Aerosol Transmission           

By Nicola Oliver and Beate Degan

COVID-19 Actuaries Response Group – Learn. Share. Educate. Influence.                                                                        http://www.covid-arg.com

Nicola

Beate Degan

Summary

In September, a group of academics published an extensive overview on the risks associated with aerosol transmission. In the form of an evidence-based FAQ, the document is more than 60 pages long. (link).

The authors state that:

‘The goal of these FAQs is to provide information to the general public in an efficient manner about how to prevent aerosol transmission of COVID-19, with the hope that this will allow more informed decision making by individuals or organizations.’

In this bulletin, we pull together the key points in an easily digestible format. We are indebted to the authors of the document for this detailed analysis.

How is the virus transmitted?

There are 3 routes for transmission of the virus that causes COVID-19.

1. Touching virus-contaminated surfaces which includes door handles and other people’s hands. This is known as the “fomite” path.
2. Receiving the virus via another person’s “large droplet” emissions which are mainly generated when coughing, sneezing, and sometimes talking. Below, these are the blue “ballistic” droplets which are relatively large and heavy and fall to the ground quickly.
3. “Aerosols” are small, are emitted from the respiratory tract, travel far and can linger in the air for hours (green, yellow and red dots below) and are exhaled when talking, singing, coughing, sneezing, and breathing.

Recent evidence suggests that the aerosol route of transmission plays a larger role than previously thought. Historic bias had prevented more careful consideration of this route of transmission, but on 5 October 2020, the CDC acknowledged that “the coronavirus can be spread through airborne particles that can linger in the air “for minutes or even hours” — even among people who are more than 6 feet apart” (link).

How do aerosols behave?

Given aerosols play an important role in transmission of COVID-19, it is not only important to maintain a rigid hygiene routine (washing hands etc.), but also to ensure you are careful regarding the air you are breathing, since aerosols dilute over time in ambient environments.

Here we can see time for particles of different sizes to settle to the ground in still air, from the height of a person.

The real boundary between ballistic droplets and aerosols is ~ 100 μm and as you can see, ballistic droplets settle much faster as they are heavier, and that very small aerosols remain airborne for hours. Evidence suggests that the SARS-CoV-2 remain infectious in aerosols for about 1-2 hours in an indoor environment.

While an individual SARS-CoV-2 virus is very small (120 nm or 0.12 microns), the aerosols in which they exist are larger, probably around a few microns (corresponding to the third of the five examples above). The viruses comprise a very small fraction of the aerosol.

Protecting ourselves from aerosol transmission

To reduce risk, avoid:

• Crowded spaces
• Close proximity to others
• Low ventilation environments
• Long durations
• Places where people are not wearing masks

The risk of transmission in different settings is displayed here. Outdoors is clearly safer than indoors, but it is still not 100% safe.

If a porous obstacle is put in the path of air that contains aerosols, some of the aerosols will end up in the obstacle – the basic physics of why we should wear a mask.

A mask should fit closely to the face, over the mouth and nose, whether clear, medical, or cloth, with universal masking providing maximum protection and least exposure.

There are many errors and misconceptions on masks, the fact is, like wearing a coat to keep you warm by preventing heat loss, wearing a mask prevents aerosol and droplet release. Vaping aerosols do not pass thorugh masks. Face shields and plexiglass are good for blocking wearer released ballistic droplets, and should be considered as an adjunct to wearing a mask.

Considerations for everyday life

Ventilation

As mentioned in the table, ventilation is important to control aerosols. Ventilation basically means diluting indoor air with outdoor air, and opening windows is the most basic method you can use. The rate of ventilation is calculated in air changes per hour, (ACH) which varies across different buildings.

A second measure is liters/second/person (L/s/p), which takes into account the number of people present, and is the most relevant parameter for preventing aerosol transmission of disease.

Estimating the ventilation rate can be done through measuring the decay rate of carbon dioxide (CO2). Relatively cheap meters are available to do this.

C0₂ levels as a proxy for ventilation quality

Measuring C0₂ levels can give a good indication of the amount of exhaled air in a space and thus the quality of the ventilation. Keeping windows continuously open allows for continuously-exhaled virus to be constantly diluted and expelled outdoors, and not allowed to accumulate indoors.

Outdoor CO₂ is about 400ppm, human exaled breath is about 40,000 ppm CO₂. There are a number of caveats to consider, but broadly speaking:

• 400-500 ppm – the ventilation is very good
• 800 ppm – 1% of the air you are breathing has already been breathed by someone in the space – considered risky.
• 4400 ppm – 10% of the air you are breathing has already been breathed by someone else – considered dangerous

This doesn’t tell exactly how many aerosols containing COVID-19 are in a room, since this depends on multiple factors (i.e. how many people are in the room, how many people with a COVID-19 infection), but it gives a good indicator.

So-called NDIR (non-dispersive infrared) CO₂ analyzers are recommended by the authors, with a price range between 100-200 USD. They can be a useful tool, especially in class rooms and alike, to help monitor ventilation, particularly in the upcoming winter period in the Northern hemisphere.

Do air filters work?

Virus exposure risk is a function of aerosol concentration and time; thus, when it is not possible to reduce our time exposure, it is recommended to reduce the concentration. This can be achieved through the use of portable air filters that are designed to remove virus relevant aerosol sizes.

High efficiency particulate air filter (HEPA) air cleaners are the best type of air cleaners and can remove more than 99.9% of aerosols in an air stream passing through them. It is important to follow manufacturer guidelines for frequency of filter replacement.

What about germicidal ultraviolet light (gUV)?

Ultraviolet (UV) germicidal air disinfection is an engineering method used to control the airborne transmission of pathogenic microorganisms in high-risk settings. Indeed, upper-room gUV was used in classrooms to combat measles and to tackle the resurgence of tuberculosis in the late 1990s. Currently, this technology is costly and is being used in some healthcare settings only.

Other points

• Portable air cleaners that are not based on filtration are not recommended
• Spraying disinfectant into the air does not remove the virus
• Humidity and temperature only matter for transmission at a distance; the biggest impact comes from increasing ventilation and air filtration

Conclusion – and a civic duty?

In conclusion, to protect yourself from aerosol transmission, avoid:

• Crowded spaces
• Close proximity to others
• Low ventilation environments
• Long durations
• Places where people are not wearing masks
• Talking, and especially loud talking / shouting / singing
• High breathing rates (e.g., indoor aerobic exercise)

These points can be represented by the mnemonic A CIVIC DUTY, per the following graphic: