Vincent J. Curtis
28 Apr 21
With Ontario, now Nova Scotia, and
prospectively Manitoba locking down, staying-at-home, and otherwise keeping
people indoors, it make me wonder if the so-called Public Health professionals
even read the latest scientific literature.
A recent paper comes from M.I.T., authored
by two real applied mathematicians who are experts at mathematically
modelling. These guys weren’t doctors
playing mathematician, they are the real deal. (They are Martin Z. Bazant and
John W.M. Bush, authors of “A guideline to limit indoor airborne transmission
of COVID-19” PNAS April 27, 2021
118(17) e2018995118; https://doi.org/10.1073/pnas.2018995118)
Their model of transmission relies on an
enclosed volume, otherwise known as “a room,” in which someone is putting virus
into the room at a rate r. In the room
with the virus generator are other people of susceptibility s, and are
breathing at rate b. They assume that
the virus is almost immediately dispersed evenly throughout the room. The question they ask is: how long does it
take for the uninfected people to breathe in enough virus for them to become
infected, given their particular susceptibility?
What is immediately obvious is that people
outdoors can never breathe in enough virus to become infected because the virus
gets dispersed, and the only pathway of transmission outdoors is for an
infected person to make a big sneeze directly into the face of the uninfected
person. Hence, there need be no limits
on outdoor gatherings, activities, or a requirement for masking.
Other things become rapidly obvious. One is that it takes time to get exposed
enough to get infected. If the room is
large, like in a church or a big box store, that infected person is going to
have to breathe a lot to fill the room with virus. Ventilation air- exchange with the outdoors -
and air filtration tend to defeat the build-up of the virus within the
room. If the uninfected aren’t breathing
heavily, the time required to inhale enough virus to overcome their
susceptibility is longer than if they’re breathing deeply and heavily. Social distancing doesn’t matter, except to
avoid being sneezed directly upon, because the virus disperses rapidly
throughout the room, and so one’s exposure to virus is the same whether one is
six feet or sixty feet away from the virus generator. Lastly, it doesn’t matter how many people are
in the room with the generator, just so long as none of them are in the room
too long.
The authors assigned a filtration factor
for masks, and found that wearing good quality masks may have a dramatic effect
in reducing transmission indoors
When you start putting realistic values to
the many parameters, quantitative guidelines can be, and were, developed that
minimize the inconvenience of lockdown restrictions. The guidelines are
published in chart form, and there is an on-line app for the convenience of
those not inclined to make detailed calculations.
The authors considered two case studies:
the classroom and the elder care facility.
With a typical American classroom designed for an occupancy of 19
students and their teacher, a modest risk tolerance, with and without masks,
the authors found a safe time of 1.2 hours after an infected person entered the
room with natural ventilation, and 7.2 hours with mechanical ventilation. With moderately effective cloth masks these
times increased to 8 and 80 hours respectively.
Hence, if daily classroom time is 6 hours, a class wearing masks with
adequate room ventilation would be safe for longer than the recovery time for
COVID-19 (7 to 14 days.). School
transmissions would be rare. The authors
assume a quiet classroom with only speaking going on and no vigorous activity.
Using standard elder care guidelines of New
York State, and accounting for the great susceptibility of older people to
COVID, the authors found a great difference between natural and mechanical
ventilation. With natural ventilation,
an uninfected elder person could get infected within 3 to 17 minutes after an
infected person entered their standard room, but it took at least 18 minutes
with mechanical ventilation. In short,
mask wearing and once-through mechanical ventilation is indicated in standard elder
care facilities.
The authors claim that standard surgical
masks can extend the critical exposure time by a factor of 400 to 10,000; while
hybrid fabric cloth face masks can extend it by 6 to 100 times. Even single layer cloth masks can extend
critical exposure time by 1.5 to 6 times.
The authors deprecated the six foot rule, arguing that a plume of
infected breath extends far beyond six feet.
Masking, on the other hand, blocks large infected droplets and redirects
the breath plume sideways and upwards.
Hence, masking indoors is preferable to a six foot distance. That said, masking obviates the need to
reduce occupancy in large rooms with decent ventilation where a person won’t
spend multiple hours.
The M.I.T. study is the opposite of fear-mongering. It offers a serious, scientific basis for its
recommendation, and some of these are at variance with off-handed repetitions
of previously failed lockdown measures.
In particular, prolonged forced stays in apartment buildings and hotels
with certain ventilation characteristics can actually increase transmission, as
happened in Wuhan, China.
Likewise, there was no need for the brutal
shutdown of Adamson’s BBQ in Toronto last December, given the ventilation of
the serving area and the short time customers spent indoors.
This study ought to inform Canada’s public
health officials on lockdowns, or at least their political masters.
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