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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
Jun 30
There is no safe level of exposure to Asbestos. For this reason, it has been banned in >60 countries across the globe.

And yet, there is an effort in the US to bring it back. In this article, I discuss why this is happening, and what it all means.

theconversation.com/the-uss-asbest…
This article is a byproduct of a previous thread I wrote about the dangers of asbestos. In it, I explain what to your body after you inhale it. In short, it doesn’t go anywhere, and causes harm for years.

In the article, I discuss the history of asbestos use, how long it took for people to understand how much harm it was causing. I also dicuss how industry spent decades covering those findings up. We are still, to this day, dealing with the consequences of their actions.
Read 6 tweets
Al Haddrell Profile picture
Jun 21
Since there is talk about bringing back ASBESTOS (this is somehow true), I thought it would be useful to describe just some what happens to you when you breath this stuff into your lungs.

In short, it’s terrible.

A 🧵 Image
What is asbestos?

Asbestos is a group of naturally occurring fibrous minerals. There are 6 types: Chrysotile, Amosite, Crocidolite, Tremolite, Actinolite and Anthophyllite.

They have some useful properties (including heat resistance, strength, durability and well insulating) Image
Because of these physical properties, humans have been using asbestos for thousands of years for a variety of purposes. In the 20th century, it began to be used as a building material.

In the 1970s, the health risks associated with asbestos exposure began to be recognized. Image
Read 19 tweets
Al Haddrell Profile picture
Jun 6
We just had an article published in ACS Central Science on the how the pH of exhaled aerosol evolves over time

As we’ve previously reported, respiratory aerosol pH (high pH!) is a driver of SARS-CoV-2 decay. Meaning, understanding the pH dynamics is important for estimating risk Image
This paper is a step in the direction of improving our undertanding of exhaled aerosol pH.

Alright, so here’s a link to the article:

pubs.acs.org/doi/full/10.10…
Apologies up front, this thread is a bit of a long one. There’s a lot of background/context to get through to appreciate why this work has been published in such a high impact journal.

In short controversy.

Enjoy the ride. Image
Read 42 tweets
Al Haddrell Profile picture
May 26
Martha asked an interesting question, so I thought I’d try to answer it.

Essentially, the question is how long to wait before one is 95% certain that the room is Covid free, given a certain set of conditions.
Disclaimer at the top: I am not a modeler, I am an experimentalist (that occasionally publishes simple models).

The data used to make these estimates are from experimental studies. The absolute values will not be exact, but the general trends and scales will be accurate.
Let’s go through these conditions one at a time.

First is the ACH. ACH describes the rate in which the air (and aerosol) in a room is removed over time. Shown below is the relationship between ACH and the time taken for 99% of the air in a room to be replaced with fresh air: Image
Read 21 tweets
Al Haddrell Profile picture
May 25
The article claiming that SARS-CoV-2 remains infectious in the air for 16 hours has been making the rounds on social media again.

I thought I would take this opportunity to discuss it, what it shows, what it doesn’t, and why I think it shouldn’t be taken seriously.

A 🧵
First off, here’s the article:

wwwnc.cdc.gov/eid/article/26…
The article is short, with only two figures. The key one being Figure 2. In it, the authors claim to show the virus surviving for 16 hours in the air. Image
Read 32 tweets
Al Haddrell Profile picture
May 19
We just had an important paper published that compares different technologies used to measure aerosolised microbes.

This resulted from a collaboration between multiple research teams in the USA and the UK. Image
Here’s a link to the article:

tandfonline.com/doi/full/10.10…
Background: Studying how long and why microbes lose viability while in the air is critical in estimating risk, and in designing effective mitigation strategies.

These measurements are extremely challenging. I discussed them in an explainer video:

Read 20 tweets

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