Avoiding an Infectious Dose

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Charles Call, Ph.D.

Did you know that your office may be one of the riskiest places you go when you leave your home? Unless you visit busy bars and restaurants, and if you work in a large office building, your office presents the highest risk to you with respect to exposure to an infectious dose of SAR-CoV-2. Let’s explore why.

A super-spreader is a person who, perhaps without showing any symptoms of illness, is spreading a high number of virus particles in exhaled breath. There is an abundance of evidence that COVID-19 is spread as an aerosol (particles suspended in air). Some people naturally and regularly produce as much as 10 times more particles in their exhaled breath than others. Talking, and even just normal ordinary breathing, typically produce hundreds, and up to thousands, of wet aerosol particles per breath. PER BREATH! Further, people generate (and spread) many more particles when they take deep breaths. So think back to the Seattle super spreader event. Choral musicians practiced music for two hours back in February when the pandemic was just taking off in the US. It is believed that one member of the choral group was infected, having flu-like symptoms 3 days prior to the practice. But after two hours of deep breathing associated with choral singing, a super spreader event had occurred, and 52 people were infected!

What happens in a crowded bar? After several hours of talking with friends, telling stories, and enjoying a night out, the risk of exposure is quite high. Loud talking translates to many more deep breaths by everyone, and thus, high levels of fine aerosol from deep in the lungs is being exhaled with each breath. If just one person in the room is shedding a relatively high load of virus particles in their breath, a large number of people could receive an infectious dose. This type of super spreader event is less likely at the office, but the breakroom and in-person staff meetings have the potential for super-spreading.

So where do you spend your time? And how much of your time at work is spent around others for whom you have little information about how they spend evenings and weekends? In other words, how much time are you spending in proximity to potential super spreaders?

We can actually control the dose of the coronavirus we receive. Your inhaled dose is proportional to the concentration of the virus in the local environment (where you are), the time you spend in that environment, and how vigilant you and co-workers are about wearing masks. If you are wearing a mask, it will filter out some of those virus particles you would have otherwise inhaled. Let’s say the efficiency of a typical cloth mask is 50%. That will cut your dose in half. If you have a high-performance “N-95” type of mask that fits reasonably well, you might be able to cut your dose by a factor of 10 or 100.

On a typical day, I might work at the office for three hours and meet with a small number of staff, drive my car for a total of about one hour, stop at the grocery store for 15 minutes, and go for a walk outside for 30 minutes. The rest of the time I am at home. So where is my risk of exposure to an infectious dose the highest? Assuming no one in my home has COVID, the concentration in my home and my car is zero. Also- the outdoor concentration is effectively zero as well because there is a near-infinite volume of air to dilute what viruses may have been exhaled by another walker. The locations where the virus concentrations are unknown, and quite possibly NOT zero, are in the grocery store and at the office. In both environments, I am spending time inhaling the air that is, in part, exhaled from people that I know little about their vigilance. But like many of us, I’ve been spending way more time at the office than in grocery stores.

For many of us, the office is probably the riskiest place we go when we leave our home. Wear your best mask, avoid crowded and under-ventilated indoor spaces, and minimize your exposure time!   

 

The views expressed in this blog are those of the author alone.

 

References:

Bake, B., Larsson, P., Ljungkvist, G. et al. Exhaled particles and small airways. Respir Res 20, 8 (2019). https://doi.org/10.1186/s12931-019-0970-9

Geggel, How a superspreader at choir practice sickened 52 people with COVID-19, LiveScience, May 14, 2020 https://www.livescience.com/covid-19-superspreader-singing.html

Morawska, G.R. Johnson, Z.D. Ristovski, M. Hargreaves, K. Mengersen, S. Corbett, C.Y.H. Chao, Y. Li, D. Katoshevski, Size distribution and sites of origin of droplets expelled from the human respiratory tract during expiratory activities, Journal of Aerosol Science, Volume 40, Issue 3, 2009, Pages 256-269, ISSN 0021-8502. https://doi.org/10.1016/j.jaerosci.2008.11.002

Morawska, and D. Milton, Clinical Infectious Diseases, Volume 71, Issue 9, 1 November 2020, Pages 2311–2313, https://doi.org/10.1093/cid/ciaa939

Patterson, B., & Wood, R. (2019). Is cough really necessary for TB transmission?. Tuberculosis (Edinburgh, Scotland), 117, 31–35. https://doi.org/10.1016/j.tube.2019.05.003

 

 

 

 

2 Comments

  1. Thank you!!1

  2. a lot lately with my father so hopefully this will get him to see my point of view. Fingers crossed! mortgage banker new york


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