respiratory aerosols
containing the virus are transported along with the carbon dioxide
produced by breathing, and are carried around a room by ventilation
flows. Insufficient ventilation can lead to high carbon dioxide
concentration, which in turn could increase the risk of exposure to the
virus." The team showed that airflow in rooms is complex and depends
on the placement of vents, windows and doors, and on convective flows
generated by heat emitted by people and equipment in a building. Other variables, such as people moving or talking, doors opening or closing, or changes in outdoor conditions for naturally ventilated buildings, affect
these flows and consequently influence the risk of exposure to the virus.



========================================================================== Ventilation, whether driven by wind or heat generated within the
building or by mechanical systems, works in one of two main modes. Mixing ventilation is the most common, where vents are placed to keep the air
in a space well mixed so that temperature and contaminant concentrations
are kept uniform throughout the space.

The second mode, displacement ventilation, has vents placed at the
bottom and the top of a room, creating a cooler lower zone and a warmer
upper zone, and warm air is extracted through the top part of the
room. As our exhaled breath is also warm, most of it accumulates in the
upper zone. Provided the interface between the zones is high enough, contaminated air can be extracted by the ventilation system rather
than breathed in by someone else. The study suggests that when designed properly, displacement ventilation could reduce the risk of mixing and cross-contamination of breath, thereby mitigating the risk of exposure.

As climate change has accelerated since the middle of the last century, buildings have been built with energy efficiency in mind. Along with
improved construction standards, this has led to buildings that are more airtight and more comfortable for the occupants. In the past few years
however, reducing indoor air pollution levels has become the primary
concern for designers of ventilation systems.

"These two concerns are related, but different, and there is tension
between them, which has been highlighted during the pandemic," said Dr
Rajesh Bhagat, also from DAMTP. "Maximising ventilation, while at the
same time keeping temperatures at a comfortable level without excessive
energy consumption is a difficult balance to strike." In light of this,
the Cambridge researchers took some of their earlier work on ventilation
for efficiency and reinterpreted it for air quality, in order to determine
the effects of ventilation on the distribution of airborne contaminants
in a space.



==========================================================================
"In order to model how the coronavirus or similar viruses spread indoors,
you need to know where people's breath goes when they exhale, and
how that changes depending on ventilation," said Linden. "Using these
data, we can estimate the risk of catching the virus while indoors."
The researchers explored a range of different modes of exhalation: nasal breathing, speaking and laughing, each both with and without a mask. By
imaging the heat associated with the exhaled breath, they could see how
it moves through the space in each case. If the person was moving around
the room, the distribution of exhaled breath was markedly different as
it became captured in their wake.

"You can see the change in temperature and density when someone
breathes out warm air -- it refracts the light and you can measure
it," said Bhagat. "When sitting still, humans give off heat, and since
hot air rises, when you exhale, the breath rises and accumulates near
the ceiling." Their results show that room flows are turbulent and
can change dramatically depending on the movement of the occupants, the
type of ventilation, the opening and closing of doors and, for naturally ventilated spaces, changes in outdoor conditions.

The researchers found that masks are effective at reducing the spread
of exhaled breath, and therefore droplets.

"One thing we could clearly see is that one of the ways that masks work is
by stopping the breath's momentum," said Linden. "While pretty much all
masks will have a certain amount of leakage through the top and sides,
it doesn't matter that much, because slowing the momentum of any exhaled contaminants reduces the chance of any direct exchange of aerosols and
droplets as the breath remains in the body's thermal plume and is carried upwards towards the ceiling.

Additionally, masks stop larger droplets, and a three-layered mask
decreases the amount of those contaminants that are recirculated through
the room by ventilation." The researchers found that laughing, in
particular, creates a large disturbance, suggesting that if an infected
person without a mask was laughing indoors, it would greatly increase
the risk of transmission.

"Keep windows open and wear a mask appears to be the best advice,"
said Linden.

"Clearly that's less of a problem in the summer months, but it's a
cause for concern in the winter months." The team are now working with
the Department for Transport looking at the impacts of ventilation on
aerosol transport in trains and with the Department for Education to
assess risks in schools this coming winter.


========================================================================== Story Source: Materials provided by University_of_Cambridge. The original
story is licensed under a Creative_Commons_License. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Rajesh K. Bhagat, M. S. Davies Wykes, Stuart B. Dalziel,
P. F. Linden.

Effects of ventilation on the indoor spread of COVID-19. Journal
of Fluid Mechanics, 2020; 903 DOI: 10.1017/jfm.2020.720 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200929130301.htm

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