A good deal of evidence indicates that wearing facial masks inhibits the spread of COVID-19.^1-4 The spread of the virus appears to occur via airborne transmission. During coughing, sneezing, talking, and breathing, humans release tiny droplets and even smaller aerosols that contain viral particles.^1-4 These droplets range in size from 1 to 500 mm.^2,3 The larger droplets (>5 micrometers) fall onto surfaces, while the smaller aerosols (< 5 micrometers) spread throughout the air; both carry viral particles that can cause infection.^1-4 Interestingly, it appears that static charge and van der Waals forces are involved in attracting aerosols and droplets to masks.^3,5 

Dean Campbell and his kids recently published an experiment in which lycopodium spores (about 30 mm in size) were sprayed toward a flame to demonstrate the effectiveness of masks at blocking the transmission of viral particles.⁶ In the absence of a mask the spores enter the flame uninhibited, creating a fireball. When blocked by a mask the spores are remarkably prevented from entering the flame and no fireball is observed.

Inspired by these experiments, I decided to test the ability of masks to block the emission of aerosolized particles in my own breath. To do so, I used liquid nitrogen to cool the tiny droplets, condensing them into larger particles that are visible to the eye. Check out the experiments I conducted in the video below:⁷

Video 1: Testing Mask Effectiveness with Liquid Nitrogen⁷

I learned quite a bit while conducting these experiments. For example, I had always thought that people exhale only gas-phase water vapor. I was unaware that we also exhale small droplets of condensed water vapor – liquid droplets too tiny to be seen. I was also quite surprised to learn that N-95 masks can filter particles that are smaller than the pore sizes in the masks themselves. It is definitely worth your time to check out the video by Minute Physics which explains much of the science behind how N-95 masks work.⁵

If you choose to try these experiments on your own, remember that quite a bit of nitrogen vapor boils off of liquid nitrogen, so you could potentially inhale increased amounts of nitrogen gas while conducting these experiments. Nitrogen gas makes up 78% of air, so there is no danger of inhaling nitrogen per se. However inhalation of too much nitrogen gas in place of air could cause oxygen deprivation, making you pass out. Therefore, do not try these experiments without someone nearby to keep an eye on you – and move to fresh air if you begin to feel dizzy. For the record, I had no trouble and felt no discomfort while doing these experiments.

Happy experimenting!

Acknowledgements:

Thanks to Michael Buratovich for helpful discussion, Abby Franklin for helpful discussion and assistance, and Christopher Wayne for helpful discussion and video advice/critique.

References (all accessed 6/29/2020):

1. Zhang, R.; Li, Y.; Zhang, A. L.; Wang, Y.; Molina, M. J. Identifying airborne transmission as the dominant route for the spread of COVID-19, PNAS, DOI: 10.1073/pnas.2009637117 https://www.pnas.org/content/pnas/early/2020/06/10/2009637117.full.pdf

2. Stadnytskyi, V.; Bax, C. E.; Bax, A.; Anfinrud, P. The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission, PNAS, 2020, 117 (22), 11875-11877. https://www.pnas.org/content/pnas/117/22/11875.full.pdf

3. Konda, A.; Prakash, A.; Moss, G. A.; Schmoldt, M.; Grant, G. D.; Guha, S. Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks, ACS Nano, 2020, 117 (22), 11875-11877. https://pubs.acs.org/doi/10.1021/acsnano.0c03252

4. Prather, K. A.; Wang, C. C.; Schooley, R. T. Reducing transmission of SARS-CoV-2, Science, 2020, 368 (6498), 1422-1424 https://science.sciencemag.org/content/368/6498/1422/tab-pdf

5. Minute Physics, The Astounding Physics of N-95 Masks https://www.youtube.com/watch?v=eAdanPfQdCA

6. Campbell, D. J.; Campbell, K.; Campbell K. Lycopodium Powder “Dragon Sneeze” Blocked with a Face Mask, ChemEd X, 2020. https://www.chemedx.org/blog/lycopodium-powder-%E2%80%9Cdragon-sneeze%E2%80%9D-blocked-face-mask

7. Tommy Technetium, Testing Mask Effectiveness with Liquid Nitrogen. https://www.youtube.com/watch?v=q99HwzAcR7k