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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