Ventilation

The SARS-COV2 virus that causes Covid-19 is airborne. This means that is moves around like invisible smoke. In rooms with poor ventilation, it can remain in the air for hours, even after the infected person has left. (Lewis, 2021 doi: 10.1038/d41586-021-00810-9; Fiorillo et al., 2020 doi: 10.3390/ijerph17093132; Zhang et al., 2020 doi: 10.1073/pnas.2009637117)

Good ventilation helps to dilute the amount of virus in the air and potentially allow the virus to leave the room.

The quality of ventilation systems in buildings will vary, but as an individual you can improve ventilation by opening windows and doors to increase air flow. (World Health Organisation. Practical guidelines for infection control in health care facilities practical guidelines for infection control in health care facilities. Geneva, Switzerland: WHO, 2004; cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/improving-ventilation-home.html)

If you have a fan, this can help to move indoor air to the outside. The best way to do this is to place it by an open window, facing outside. Don't point the fan towards people, as it can blow contaminated air directly at them. (cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/improving-ventilation-home.html)

A study in Italian schools found that when mechanical ventilation systems were installed, risk of Covid-19 infection in students decreased by at least 74%. (Buonanno et al., 2022 doi: 10.3389/fpubh.2022.1087087)

The American Centre for Disease Control (CDC) has an Interactive Home Ventilation tool that can help to estimate ventilation quality in your home.

Click Here to Open It

Stop Covid-19 Hanging Around
A good visualisation of Covid viral particles in the air

Image of an Aranet4 CO2 monitor from aranet.com/products/aranet4

CO2 Monitors

CO2 monitors can help to roughly estimate how well a room is ventilated. This is because we breathe out CO2, and so it builds up more in spaces with poor ventilation and air flow. (Burridge et al., 2021 doi: 10.1177/1420326X211043564)

CO2 level is measured in parts per million (ppm). A space is considered well ventilated when CO2 level is below 800ppm and poorly ventilated if CO2 level is above 1500ppm. (hse.gov.uk/ventilation/using-co2-monitors.html; Baraniuk 2022 doi: 10.1136/bmj.o736)

However, it is important to understand that although a low CO2 level means a space is well ventilated, it does not mean Covid-19 risk is removed. It is still possible to be infected in a well ventilated space, especially if there are many infected people or if infected people are very close to you. (Baraniuk 2022 doi: 10.1136/bmj.o736)

Air Filtration

Air filtration is when particles are removed from the air, such as SARS-COV2 virus particles. There is abundant evidence that this can reduce risk of Covid-19 infection:

A simulation of an infected person meeting three uninfected people showed that 2 HEPA filters close to the infected person reduced Covid exposure of the others by up to 65%. When combined with masking, Covid exposure by reduced by 90%. (Lindsley et al., 2021 doi: 10.15585/mmwr.mm7027e1)

Research in US high school classrooms found that when four HEPA filters produced 5.5 air changes per hour, aerosol concentrations were reduced by >90% within 30 minutes. The researchers then used this to calculate the maximum concentration of virus-containing aerosol in classrooms with and without HEPA filters, concluding that they are able to substantially reduce risk of Covid-19 airborne transmission. (Curtis and Schrod 2020 doi: 10.1080/02786826.2021.1877257)

Research in Georgia, USA showed a 35% decreased incidence of Covid-19 in classrooms with improved ventilation, and a 48% decreased incidence when this was combined with HEPA filtration and/or UV irradiation. (Gettings et al., 2021 doi: 10.15585/mmwr.mm7021e1)

Use of an aerosol transmission model showed that in a 160m3 classroom, two HEPA filters results in a 5-fold decrease in concentration of viral particles. Combined interventions of natural ventilation, masks, and HEPA filtration were the most effective, resulting in at least a 25-fold decrease in viral particle concentration. (Villers et al., 2022 doi: 10.4414/SMW.2022.w30178)

Key Terms

CADR and ACH

Air filtration is measured in terms of CADR or ACH:

CADR is the clean air delivery rate, i.e. the volume of clean air delivered per minute.
ACH is air changes per hour. This is how many times the total volume of air in the room is changed per hour.

HEPA Filters

HEPA stands for High Efficiency Particulate Air (filters). As the name suggests these are very efficient types of air filter, which should theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and any airborne particles with a size of 0.3 microns.

HEPA Filters are widely available to buy at various prices, in various sizes and may be battery or mains powered.

MERV

MERV stands for minimum efficiency reporting value. It is a way of describing how well any filter (HEPA or less efficient filters) removes particles. MERV ratings go from 1 to 16, with MERV 16 meaning better filtration, as more particles are captured with each passage through the filter.

Air Filter Recommendation Tool

Clean Air Stars offer an Air Filter Recommendation Tool, where you can enter information such as acceptable noise limit, desired ACH, room volume and electricity tarriff and it will show all air filters available that match your search.
It lists the price of the unit, replacement filters and yearly electricity costs in different scenarios.

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DIY Air Filtration: The Corsi-Rosenthal Box

Named after its two designers, the Corsi-Rosenthal Box offers a DIY, lower cost alternative to comercial air filters. It works using MERV-13 filters and a box fan to move air through them and research has validated their efficacy in reducing concentrations of suspended particles within the air.

(Porto et al., 2022 doi: 10.1080/02786826.2022.2054674)

For more information, a good resource is cleanaircrew.org

A summary of how to build your own Corsi-Rosenthal Box is found below:

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