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April 2, 2022
A Guest Post by Tim Henthorne, B.A.Sc., P. Eng
Over the past two years, we have been flooded with opinions regarding the wearing of masks, all while trying to learn how to deal with the existential threat of COVID-19.
Unfortunately, much confusion has come from Public Health itself with its frequently changing recommendations. Direction has ranged from cautioning against wearing a mask (in case it is "worn improperly") to mandating use of masks. This has come with little guidance on how to choose a mask or how masks can protect from both viral transmission and infection.
Are cloth, surgical/medical, and N95 masks (or their equivalent KN95, KF 94 and FFP 2) better or worse than one another? Are they equally effective? Equally useless?
I hope to clear up some of this confusion around masks so that we can all make more informed decisions.
This is especially important now as many jurisdictions, including BC, are lifting mask mandates in most public spaces. This is happening despite Omicron still spreading in our communities, and BA.2 (an even more transmissible sub-variant) gaining ground around the world.
A mask is not a really fine strainer but rather a series of sticky spider webs
As I was considering how to present the physics of masks in an accurate but readily understandable way, I discovered a 6 minute video. The Astounding Physics of N95 Masks is a snappy, fast paced, illustrated explanation. It is clever, engaging and easy to share. Please consider watching it now.
For those who prefer to read and absorb information at a more ‘thoughtful’ pace, you’ll find a complete transcript of the video here.
Droplets and Aerosols
As the video explains, the size of the virus itself isn’t particularly relevant even though it is generally much smaller than the gaps (or pores) between the fibres in the mask. This is not to say that the size of the pathogen is irrelevant to how it is spread or what we need for protection. Very small sized particles are usually measured in microns which are one millionth of a metre (or one thousandth of a millimetre) and aerosols and droplets can be very small indeed; an aerosol can be smaller even than a virus particle.
The schema below shows the relative, approximate size of a micron compared to a variety of other substances. Although you’ll notice the limit of visibility is larger than a single smoke particle, smoke and some other very small sized particles can be visible when present in sufficient quantities.
Figure 1: How small is a Micron?
Source: Lanz
Germs that cause airborne illnesses need moisture to survive outside the human body and so, as noted in the video, viral particles do not usually travel through the air by themselves but rather in exhaled respiratory droplets and aerosols.
Droplets vary in size and are usually thought to be 100 microns or more. They fall to the ground or onto surfaces rapidly (within seconds or minutes) and most often near the infected person although, if people cough or sneeze violently droplets can travel further.
Particles less than 100 microns are called aerosols and can be thought of as resembling mist or cigarette smoke. Aerosols linger for much longer periods of time, even hours and accumulate in the air, especially in poorly ventilated indoor spaces, and can travel much further distances than droplets.
Graphic by Professor Linsey Marr
Figure 2. Range of respiratory particles and potential spread over distance.
Blue particles represent droplets, typically >100 microns diameter. Red particles represent aerosols, typically <100 microns.
Hippocrates, the Greek “father of modern medicine” is quoted as saying:
Whenever many men are attacked by one disease at the same time, the cause should be assigned to that which is most common, and which we all use most. This it is: which we breathe in.”
Despite this sage advice, the medical profession generally, including BC Public Health, has been slow to accept that SARS-CoV-2, the virus causing COVID-19, is airborne and spreads through aerosols.
Thus, in BC, public health measures have focused on droplets (hence the two metre rule and plexiglass) and contact spread (thus washing of hands and surfaces) and have failed to emphasize risk reduction from infectious aerosols, such as ventilation and air filtration in indoor settings or individual protection offered by ‘state of the art’ masks.
In this article, I will be using the word “mask” where others might favour “respirator”.
Please be aware that while respirators are well fitted, “specialized filtering masks” not all masks are respirators.
What does N95 mean?
The N95 mask was first introduced in 1970 by the American multinational company, 3M to protect workers from airborne contaminants in industrial settings. In 1995, Peter Tsai, a Taiwanese American Scientist, patented an ‘improved’ N95 mask with filtration material capable of blocking viruses and other pathogens, making it useful in medical settings.
Filters are designed to trap as many particles as possible and all masks have a Most Penetrating Particle Size (MPPS) due to the physics of filter technology. For example, the N95 mask is designed to trap 95% of particles at 0.3 micron. This is the particle size which will most readily pass through the mask (although 95% will be trapped). Entrapment of particles larger or smaller than 0.3 micron will be higher than 95%. This effect is modelled below; the dashed line shows the mask’s cumulative filtration efficiency.
Figure 3. Filter Efficiency per particle size (um=micron)
Red arrow highlights Most Penetrating Particle Size (MPPS) of 0.3 micron with 95% filter efficiency
(Source: Simon J. Smith, Selecting Effective Respiratory Protective Equipment For SARS CoV-2, OHCOW 2020)
Figure 4. Principles of Filtration
(Source Simon J. Smith, Selecting Effective Protective Respiratory Equipment for SARS CoV-2, OHCOW 2020)
To summarize, the principles of filtration are:
- Inertial impaction (straight line motion momentum) and
- Brownian diffusion (zigzag motion) provide the mechanisms which move the particles to the mask fibres
- Interception occurs when a particle travels past a fibre within one particle radius distance and is attracted and captured
- Sieving blocks particles larger than the pore size
- Electrostatic forces attract the particles to the mask fibres
and finally
- Electrostatic attraction and microscopic attraction between the particles and the mask fibres keep the particles stuck
Taken together, these processes ensure that the N95 mask will trap a significant percentage of droplets and aerosols, 95% or better, which are believed to contain exhaled viral material.
In other words, all sizes of aerosols of concern will be filtered at a rate of at least 95%. Molecules of oxygen and the like will still pass through freely.
What about valves?
Many industrial N95 masks have a one-way valve which makes exhalation easier but should not be used with an infectious aerosol as the valve would expel the user’s unfiltered and potentially contaminated breath onto others.
Can I reuse an N95 mask?
Yes, many times. Pascal S.C. Juang and Peter Tsai, have provided some guidance on how to reuse these masks which can be as simple as rotating through three masks, allowing a used mask to “air” for two days before reuse.
Guidance from Aaron Collins, a mechanical engineer and self-described mask nerd, is that N95 (and equivalent masks) can be worn for up to 40 hours, unless wet, soiled or the elastics having stretched such that they do not provide a snug fit.
What about fit?
As explained in the The Astounding Physics of N95 Masks, a mask is only as good as its fit. The mask must seal around the face. Leaks around the edges of the mask compromise both the wearer and bystanders near and far. The N95 mask provides a very snug fit which may cause some reluctance to wear one, but from a transmission point of view without a snug fit, the wearer and the people sharing the same air are more at risk of aerosol leaks and thus infection. N95 are in fact easy to breathe through and comfortable. Aaron Collins, the mask nerd, provides useful tips on how to assess your mask fit.
(The testing standard in all masks applies to the material only and not the fitted mask).
Masks Filter and Fit and Effectiveness
Figure 5. The 3Fs of Masks
Source: Masks 4 Canada
Isn’t a surgical or medical mask good enough?
Not all masks are created equal; even surgical masks as we will see below are not good enough to stop significant aerosol transmission.
For a simple illustration of the effectiveness of surgical masks in a clinical setting (where surgical masks are still ubiquitous), consider how often we have been told that there is a crisis in health care facilities because so many of the staff are off sick with COVID-19.
In a pandemic, where the contaminants are largely contained in aerosols, surgical masks have three significant weaknesses compared to the N95:
1. Surgical masks lack a tight fit around the whole face, increasing transmission of contaminants through gaps and effectively bypassing the mask’s filtration altogether.
2. Surgical masks look and feel ‘flimsy’ compared with thicker and more rigid N95 masks. N95s have greatly increased volume of plastic fibres (usually polypropylene) that account for many layers upon layers of much stickier ‘spider webs’. The more fibres, the more particles the mask will catch - and not release.
3. Surgical masks date back to the turn of the 20th century and were designed as a barrier against larger droplets to protect both surgeon and patient.
Surgical masks are much less effective in protecting both the wearer and others from infectious aerosols they breathe in or out.
Theoretically, all masks filter particles by one or more of the mechanisms described in the video but in the case of cloth masks with only a few layers of unknown fabric and unlikely to contain an electrostatic layer, the medium is simply inadequate to permit the ‘genius of physics’ to operate. This was confirmed in a real world study of the effectiveness of face mask or respirator use in indoor public settings for preventing SARS-CoV-2 infection. The study was carried out in California from February to December 2021, pre-Omicron.
Figure 5: Indoor Mask Type Used and Odds of Testing Positive for COVID-19
Source: CDC Morbidity and Mortality Weekly Report, February 11, 2022
For aspiring mask nerds, a paper in Nature contains some very good images and measurements of the interior of the materials and pore size distributions of cloth, surgical and N95 masks.
But if a mask doesn’t trap 100% of the virus, is it of any use?
Clearly, no practical face mask will filter every virus particle which brings us to the false argument that if the mask lets any virus through, it is useless and thus there is no point in wearing one. Consider what Paracelsus said in 1538 and is no less relevant today,
dosis sola facit venenum”
“only the dose makes the poison”
In the case of COVID-19, the “dose” can be thought of as the concentration of virus combined with the length of exposure. The exposure dose is thought to be correlated with the subsequent risk of infection and the severity of the illness.
The choice is yours; do you want a potentially lethal dose or a much reduced one?
We all need to take airborne transmission of SARS-CoV-2 seriously and understand the critical role that ventilation, air filtration and effective masking can play in disease prevention for both acute COVID-19 and Long COVID.
Replacing surgical/ medical masks with an N95 mask (or equivalent) is a simple, prudent and highly effective means of increasing personal protection for everyone and can be especially empowering and reassuring for individuals at high risk or with high risk family members and everyone else.
Many Public Health agencies including BC, have dropped masking protection in indoor public places. This will lead in many instances, to one-way masking which we know is not as effective as universal masking.
In many jurisdictions including in BC, masking protections are also being dropped before improved indoor air quality standards are brought in and while BA.2 is becoming the dominant sub-variant, thus wearing the best mask possible in terms of fit and filtration is more important now than ever.
About the Author
The author is a Vancouver, B.C. professional engineer (mechanical) with 35 years of experience analyzing and designing mechanical systems in the fields of aviation, high tech, automation, and laser imaging. He has designed cleanrooms (and devices which must operate in cleanrooms) where extremely clean airflow and filtration allow for the production of critical mechanical and electronic components including computer chips. This work included the study and design of particle measuring, air flow and filtration.
Additional resources:
1.How to avoid counterfeit N95 or equivalent mask?
US CDC lists all N95 masks approved by NIOSH (National Institute for Occupational Safety and Health)
How to find a Quality Mask (and Avoid Counterfeits)
Mask Regulatory Consideration and Standards
2. Where can you purchase masks online purchase? (suggestions only, not endorsement)
