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Al Haddrell Profile picture
@ukhadds
Aug 5 • 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
Aug 3
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🧵:

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. Image
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. Image
Read 14 tweets
Al Haddrell Profile picture
Jun 26
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. 🧵 Image
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…
The amount of aerosol a person exhales is correlated with how loud they are talking/singing. Perhaps a reason why there have been no super spreader events reported in a library (?) Image
Read 12 tweets
Al Haddrell Profile picture
May 25
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…
Image
Previously, we’ve reported that when aerosol containing SARS-CoV-2 are exhaled into dry air, the aerosol changes phase (effloresces). This results in a rapid loss of ~50% of the infectious viral load.

Understanding aerosol phase dynamics is important!

royalsocietypublishing.org/doi/10.1098/rs…
Image
Read 10 tweets
Al Haddrell Profile picture
May 18
@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: Image
@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…
@ToshiAkima After 2 minutes, the pH difference between the 2 would become different. The expected result would be at lower CO2, >99% of the infectious viral load would be gone faster. Much in the same way we show with 3000 ppm vs 500 ppm data: Image
Read 5 tweets
Al Haddrell Profile picture
Apr 26
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. Image
In our first paper we reported that the decay dynamics of the original strain. We reported that the viral decay rate is faster than initially reported: 90% of the viral load is inactivated in 20 min. We proposed aerosol pH was playing a role in the decay

pnas.org/doi/full/10.10…
Read 64 tweets
Al Haddrell Profile picture
Feb 17
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. Image
@robbie_groth To study viral decay in air, the virus is aerosolised, contained in a controlled environment, sampled and measured. Every aspect of this process affects viral decay to some degree. Thus, to compare results between labs, identical techniques/benchmarks, etc must be used. Image
Read 8 tweets

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