Given the unusually high number of swimmers catching Covid in the Olympics, many have hypothesized as to why. I’ve seen a few people point to some work we published on how environmental factors affect SARS-CoV-2’s aerostability.
When respiratory aerosol is exhaled, the dissolved CO2 in the fluid (in the form of HCO3) leaves the aerosol over the course of a couple of minutes. When the CO2 leaves, the pH of the aerosol reaches >10.3.
The high pH drives viral decay.
We have reported that anything that can limit this increase in aerosol pH, such as nitric acid or CO2, slows the airborne viral decay rate. This, in turn, will increase the risk of transmission.
The net effect is more impactful over longer time periods. Elevating the CO2 from 500 to 3000 ppm leads to a 10-fold increase in the airborne viral load over 40 minutes. Likewise, increasing nitric acid from 0 to ~50 ppb leads to 2-fold increase.
The decay rate of the virus in the aerosol slows over time (left). The reason for this is that the trace acidic vapor in the air (normal air pollution) will slowly neutralize the aerosol. As this happens, the aerosol becomes more and more hospitable for the virus (right).
Okay, so what does this have to do with a swimming pool? Well, the swimming pools are disinfected with chlorine. Chlorine vapourwill react with the water in the respiratory aerosol to form acid. This will reduce the aerosol pH.
This suggests that the chlorine in the air around the pool will lead to the virus remaining infectious in the air longer, leading to higher transmission risk.
Theory 1: The chlorine above the pool neutralizes the aerosol, leading to the virus remaining infectious in the air longer.
Theory 2: The concentration of chlorine above the pool is so high that the pH in the aerosol actually becomesacidic, and the acidity inactivates the virus.
Which theory is correct? 1 or 2?
Unfortunately, currently, we simply don’t know. The measurements have yet to be made. I could speculate, but that wouldn’t be all that helpful (if not harmful).
A link to the study where we first explore the interplay between air acidity and aerostabilityis here:
It’s also important to note that there are numerous factors that ALL play a role in airborne viral transmission. What is happening in the Olympic pool could be due a factor other than aerostability, or even a combination of multiple factors. We need to make measurements to know.
Since there were people discussing this, I thought it would be helpful for people to have a better understanding of the underlying processes that are in play.
If you have any questions, I would be happy to try to answer them.
I suppose? Humidity is known to affect mucosal immunity, perhaps this is something similar(?).
Maybe someone more familiar with this end of things can add some insights… 🙏
• • •
Missing some Tweet in this thread? You can try to
force a refresh
The work done in this study is in bulk, meaning that the measurements were not made in aerosol. There is no doubt that biological molecules will be affected by highly acidic conditions. The question is what that has to do with respiratory aerosol.
I did my PhD in Simon Fraser University in Vancouver, Canada. As part of the project, I ended up working in St Paul’s hospital, specifically within the James Hogg iCapture Centre.
They have been rebranded, and remain a world leading research facility.
During that time, I had the privilege of attending weekly meetings with James Hogg. He was late into his career, and had worked with groups from around the globe. As a result, he always had stories to share about various studies, some published, others not cdnmedhall.ca/laureates/jame…
One year ago today, I officially started a YouTube Channel.
I thought I’d take the opportunity to reflect on the year, and talk a little about what the channel is, what it isn’t, and what I’m hoping to accomplish with it.
When scientists say things like, "opening a window lowers the risk of transmission by 38%", where do they get these numbers? Well, they use airborne transmission models.
In my latest explainer video I walk through the history of these models, what they do, and what the don't.
Here's a link to the video:
The aim of this video is not dive into exactly how these models are written, etc. Rather, my focus is simply on the types of models people use, and the advantages and disadvantges of each.
This article was published recently that reports the aerostability of SARS-CoV-2. I’ve been asked for my thoughts, and given that this is in my wheelhouse, I have a few.
In the study, the authors aimed to explore how long SC2 remained viable in the air, on a surface and the combination the 2. To measure this, they built an aerosolization chamber. Chambers like this have been used for decades, and extensively to study SC2
How poor experimental design coupled with media sensationalism undermines physical and engineering solutions to limit the spread of airborne diseases
A 🧵
Airborne disease transmission is a complex, and multidisciplinary process. As a result, understanding how various factors affects transmission rates is exceedingly difficult.
Consequently, designing effective physical mitigation strategies for this process remains a challenge.
Various strategies have been implemented with various degrees of success. Masking, ventilation, filtration, using CO2 monitors, etc.
The challenge is, how does one test how well do they limit transmission?