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 mm) fall onto surfaces, while the smaller aerosols (< 5 mm) 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.
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
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
Safety: Video Demonstration
Safety: Video Demonstration
Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.
Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.
Anyone thinking about performing a chemistry demonstration should first read and then adhere to the ACS Safety Guidelines for Chemical Demonstrations (2016) These guidelines are also available at ChemEd X.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
Evaluate a Solution to a Real World Problem is a performance expectation related to Engineering Design HS-ETS1.
Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.
Evaluating potential solutions-In their evaluation of a complex real-world problem, students: Generate a list of three or more realistic criteria and two or more constraints, including such relevant factors as cost, safety, reliability, and aesthetics that specifies an acceptable solution to a complex real-world problem; Assign priorities for each criterion and constraint that allows for a logical and systematic evaluation of alternative solution proposals; Analyze (quantitatively where appropriate) and describe* the strengths and weaknesses of the solution with respect to each criterion and constraint, as well as social and cultural acceptability and environmental impacts; Describe possible barriers to implementing each solution, such as cultural, economic, or other sources of resistance to potential solutions; and Provide an evidence-based decision of which solution is optimum, based on prioritized criteria, analysis of the strengths and weaknesses (costs and benefits) of each solution, and barriers to be overcome.
Refining and/or optimizing the design solution: In their evaluation, students describe which parts of the complex real-world problem may remain even if the proposed solution is implemented.