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[20210529_std001]

Covid transmission
Improving ventilation will help curb SARS-CoV-2

There has been a misunderstanding about how the virus spreads

Science & technologyMay 29th 2021 edition
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May 26th 2021

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ON JANUARY 24TH 2020 three families, together numbering 21 people,
came independently to eat lunch at a restaurant in Guangzhou. It was
the eve of the Chinese New Year. Extra seating had been squeezed in
to accommodate more patrons than usual, and these families were
crowded onto neighbouring tables along one wall of the windowless
room (see plan). The largest of them--a party of ten who had arrived
the day before from Wuhan--sat around the middle table. Later that
day, one of their number developed fever and a cough and, at a
hospital, was diagnosed with covid-19. Within two weeks, ten of the
21 were confirmed as being infected with SARS-CoV-2.

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The families involved had never met and video footage showed they had
no close contact during the lunch. An initial analysis by the
Guangzhou Centre for Disease Control and Prevention proposed that the
infection had spread via respiratory "droplets". But medical lore has
it that such droplets--defined as particles expelled while breathing
that are more than five microns across--cannot travel more than a
couple of metres after they have been exhaled. And some of those who
became infected during the lunch were farther than that from the
"index" patient.

[20210529_s]

It made no sense. How could a single infected person transmit the
virus to nine others in just an hour when there had been no direct
contact between them?

Current thinking

The outbreak at the Guangzhou restaurant was the first recorded
"superspreading" event of the pandemic. Superspreading is loosely
defined as being when a single person infects many others in a short
space of time. More than 2,000 cases of it have now been recorded--in
places as varied as slaughterhouses, megachurches, fitness centres
and nightclubs--and many scientists argue that it is the main means by
which covid-19 is transmitted.

In cracking the puzzle of superspreading, researchers have had to
re-evaluate their understanding of SARS-CoV-2's transmission. Most
documented superspreadings have happened indoors and involved large
groups gathered in poorly ventilated spaces. That points to SARS-CoV
-2 being a virus which travels easily through the air, in
contradistinction to the early belief that short-range encounters and
infected surfaces were the main risks. This, in turn, suggests that
paying attention to the need for good ventilation will be important
in managing the next phase of the pandemic, as people return to
mixing with each other inside homes, offices, gyms, restaurants and
other enclosed spaces.

It has taken a long time for public-health experts to acknowledge
that covid-19 routinely spreads through the air in this way. Social
distancing and mask-wearing were recommended with the intention of
cutting direct, close-range transmission by virus-carrying droplets
of mucus or saliva breathed out by infected individuals. The main
risk of spreading the illness indirectly was thought to come not from
these droplets being carried long distances by air currents, but
rather by their landing on nearby surfaces, on which viruses they
were harbouring might survive for hours, or even days. Anyone who
touched such an infected surface could then transfer those viruses,
via their fingers, to their mouth, eyes or nose. This makes sense if 
SARS-CoV-2 spreads in the same way as influenza--which was indeed the
hypothesis in March 2020, when the World Health Organisation ( WHO)
declared the start of the covid-19 pandemic. Hence the advice to
disinfect surfaces and wash hands frequently.

Doctors did know at the time that not all respiratory particles fall
fast. Those smaller than five microns can become aerosols, staying
aloft for hours and potentially travelling much farther than
droplets, or simply accumulating in the air within a closed room.
Anyone inhaling these aerosols could then become infected. But this
was assumed not to matter, because aerosols were thought to be
relevant only in specialist medical settings, such as when patients
are attached to a ventilator in an intensive-care unit. Intubation,
as this process is known, does indeed create aerosols, as the
breathing tube is forced down a patient's trachea. But a wider risk
was not perceived. The WHO therefore played down the risks of
aerosols, issuing guidance via its Twitter and Facebook pages at the
end of March 2020 that the general public need not worry. "FACT: #
COVID19 is NOT airborne," it said, adding that any claims to the
contrary were "misinformation".

Physics envy

Researchers outside the medical world, however--especially those who
study the physics of particles in the air--felt the evidence pointed
in a different direction. The Guangzhou restaurant outbreak was an
early warning. Around the same time, 1,300km across the country in
Ningbo, 23 of 68 passengers on a bus fitted with an air-recycling
system had been infected during a one-and-a-half-hour journey. But
the worst known case of superspreading early in the pandemic was
American. This happened at a choir practice in Skagit Valley,
Washington State, in March 2020. Of the 61 people present during a
two-and-a-half-hour meeting, 53 became infected. In all these cases,
investigation showed that those infected were not necessarily the
people closest to the index patients, as might be expected if
transmission had been by droplet or surface contact.

None of this surprised Lidia Morawska, a physicist at the Queensland
University of Technology, in Brisbane, Australia. She had spent much
of her career studying how pollution caused by so-called particulate
matter, such as dust and smog, affects air quality. After the
original SARS outbreak, which happened in 2003, she began experiments
to show how respiratory particles are generated in people's throats
and then transported through the air.

[20210529_s]

She demonstrated that received medical wisdom is wrong. Because
exhaled breath is a moist, hot, turbulent cloud of air, a
five-micron-wide droplet released at a height of one and a half
metres (about the distance above ground of the average mouth or nose)
can easily be carried dozens of metres before settling. Also, the
generation of respiratory particles is not restricted to medical
settings. Liquid drops of all sizes--including those defined as
aerosols--are continuously shed while people are breathing, talking,
sneezing or singing (see chart).

In July 2020 Dr Morawska wanted to bring this work to the attention
of public-health agencies. She assembled a group of 36 experts on
aerosols and air quality to write an open letter outlining their
evidence for infection by smaller liquid drops and calling on the WHO
to change its tune on airborne transmission. "We appeal to the
medical community and to the relevant national and international
bodies to recognise the potential for airborne spread of coronavirus
disease 2019 (covid-19)," they wrote in Clinical Infectious Diseases.
"There is significant potential for inhalation exposure to viruses in
microscopic respiratory droplets (microdroplets) at short to medium
distances (up to several metres, or room scale), and we are
advocating for the use of preventive measures to mitigate this route
of airborne transmission." More than 200 other researchers from 32
countries also signed the letter.

One signatory was Jose-Luis Jimenez, an atmospheric chemist at the
University of Colorado, Boulder. He says that the confusion in health
circles over whether or not airborne transmission of SARS-CoV-2 is
important can be traced back to medical textbooks that still contain
outdated descriptions of how respiratory particles are produced and
move.

But the widespread assertion, still stubbornly promulgated by the WHO
, that droplets above five microns in diameter do not stay airborne,
but rather settle close to their source, is a dodgy foundation on
which to build public-health advice. According to Dr Jimenez,
physicists have shown that any particle less than 100 microns across
can become airborne in the right circumstances. All of this matters
because hand-washing and social distancing, though they remain
important, are not enough to stop an airborne virus spreading,
especially indoors. Masks will help, by slowing down and partially
filtering an infectious person's exhalations. But to keep offices,
schools, hospitals, care homes and so on safe also requires
improvements in their ventilation.

Fan-tastic

Under pressure from physicists, the WHO recently acknowledged that
better ventilation should be used to help prevent covid-19's
spread--and in March it published a "roadmap" to that effect. But the
document fell far short of properly recognising the hazard of
airborne transmission and, therefore, the need to control it. Despite
overwhelming evidence that it happens, the agency still maintained
that SARS-CoV-2 "mainly spreads between people when an infected
person is in close contact with another person".

Others, though, are acting on the new knowledge. Martin Bazant, a
chemical engineer, and John Bush, a mathematician, both at the
Massachusetts Institute of Technology, have devised a way to
calculate how long it would be safe to stay within a room that
contains an infected person. The pair described their model in a
paper in a recent issue of the Proceedings of the National Academy of
Sciences.

Applied to a typical American school class of 19 pupils and a
teacher, the safe time after an infected individual enters a
classroom that is naturally ventilated (that is, how long before the
risk of infection is unacceptably high) is 72 minutes. This period
can, though, be extended in two ways. One is by mechanical
ventilation of the room, which increases the safe time to 7.2 hours.
The other is by everyone wearing masks. In the absence of mechanical
ventilation, mask-wearing increases the safe time to eight hours. But
the real benefit comes from combining these approaches. That pushes
the safe time up to 80 hours--almost 14 days if a school day is six
hours long. Add in intervening weekends and a class wearing masks in
a school room with adequate ventilation would thereby be safe for
longer than the time it takes to recover from covid-19, which is
typically between one and two weeks. School transmissions would thus
be rare.

A caveat is that the modelling assumed a classroom with minimal
talking, physical activity or singing by the pupils. But games
lessons would usually be outdoors and singing lessons could be. As to
too much talking, teachers might welcome an unimpeachable reason to
tell pupils to keep quiet in class.

Infection risk will not always be distributed evenly around a room.
Jiarong Hong, a mechanical engineer at the University of Minnesota,
Minneapolis, therefore used computer models to study how aerosols
would spread in a classroom, according to the location of an infected
individual and the position of nearby fans or air filters. Assuming
the teacher was infected, and so was releasing virus-laden aerosols
at the front of the class, Dr Hong's modelling shows that placing an
air cleaner or extractor fan at the front of the room sets up an
airflow which prevents the movement of such aerosols towards the
pupils. An even better aerosol-cleansing effect is achieved when the
fans and filters are elevated above the people in the room. This
takes advantage of the rising air plumes created by body heat, which
mean that exhaled aerosols tend to float upwards. Dr Hong's modelling
shows that even small, cheap box fans mounted in this way would do a
good job of keeping classrooms safe and preventing aerosols from
building up to dangerous levels.

Dr Hong has also modelled the air flow in the Guangzhou restaurant
outbreak of January 2020. As the plan shows, he found that the
movements of virus-laden aerosols around the three affected families
of diners matched the seating positions of the people who eventually
became sick. The outbreak occurred because there was no source of
external fresh air and a nearby recirculating air conditioner
redistributed aerosols from the infected person to the other tables,
creating a contaminated bubble of air that was increasingly burdened
with viruses over the course of the lunch.

The risk, then, is real. But how can the occupants of a room know
whether it is well-ventilated? Just because a room feels spacious and
an air conditioner is operating does not mean the air inside it is
clean.

Here, Dr Morawska has a suggestion. In a (non-scientific) experiment
last year, she took a carbon-dioxide meter into a large,
high-ceilinged, air-conditioned restaurant near her home. CO[2]
concentrations can be a useful proxy for clean air. Outdoor air
contains around 400 parts per million (ppm) of the gas, and people's
exhaled breath contains around 40,000ppm. Exhaling into a room
therefore gradually raises its CO[2] concentration unless the
ventilation is good enough to remove the excess.

According to experts on air quality, anything below 500ppm in a room
means the ventilation is good. At 800ppm, 1% of the air someone is
breathing has already been exhaled recently by someone else. At
4,400ppm, this rises to 10%, and would be classed as dangerous. These
sorts of levels are seen only in crowded spaces with poor airflow. To
keep the risk of covid-19 low, CO[2] levels should be well below
700ppm.

When Dr Morawska conducted her experiment, the restaurant had ten
people in it--far fewer than would normally be allowed--and the CO[2]
concentration was already 1,000ppm when she arrived. Within an hour
it had jumped to 2,000ppm. "We continued sitting during the dinner
for another hour or so," she says. "So if there was someone infected
there, well this could have been a problem."

[20210529_s]

Though anecdotal, that tale indicates a serious risk--and one which
resonates beyond covid-19. All sorts of symptoms, from headaches,
fatigue and shortness of breath to skin-irritation, dizziness and
nausea, are linked to poor ventilation. It has also been connected
with more absences from work and lower productivity.

The ventilation measures needed to deal with all this are not
difficult, but existing regulations and design standards often have
different objectives--particularly, these days, conserving heat and
thus reducing energy consumption. That often means recirculating air,
rather than exchanging it with fresh air from the outside world. (An
exception is passenger aircraft, which refresh cabin air frequently.)

In situations where it is not possible to reduce health risks by
ventilation alone--for example, places like nightclubs, where there
are lots of people crowded together, or gyms, where they are
breathing heavily--air filtration could easily be incorporated into
ventilation systems. Air could also be disinfected, using germicidal
ultraviolet lamps placed within air-conditioning systems or near
ceilings in rooms.

All change

And then there is public awareness. "Before this pandemic it was
completely socially acceptable to come to the office coughing,
sneezing, spreading viruses around," says Dr Morawska. "No one would
say anything--even people educated to understand how infections are
transmitted."

That insouciance must be corrected, she says. The WHO must
acknowledge the need to control airborne pathogens and governments
must agree and enforce comprehensive standards for indoor air quality
that keeps people healthy. One way to ensure compliance might be to
issue ventilation certificates for buildings, similar to the
food-hygiene certificates which already exist for restaurants.
Occupants should also be given information about air quality
routinely, she adds, through the use of monitors and sensors that can
display a room's carbon-dioxide levels or other relevant measures.

For new buildings this should not cost much extra, though replacing
exiting ventilation systems might be costly. But not as costly as
covid-19 has been. And if improvements in indoor air quality also
reduced absenteeism and improved productivity, those gains might
cover that cost. "Although detailed economic analyses remain to be
done," wrote Dr Morawska in a recent edition of Science, "the
existing evidence suggests that controlling airborne infections can
cost society less than it would to bear them." #

Dig deeper

All our stories relating to the pandemic and the vaccines can be
found on our coronavirus hub. You can also listen to The Jab, our
podcast on the race between injections and infections, and find
trackers showing the global roll-out of vaccines, excess deaths by
country and the virus's spread across Europe and America.

A version of this article was published online on May 26th, 2021

This article appeared in the Science & technology section of the
print edition under the headline "Surface to air"

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