This study made some waves on social media yesterday. It purports to show that CO2 concentration doesn’t correlate with covid transmission.

Let’s talk about this, I have some thoughts.

sciencedirect.com/science/articl… Context: Why do people think knowing the CO2 concentration is important when it comes to Covid transmission?

There are 3 unique reasons.

You have no idea. Ironically, I found out about this letter when I was on my way to a dinner to celebrate the end of a 5 year covid project I was on. This project resulted in 14 papers in high impact journals where we explored how SARS-CoV-2 is aerosolised through talking/singing

https://twitter.com/drseanmullen/status/1880829988871803062

Needless to say, to find out right then that the government had produced this letter was bizarre and upsetting. Like, why are we doing all of this research if they are just going to ignore it?

When I shared the letter with others on the team, it got a little awkward....

If you got a CO2 monitor for Christmas and are wondering what exactly the device is used for, I’ve put together a video to walk you through some ways you can use it improve your indoor air quality, and to lower your risk of catching airborne viruses (C19)
With New Years coming up, you may be having friends over. You can use a CO2 monitor to see how best to ventilate your home, to make sure that your friends and family start the year off healthy! I walk you through how to measure your home's ventilation here

I was asked to walk through why the work published earlier in the year by the Lund group is so important. I’ve put together a thread to walk through some of my thoughts.

https://twitter.com/DrInfoSec/status/1872658552852238579

First, here’s a link to the study.
nature.com/articles/s4159…

Thanks for your interest in our work. You’ve asked a lot of “out of the box” questions here, and I’ll address some of them.

https://twitter.com/ShaneBond77/status/1865713082237874479

Exhaled aerosol has a water activity of ~0.995. The RHof fog could be higher: the the aerosol would simply take up water from the gas phase. For viruses such as influenza, where the salt concentration seems to be very important, this would lead to an reduction in the decay rate.

A study came out last month looking at how long influenza remains viable in the air.

In the article, it highlights some of my larger concerns in what is happening in the field of airborne viral decay.

First off, here’s the article:

frontiersin.org/journals/micro… Out of the gate, the article is fine.

My largest critique is the assertion that 99% decay means something. Why 99%? Why not 98%? Or 97… or 92… 87%? 83.5%?

How does humidity affect the transmission of SARS-CoV-2?

There's a lot of confusion around this question. Is dry air or wet air better? Somewhere in between? In this explainer video I dive into this and go into what we know, and what we don't. This is the first part of a (what I expect to be) a two part series. In this video I discuss how humdity affects transmission. In the followup I will dive into why humdity does, or does not, have an effect.

Are respiratory aerosol liquid, solid, neither or both? What about the virus, where does it go? Let's get into this.

https://twitter.com/open_erv/status/1835736015794798823

We published some work on this previously where we studied how respiratory aerosol changes once it is exhaled.

tandfonline.com/doi/full/10.10…

There's been a lot of discussion about the size of exhaled aerosol that contains the most virus. For the most part, it's thought to be in the ~1 to ~5 micron range. There's a little bit of variation between studies, but that's roughly the size of concern (“Viral”).

https://twitter.com/piotr_wasik/status/1835253421584089103

If you are curious, this is due to a combination of the size distribution of exhaled aerosol and maximum conc that the virus can grow in the respiratory fluid. Here we looked at aerosol size, others have looked into the viral load as a function of size.tandfonline.com/doi/full/10.10…

Huge paper exploring the relationship between exhalation aerosol counts and CO2 has just been published.

Take home message: CO2 and aerosol strongly correlate in silence. Vocalisation causes this relationship to breakdown (way more aerosol than CO2).
pubs.acs.org/doi/10.1021/ac… This has huge implications on how CO2 can be used to estimate the aerosol counts in a room. Noise matters!!

I've mentioned this work previously, it's great to see it finally published so everyone can have a good look.

"How does humidity affect SARS-CoV-2 transmission?"

Whenever this question comes up, the answer I give is along the lines of “it’s complicated”.

So, what exactly do I mean (a 🧵)?

https://x.com/HMogan24/status/1820211262858432569

Context: When considering airborne transmission of a respiratory virus, numerous factors are involved.

They ALL matter.

Moreover, they are all independent. Meaning, a certain parameter may affect each factor differently.

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.

Some thoughts🧵:

https://x.com/Ikat0/status/1819526475587973354

Context:

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.

Given that the COVID numbers are on the rise, I thought I’d it useful to share some our research team’s work looking at the interplay between CO2, aerosol, SARS-CoV-2, and airborne transmission. 🧵 SARSCoV2 is spread primarily through the air via aerosol. Meaning, the amount of aerosol a person produces will to some degree correlate with the amount of virus exhaled. Our group has done of studies into how different activities affect aerosol production
tandfonline.com/doi/full/10.10…

Understanding the rate aerosol particles change phase is critical, from drug delivery to the lung or atmospheric processes, to disease transmission.

Our work looking into the dissolution and crystallization dynamics of aerosol has just been published:
pubs.acs.org/doi/10.1021/ac… For example, the size of a particle will influence where it is deposited in the airway. Likewise, the rate in which a solid particle dissolves will affect both where the particle is deposited as well as the structure of the particle when it lands: () pubs.acs.org/doi/10.1021/ac…

@ToshiAkima Great question! The maximum pH respiratory aerosol will reach is highly dependent on CO2. The evaporation of CO2 from the droplet is relatively slow, taking 10s of seconds to minutes. One result of this is at 2 minutes, we saw no effect on aerostability between 500ppm and 0ppm: @ToshiAkima Thus, at 2 minutes the pH of the 500 and 0 ppm droplets were still the same. The result of this is that for short distance transmission, the increase in outdoor CO2 concentration over the past hundred or so years would be expected to have no effect. HOWEVER…

I’m excited to discuss our latest research on how ambient CO2 affects how long #SARSCoV2 remains infectious in air. We report that even subtle increases in CO2 affects both how long #COVIDisAirborne and transmission risk. Here’s a🧵going over the findings
nature.com/articles/s4146… Somebroader context: this study is the third part of our SARS-CoV-2 (SARS-Wars(?)) trilogy.

Measuring how long viruses remain infectious in the air is very difficult. A review article discussing the challenges of making these measurements, and the need for standardization, has just been published! The paper was the brainchild of @robbie_groth

pubs.acs.org/doi/10.1021/ac… @robbie_groth For any measurement, standardization is critical. Without it, it’s impossible to compare results between laboratories. For example, for something as simple as mass, people use calibration weights to ensure their scale is accurate.

The length of time viruses and bacteria remain viable in the air is dependent on the conditions they experience in an aerosol droplet. In our latest study, we explore how the size/phase/composition of respiratory aerosol changes in a range of conditions. tandfonline.com/doi/full/10.10… This work was headed up by Jianghan Tian (@TianJianghan96).

In the study, individual droplets are generated, levitated and probed with an electrodynamic trap. A laser probes the droplet. From the scattered light, both the droplet size and morphology can be measured over time.