Some simple ventilation and filtering measures, combined with strategies such as mask wearing, will reduce the risk of Covid-19 transmission in confined spaces, a scientist says.
Joe Allen is an assistant professor of exposure assessment science and director of the Healthy Buildings program at Harvard University
He's interested in the way our offices, homes and other enclosed spaces could be contributing to the spread of Covid-19.
An early advocate of facemasks to limit Covid's spread, he's a critic of needless 'hygiene theatre" at the expense of tackling what he sees as the single most important issue: airborne transmission.
“There is no one strategy sufficient to control Covid, Allen says, rather a “layered defence approach” is needed.
“There is no doubt that masking helps and works, when there’s universal masking the virus has to go through two filters, it has to go through my mask and your mask plus it has to travel through space and time.”
But there is an additional control measure which compared to masking, distancing and hand washing has not got the attention it deserves, he says.
Airborne transmission of the virus has been underestimated, Allen says.
“It goes back decades, the medical community and this is codified in text books, there is an assumption that 5 micron particles settle out of the air within 2 metres
When you talk or breathe or sing you release a continuum of different particle sizes and most are in a 1 to 10 micron range so the medical community for a long tome had thought most of these particles settle out of the air quickly, thus the six foot or 2 metre rule.”
This is untrue, he says.
Aerosol physics science tells you that a 5 micron particle, travels beyond six feet, in fact it will travel across the room and it stays aloft for 30 minutes at least.
“And smaller particles 1-micron particles these can stay aloft indefinitely for hours in fact they’ll stay aloft until one of those two removal mechanisms occurs.”
Those two measures are ventilation and filtering, he says.
A super-spreading event in Guangzhou, China illustrates this.
“If you look at all the major outbreaks globally, the clustering events, the super-spreader events, in restaurants in schools in camps on a bus … where ever they are, it’s less about the location and more about the physical characteristics and what was happening or wasn’t happening in that space.
“So, it’s always the case that people are not masked and they have low or no ventilation.”
In Guangzhou one infected person infested many other diners at a restaurant.
“They had an air conditioning system, but it was only recirculating air, no fresh outdoor air, so that means no dilution and no filtration.
“So, what you had was a steady build-up of viral air particles from this one infected person who is contagious and it was moving the air but just moving it around and around in the same room and no replacing that air with clean air.”
Filtration and ventilation needn’t be expensive, he says.
“These don’t have to cost a lot of money, it can be a simple as opening up the windows and working on that air flow.
“For buildings that have a central mechanical system, it just means running your fans a bit harder letting in more outdoor air and increasing or improving that type of filter you are using.”
Mask wearing remains a simple yet effective way to reduce airborne transmission, he says.
“Let’s say I have a 60 percent efficient mask, you have a 60 percent efficient mask and let’s say 100 particles come out of my mouth
“If it goes through my mask at 60 percent there are only 40 of those particles left it goes through your 60 percent efficient mask you get a combined effect of 84 percent … that’s pretty good just for masking.”
Engineering controls can then take care of what remains, he says.