Airborne Particles — what does it mean really?
Prof S. Curran, Prof K. Russell, and B. Scott
Several months into the pandemic, we have been well educated about the 6 feet apart rule to keep social distancing. It’s worth asking, however: “Where did this rule come from?” Critically, it is based on work that was done in the 1930s by William F. Wells, et al(1) who measured the duration and distance it takes for large droplets to fall to the ground — on the basis of understanding the spread of tuberculosis. In fact, the presumption then was that the droplets were going to dry out and leave a virus exposed to the elements — desiccating within seconds. However, TB has proven to be very air stable. The distance calculated by the six-foot rule, also known as the d2 law, can only be applied to an isolated spherical water droplet.
In the decades since, there has been little push to examine or extend these ideas and Wells’ work became the rule of thumb for all medical agencies — including the WHO — in part because of necessity. Compounding all of this is the fact that everyone will go back inside as the winter months approach. We must have a look at the Wells assumptions again. In fact, we understand now that when someone coughs, sneezes, and breathes, significant numbers of droplets are smaller than 10 micron. In non-technical terms, that’s about 1/5 the diameter of a human hair, so small the human eye cannot really see them. We also know those droplets can travel up to 17 meters — and beyond — indoors, while any droplets smaller than 5 micron (1/10 the diameter of a human hair, if we are now splitting hairs) do not fall to the ground at all. They will travel in air currents until they are sucked into someone’s lungs, land and remain on a random surface, or find a circulating path through a ventilation system.
As small particles dry out, we must also remember that the relative humidity will also have an effect on how quickly the virus dries out. According to a paper from K.L Cheong, et al(2) recently, the Wells model is not very effective when it comes to the SARS-CoV-2 virus and practically any other similar-sized and respired types of viruses. So, we can discern that SARS-CoV-2 will stay airborne for a prolonged period and will still have an outer layer of saliva (as well as salts, proteins, and other inorganic and organic matter). They will form nuclei and aggregate in numbers, which in itself will likewise alter the evaporation rates(3). Until the airborne viral particles reach HVAC filters, they have the potential to be active and infect those not wearing proper protective masks. This remains an area of active debate, however, as arguments ensue about how infectious these small particles really are.
Where does this leave us?
It is better to remain outside the line of fire of course, so keeping as distanced as possible from other individuals is a must. It should be reinforced that 6 feet is not a guarantee of safety but more likely the least distance as you approach individuals, especially — and in particular — indoors. For the foreseeable future, wearing masks is going to be necessity.
What about indoors — am I protected by filters?
The answer is, of course, yes and no. If you have HEPA filters, that’s an awesome way to keep air cleaner, but not completely clean from viral loads. But, with standard cloth filters (polymer or cotton), they all have a Minimum Efficiency Reporting Value (MERV) rating which range from 1 (least porous) to 20 (most porous) — this is something most people have never come across.
So, yes it’s good to filter the air, but not all filters are capable of stopping the virus. According to ASHRAE, with a MERV 14 filter it can take 4 or 5 flushes of air — recycling the same air — before air quality improves. Most homes use MERV 4–6 depending on tolerance for pollen, dust, and dander; schools and office areas may use as high as MERV 8; and for the most filtered air outside of HEPA filters, there is MERV 16.
What do we do now — what is really safe?
The debate remains as to why 6 feet apart has become the standard even after 86 years. That’s not saying the studies were bad, they were advanced for their time, like comparing a propeller plane with a space shuttle. Today, we know much more about flow dynamics, viruses, and nano particles (even modeling exactly how they work). Feynman first introduced the “idea” of nanotechnology in 1959, 15 years after the 6 foot “rule” was brought into place(4). It would be another 40 years before instruments could readily reveal these viruses. The world must wake to — and listen to — scientists warning that SARS-CoV-2 can travel across rooms and between open spaces in buildings.
Until we tackle this, we are trying to perform delicate surgery with bolt cutters and a saw.
1. Wells, W. F. (1934). “On air-borne infection: study II. Droplets and droplet nuclei”. American Journal of Epidemiology. 20 (3): 611–618. doi:10.1093/oxfordjournals.aje.a118097
2. Kai et al, https://www.medrxiv.org/content/10.1101/2020.08.04.20168468v1
3. T.Yamada, N. Sasagawa and K. Sakai, ‘Accurate determination of volume and evaporation rate of micron-size liquid particle’ Journal of Applied Physics 108, 063522 (2010)
4. Feynman, Richard (1959, December 29). There’s plenty of room at bottom: an invitation to enter a new field of physics[Lecture]. American Physical Society annual meeting, Caltech.
About the authors:
Shay Curran, PhD
Dr. Shay Curran is a dynamic leader and innovator and excels at scaling up base technologies into full-fledged marketable products. He’s also a Physics Research Professor at University of Houston and heads the Curran Nanotech Transformation Group (CNTG) at UH.
•Recent winner of the highly coveted National Academy of Inventors Fellowship
•2020 Silicon Valley 50 Award
•19 patents
•Recent Nature article (optical limiting applications of specialized nanocoating)
Kenneth Russell, PhD
Dr. Ken Russell is a respected technology leader/coach and organization builder, as well as a Research Professor at University of Houston and Chief Innovation Officer at CNTG.
•Respected and award-winning leader, technologist, coach, author, and innovation amplifier
•Executive roles at Duke Energy, Bank of America, North Carolina Research Campus, and Cisco
•Known for successfully leading, stabilizing, and delivering in turbulent environments
• Valued voice and key influencer
•IoT, Business Intelligence, AI/ML, DLT, M&A, and next generation Innovation
Becky Scott, MBA
Becky Scott is a leader in organization transformation and helps Curran Biotech connect with members, partners, and sponsors. With successes at Cisco, Khoros, Cloudera, TEDxSanDiego, and many others, she understands how marketing in the digital age is constantly changing.
•Forrester Groundswell Award for most innovative service at Cisco
•Featured speaker, consultant, and social strategist
•Adept at integrating traditional and innovative programs into actionable business successes
