Aerosol distribution in rooms and the importance of proper ventilation
Summary of ENEtwork presentation, 29.10.2020
(Link to presentation: /media/61482/edit)
What are aerosols:
Aerosols are liquid or solid particles suspended in the air sufficiently long to be measured
For us (the LAC) aerosols is a synonym to particulate matter, PM, or in German Feinstaub. Only sizes up to 10 micro m are relevant
Droplets for the inhalation and pulmonary deposition of pharmaceutical drugs are also called aerosols; these droplets are also typically smaller than 10 micro m
There is multiple and confirmed evidence that aerosols are relevant in the transmission of Covid-19.
We all emit aerosols in the size range between 1 and 10 micro m, even during breathing (when speaking, sneezing or coughing we also emit much larger droplets up to 1 mm, these settle to the ground within about 1.5 m, which defines the distance rule of 1.5 m). The louder our voice, the more aerosols we emit. This is especially evident for singing. The Covid-19 virus has a size of ~100 nm and comprises only a very small fraction of the total volume of these aerosols: There are typically ~0.1-1 virus copies per 10 micro m droplet. Also, about 100 – 1000 viral copies are needed for a 50% probability to be infected. Speaking results in ~100-1000 aerosols per liter of air. Thus, 1 liter of air (= 1 to 2 undiluted breath exhalations) can be sufficient for an infection (please take all these numbers as rough estimates!).
After emission, aerosols shrink slightly by loosing water to adapt to the lower relative humidity, such that we now measure a size of 1-2 micro m. These particles settle very slowly and can stay in the atmosphere for many hours. Unlike for the large droplets, a distance of 1.5 m is therefore no protection against aerosols.
The precautions against covid infection via aerosols are therefore:
Stay outside as there the dilution is the highest (but the concentrations are not zero, as you still smell a cigarette from somebody nearby also when you are outside)
Use active ventilation (lüften, lüften, lüften!) when you are inside
Your ventilation system should have an efficient filter (ideally a HEPA filter, which eliminates aerosols and with them viruses efficiently). Alternatively one can also use UV lights to destroy the viruses, obviously this needs eye protection)
Wear masks whenever you are inside with other people that are not from your household: masks reduce the emission of aerosols (and efficiently eliminate large droplets) and reduce the aerosol load in the inhaled air. However, the standard hygiene masks have a limited efficiency; the combined reduction of the risk of being infected (exhalation and inhalation) is maybe about a factor of 6.
Lelieveld et al. (2020) present numbers for risk of being infected in different environments (office, classroom choir, reception, see a preprint at doi: https://doi.org/10.1101/2020.09.22.20199489). This paper also provides an excel spreadsheet where you can do your own calculations. As an example: Being in the same room for two days with a superspreader with only passive ventilation and without masks results in a chance of 88% of being infected.
This is why we wear masks at PSI when being indoors together with other people.
Note: also on 29.10.2020 the Swiss Covid Taskforce published a document on ‘The role of Aerosols in SARS-CoV-2 Transmission’. This document (in English), found at https://ncs-tf.ch/de/policy-briefs/the-role-of-aerosols-in-sars-cov-2-t…, is a very good and detailed document that I recommend to read. The statements are fully in line with the ones of my presentation.
Urs Baltensperger, firstname.lastname@example.org, 056 310 2408