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Airborne Transmission of Respiratory Viruses

The COVID-19 pandemic has focused unprecedented attention by scientists onto the mechanisms of virus transmission.  While much of the guidance early in the pandemic was based on stopping the spread by blocking large droplets (for example those from coughs and sneezes) and by sanitizing contaminated surfaces, there is now an overwhelming scientific consensus about the importance of an airborne transmission route, not just for SARS-CoV-2 but also other common viruses such as flu and rhinovirus.  Airborne transmission by aerosols occurs over much longer distances than the standard 6-foot rule that applies to droplets, and these aerosols are the types of particles that the DIY air filters aim to remove from the air.

In this review of the recent scientific literature, seven of the leading experts in the world on this topic argue that the airborne transmission route may in fact be the dominant one, not just for SARS-CoV-2 but also for other viruses.  This article is the most up-to-date and comprehensive summary of current scientific understanding of airborne virus transmission.  

Minimum Sizes of Respiratory Particles Carrying SARS-CoV-2 and the Possibility of Aerosol Generation

This study calculates and elucidates the minimum size of respiratory particles that are potential carriers of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); furthermore, it evaluates the aerosol generation potential of SARS-CoV-2. The calculations are based on experimental results and theoretical models. In the case of maximum viral-loading derived from experimental data of COVID-19 patients, 7.18 × 10−4% of a respiratory fluid particle from a COVID-19 patient is occupied by SARS-CoV-2. Hence, the minimum size of a respiratory particle that can contain SARS-CoV-2 is calculated to be approximately 4.7 μm. The minimum size of the particles can decrease due to the evaporation of water on the particle surfaces. There are limitations to this analysis: (a) assumption that the viruses are homogeneously distributed in respiratory fluid particles and (b) considering a gene copy as a single virion in unit conversions. However, the study shows that high viral loads can decrease the minimum size of respiratory particles containing SARS-CoV-2, thereby increasing the probability of aerosol generation of the viruses. The aerosol generation theory created in this study for COVID-19 has the potential to be applied to other contagious diseases that are caused by respiratory infectious microorganisms.

CDC Ventilation in Buildings

Research shows that the particle size of SARS-CoV-2 is around 0.1 micrometer (µm). However, the virus generally does not travel through the air by itself. These viral particles are human-generated, so the virus is trapped in respiratory droplets and droplet nuclei (dried respiratory droplets) that are larger than an individual virus. Most of the respiratory droplets and particles exhaled during talking, singing, breathing, and coughing are less than 5 µm in size. CDC recommends using the highest efficiency ventilation filters possible, without having detrimental effects on overall HVAC system performance. ASHRAE has similar guidance; however, they recommend a minimum filtration efficiency target of MERV 13, provided there are not substantial negative impacts on the HVAC system performance and occupant comfort. A MERV 13 filter is at least 50% efficient at capturing particles in the 0.3 µm to 1.0 µm size range and 85% efficient at capturing particles in the 1 µm to 3 µm size range. Collectively these particles are capable of remaining airborne for hours and are most associated with deep lung penetration. A MERV 14 filter is at least 75% and 90% efficient, respectively, at capturing those same particles. Efficiencies for MERV 15 and MERV 16 filters are even higher. Thus, the recommended filters are significantly more efficient at capturing particles of concern than a typical MERV 8 filter, which is only around 20% efficient in the 1 µm to 3 µm size range and is not rated for capture efficiency of the smaller 0.3 µm to 1.0 µm particles.

Wired: Could a Janky, Jury-Rigged Air Purifier Help Fight Covid-19?

THE IDEA SEEMS pretty simple, once someone explains it. The virus that causes the pandemic disease Covid-19 passes from human to human on tiny droplets of spittle, through the air. Masks block some of them. But what if—and I am literally spitballing here—you could clean those particles from the air itself?

Ford DIY Air Filtration Kit That Reduces COVID-19 in the Air Validated by Peer-Reviewed Scientific Research Journal

Ford’s latest innovation in the fight against COVID-19 is an air filtration kit, co-developed with Lasko, that you can assemble at home or in school – and the science behind it is backed up by a peer-reviewed research article appearing this month in a prominent scientific journal.

Ford and Lasko are donating 20,000 of these easy-to-make kits to underserved communities. Featuring an open-source design, the kit can be used by others interested in making their own to help reduce COVID-19 in rooms and other enclosed spaces.

Other Resources About DIY Air Filters

Here are some of the best resources we know about with additional information about DIY Air Filters:

 UC Davis Western Cooling Efficiency Center

Clean Air Crew

 Philip Neustrom’s page

Edge Collective (LOTS of information and data, scroll to the bottom for the “chicken wire shroud”!)


Featured Downloads

Box fan filter stand

The EPA recommends air purifiers to blow upward, so the single-filter version needs a stand.  We revised a design by Ford (linked above) that allows you to make a sturdy stand using the box your fan came in.  Download instructions below!