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Al Haddrell Profile picture
@ukhadds
Aug 5, 2024 • 15 tweets • 6 min read • Read on X
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"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

Al Haddrell Profile picture
Mar 21
Aerosols are very small, have a high surface to volume ratio and can readily reach super saturation. This combination makes them a unique environment for chemical reactions. We had a paper published in JACS that looked into the rate of chemical reactions within and on aerosols. Image
This work has implications from material fabrication through to understanding disease transmission.

The project was headed by Bryan Bzdek (University of Bristol, whose office is 3 doors down from mine).

Here’s a link to the article:

pubs.acs.org/doi/10.1021/ja…
Context: The chemistry at the air-liquid interface of a sample will be different than in the bulk. There are many reasons for this, such as the molecular orientation, higher solute concentration (due to evaporation of the solvent at the surface) and unusual acidity/basicity. Image
Read 16 tweets
Al Haddrell Profile picture
Mar 15
A preprint came out that measured how acidic vapour affects influenza aerostability.

How the pH of exhaled aerosol evolves over time will dictate both how long microbes remain viable in the air, but also how effective different mitigation strategies will work.

Let’s discuss. Image
Here’s a link to the preprint:

researchsquare.com/article/rs-902…
Context: Our team has studied how long numerous microbes remain viable in the air. In these studies, we found that the alkalinity of exhaled aerosol drove the decay of SARS-CoV-2. Consequently, elevated levels of CO2 results in SC2 remaining viable in the air for a longer period. Image
Read 30 tweets
Al Haddrell Profile picture
Mar 1
An article came out recently that looked at the effect of CO2 on influenza decay in sessile saliva droplets.

The study confirms what we have been saying since 2022: respiratory fluids become highly alkaline when they leave the body, and this in turn affects microbe viability Image
Given that this is a research area where our team has made a lot of waves, I thought I should share my thoughts.

Here’s a link to the article:

pubs.acs.org/doi/10.1021/ac…
Context: Prior to 2020, the assumption in the literature around respiratory aerosol pH was that was acidic. There were a few reasons for this, but it largely came down to the fact that almost all environmental aerosol is acidic, so it was assumed respiratory would be the same Image
Read 18 tweets
Al Haddrell Profile picture
Feb 23
I got this question over on BlueSky that I thought some might find interesting.

In short, why does the CO2 levels in a hockey arena trend upwards even though it has an upgraded HVAC? Image
When HVAC systems are installed, certain assumptions are made, largely because they need to be. One is that the air within the space is evenly mixed.

The problem is that it rarely is. Image
In a perfectly designed space, for every litre of fresh air brought into a space, 1 litre of air would be removed. For this to occur, a room would have be essentially a pipe. Image
Read 9 tweets
Al Haddrell Profile picture
Feb 7
The risk of the airborne transmission of disease correlates with the amount of infectious exhaled aerosol. Since people exhale CO2 with aerosol, its conc has been used as proxy for exhaled aerosol

In this article, researchers propose a new way to estimate risk of transmission Image
Here’s a link to the article (the first author is Henry Oswin, a former PhD student from our group who is currently working with Lidia Morawska):

sciencedirect.com/science/articl…
For a variety of reasons, CO2 may not be a good proxy for exhaled aerosol. eg, it will underestimate the risk when people are talking, or overestimate when filtration is used.

I walked through some of this in my explainer video (excerpt shown below):

Read 10 tweets
Al Haddrell Profile picture
Jan 22
Not so fun fact: Tear gas isn’t a “gas”

If it’s not a gas, then what is it?

Answer: it’s an aerosol. And this distinction matters.

Let’s discuss 🧵 Image
The burning sensation of tear gas is caused by the compound 2-chlorobenzalmalononitrile.

Rather than discussing how this chemical affects the body biologically, let’s go over how this chemical is dispersed physically, and why that matters (aerosol science!). Image
Tear gas is delivered a couple different ways.

1) Pyrotechnic canister where the device produces a cloud of hot smoke.

2) Aerosol spray devices where the chemical is dissolved in a solvent and then sprayed. Image
Read 18 tweets

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