The locations where mass infections were confirmed so far are
places where the following three conditions were met simultaneously: (1)
closed space with poor ventilation, (2) crowded with many people and (3)
conversations and vocalization in close proximity (within arm’s reach of
one another). It is believed that more people were infected in such
places. Therefore, we ask that you predict locations and settings where
these three conditions could occur simultaneously and avoid them.
We do not have enough scientific evidence yet on how significantly
such actions can reduce the risk of spreading infection. However, since
places with poor ventilation and crowded places are increasing
infections, we ask that you take precautions even before scientific
evidence for clear standards is found.” (page 2)
This narrative assumes the possibility of airborne transmission
and asks citizens to share the uncertainty and act in a precautionary
way.
The droplet-but-not-airborne narrative emphasised randomised control
trial evidence (see appendix on bmj.com);22 it drew
implicitly on the hierarchy of evidence —a formalisation of the
assumed superiority of randomised trials,23 which
“… typically serve[s] the needs and realities of clinical
medicine, but not necessarily public policy” (page
665).24 It did not acknowledge the hierarchy of
controls —a more public health-oriented framework which favours
system-level interventions aimed at pathogen elimination, followed by
environmental controls aimed at making air and water safe, and both of
these over interventions to influence human
behaviour.5 Since randomised trials are not considered
appropriate in the science of building design and
ventilation,25 this mindset led policymakers to reject
a wealth of wider evidence which we discuss in the next section.
“Covid is unequivocally airborne”
Aerosol scientists study how fluids and particles travel in the air.
Some had specialised in how respiratory pathogens—including
tuberculosis, influenza and other coronaviruses such as SARS and
MERS—travel. They had shown, using laboratory studies, real-world case
studies and computer modelling, that these pathogens are transmitted by
aerosols and require airborne mitigation measures (Table 1, column
b),26 27 and that coughs and sneezes generate
turbulent gas clouds different-sized particles which can travel long
distances.28 29
From early 2020, evidence accumulated from a range of study designs to
support the hypothesis that, like most other respiratory pathogens—and
perhaps more so than other coronaviruses—SARS-CoV-2 is transmitted
through the air (Box 2).18 30-32
Box 2: 10 streams of evidence in support of airborne
transmission of
SARS-CoV-2