Al Haddrell Profile picture
Aug 5, 2024 15 tweets 6 min read Read on X
"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 🧵)?

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. Image
Since the dawn of the field (1950s/60s), the airborne survival of viruses has been measured as a function of relative humidity (RH) and temperature. There are numerous reasons for this, such as to understand viral transmission and to inform about why the virus decays. Image
Another reason there was a focus on temperature and humidity was that people can both feel, as well as control, them. By understanding transmission via these parameters, it becomes readily possible to mitigate spread. Image
For SARS-CoV-2, numerous epidemiological studies have shown that transmission INCREASES at HIGH humidity.



nature.com/articles/s4159…
publichealth.jmir.org/2021/1/e20495/
ncbi.nlm.nih.gov/pmc/articles/P…
For SARS-CoV-2, numerous epidemiological studies have shown that transmission INCREASES at LOW humidity.



aaqr.org/articles/aaqr-…
ncbi.nlm.nih.gov/pmc/articles/P…
sciencedirect.com/science/articl…
So, what is going on here? Both of thesethings can not be true.

More curious is the specificity of the claims. For example, there has been reported both a strong increase and decrease below an RH of ~70%. Image
To understand what is happening, consider the following figure. Of the numerous processes involved in airborne transmission of a virus, RH affects a significant fraction. Moreover, the effect is often contradictory. Image
Consider just what is happening within the aerosol.
At high humidity:
-SARS-CoV-2 remains infectious longer
-the aerosol itself is larger
-the larger size causes it to settle out of the air faster

These processes are contradictory Image
Consider the effect of RH on behavior. It the room gets too humid (or even too dry), people will proactively change their environment. For example, they may open a window leading to improved ventilation which in turn lowers the risk. Image
The body’s first line of defense to stop a respiratory infection is the layer of mucus and cilia on the surface of the bronchus epithelia. In dry air, the efficiency of this defense mechanism is lowered.

pnas.org/doi/full/10.10…
Mechanistically, there are reasons that high humidity both increases, and decreases, SARS-CoV-2 transmission. Likewise for low humidity.

As a result, it is unsurprising that both positive and negative correlations have been reported.
In short, the effect of humidity on SARS-CoV-2 transmission is a mess.

If you have any questions, I’d be happy to try to answer them. Image
I should also add that each of these general factors can be massively expanded. For example, "Immunity" encompasses all of the myriad of different virus/cell interactions.
@serehfas For example, people will turn on the AC in hot/humid conditions. Some AC units ventilate, others just push the, now cooler, air around more. Same action, wildly different changes in long distance transmission risk.

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More from @ukhadds

Sep 8
Evidently this needs to be said:

Do not spray nitric acid into the air in the hopes of limiting viral transmission.

More pollution is not the solution. At all.
This is in response to a paper that came out recently: nature.com/articles/s4200…
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.
Read 9 tweets
Sep 6
Got this question, and it reminded me of a story I heard back during my PhD days.

The question is essentially, is there a problem with breathing too clean of air?

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.

hli.ubc.ca/who-we-are/
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…
Read 11 tweets
Aug 31
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. Image
First off, here’s a link to the channel.

youtube.com/@AlHaddrell/fe…
The pandemic made it very clear that aerosol science is important. That said, aerosol science is also weird and completely unintuitive.

For example, did you know that a pure evaporating ethanol droplet will become a pure water droplet in a fraction of second (weird, right!?). Image
Read 18 tweets
Aug 28
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. Image
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.
Read 7 tweets
Aug 19
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.

First off, here’s the article:

nature.com/articles/s4429…
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

pmc.ncbi.nlm.nih.gov/articles/PMC90…
Read 30 tweets
Aug 13
Straw Man Science and Covid Mitigation:

How poor experimental design coupled with media sensationalism undermines physical and engineering solutions to limit the spread of airborne diseases

A 🧵 Image
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. Image
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? Image
Read 51 tweets

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