What became the 3M N95 respirator was the brainchild of Sara Little Turnbull, one of America's first female industrial designers, whose curiosity took her on anthropological research trips to Borneo, Malaysia, the Philippines, India, and Kenya.
Her design for a pot lid was inspired by observing cheetahs grasping their prey in the wild. “It always starts with a fundamental curiosity,” she said of her quest for innovative product design. “When I can't find the answer in a book, I go out and search for it. The excitement of my life is that I have always jumped into the unknown to find what I needed to know.” In another case, she began the design process for a burglar-proof lock by interviewing thieves in jail.
The same curiosity lead to her making the connection that melt-blown fibers used in ribbons and bras could be applied to respirators, which even retained their original bra cup shape:
The final step in the process that made N95 masks a healthcare standard was a crucial innovation by University of Tennessee professor Peter Tsai, which made the respirators effective at blocking very small particles:
In producing the masks, whose filters help block and contain a minimum of 95 percent of all particles, Tsai used a method called corona electrostatic charging...
The technique, which earned him a US patent in 1995, uses an electric field to ionize the neutral air to generate ions and electrons, which then charge the nonwoven fibers through field and induction. Using his innovative approach, the charged nonwoven fabric can filter particles in the air ten times more efficiently than uncharged fabrics without adversely increasing the air resistance. It was one of five patents Tsai earned that year.
In this guide, we'll focus on common types of respirator masks useful for everyday wear - disposable N95-equivalent, medical masks, and reusable cloth masks. We'll also talk about fit and cleaning.
For purposes of slowing the spread of COVID-19, any face covering is better than none.
Masks vs. respirators
3M explains that
respirators are designed to help reduce the wearer’s exposure to airborne particles. The primary purpose of a surgical mask is to help prevent biological particles (e.g. bacteria and viruses) from being expelled by the wearer into the environment.
That is, respirators are designed to protect the wearer, and masks, to protect others from the wearer. This distinction matters in many of the cases these devices are designed for - say, mining or painting cars or open-heart surgery - but not for us, as both are nearly equally effective in reducing viral transmission through the air. And while the focus of respirator design has been on protecting the mask wearer in industrial environments, there is a special class of medical-oriented respirators.
Other nuances are described by 3M in detail, Respirators and Surgical Masks: A Comparison.
Common shapes and types
The WHO outlines forms of respirators with these technical specifications:
Valve vs. non-valve
Are two masks better than one?
Some researchers at Huazhong University of Science and Technology have done related research, using a single disposable medical mask as a double-layer disposable medical mask to superimpose and compare the filtration efficiency of the two particles at different flow rates.
The results show that at low suction flow rate (10-70 L / min) there is almost no difference in particle filtration efficiency between the two groups. At higher suction flow rate (80-100 L / min), the filtration efficiency of the double mask Significantly higher than single-layer masks.
Table 2: Particulate filtration efficiency of different types of masks under different suction flow rates
However, in actual wearing, it is unlikely to have a flow rate of 80-100 L per minute! The multi-layer mask not only does not enhance the protective effect, but also reduces the wearing comfort.
Fitting and wearing masks
In an ideal scenario, and what's recommended for medical professionals:
When a disposable particulate respirator is put on, it should be fit-tested and a seal check should be done. If used with goggles, the particulate respirator should be fluid-resistant. Fluid resistance is not required if the particulate respirator is used together with a face shield. Not all particulate respirators are fluid-resistant; for example, N95 respirators are fluid-resistant only if they are labelled as “surgical N95 respirator”
A respirator should fit tightly, forming a complete air seal. More on fit tests for N95 respirators, below.
Masks do come in kid's sizes.
Note that sizing and fit may be different for N95 masks and Chinese-standard KN95s, where the design is informed by findings that facial characteristics in China vary widely from the American reference model.
Putting on (donning) a respirator
Putting on a mask is known as donning.
First, you need to conduct a fit test and a seal test to make sure the respirator sits as tightly as possible and does not leak. This is far easier said than done and these very boring instructions are surprisingly hard to execute consistently and carefully; even when executed properly, it's not a guarantee that the seal is perfect.
There are two types of fit tests: qualitative and quantitative.
Qualitative fit testing is a pass/fail test method that uses your sense of taste or smell, or your reaction to an irritant in order to detect leakage into the respirator facepiece. Qualitative fit testing does not measure the actual amount of leakage. Whether the respirator passes or fails the test is based simply on you detecting leakage of the test substance into your facepiece. There are four qualitative fit test methods accepted by OSHA:
- Isoamyl acetate, which smells like bananas
- Saccharin, which leaves a sweet taste in your mouth
- Bitrex, which leaves a bitter taste in your mouth
- Irritant smoke, which can cause coughing.
At home, you can try smelling something like chilis, or burning sugar, anything smoky or strong. The mask should block or significantly reduce the odor.
The CDC's step-by-step instructions, Donning PPE: Put on N95 Respirator:
- Put on the N95 respirator.
- Hold the respirator in the palm of your hand with the straps facing the floor.
- Place the N95 respirator on your face covering your nose and mouth.
- Pull the bottom strap up and over top of your head, and put it behind your head below your ears.
- Take the upper strap and put it behind your head towards the crown of your head.
- Mold the nose piece of the respirator over the bridge of your nose to obtain a tight seal.
Fit/Seal Check (via the New York State Department of Health):
- Negative pressure check
- Place both hands completely over the mask and inhale sharply. Be careful not to disturb the position of the mask. The mask should pull into your face. If air leaks around your face or eyes, adjust the nosepiece and straps and repeat the positive pressure check.
- Positive pressure check
- Put your hands over the mask and breathe out sharply. If your mask has an exhalation valve (like the one pictured above) be sure to cover the exhalation valve when you exhale. No air should leak out of the mask if it fits properly. If air leaks out, readjust the nosepiece and straps and repeat the negative pressure check.
Here's another good video tutorial.
Note that fit tests are only needed for tightly sealing respirators, not surgical masks. A seal test verifies that a complete seal is made. This especially challenging for children (who need smaller respirators, which are more difficult to find) and those with beards.
We're including multiple videos here, because most people do not wear respirators correctly and the seal tests are difficult to execute accurately:
The most likely explanation is: doctors aren’t much better at using respirators than anyone else. In a California study of tuberculosis precautions, 65% of health care workers used their respirators incorrectly. That’s little better than the general public, who have a 76% failure rate. Bunyan et al note... Fit-testing is a laborious task, taking around 30 min to do properly... self-checking for a seal has been demonstrated to be a highly unreliable technique.
Another study found that
all subjects failed the unassisted qualitative fit test on the first exercise (normal breathing). Eighteen subjects failed the assisted qualitative fit tests; 60% failed on the first exercise.
This problem is exacerbated by conditions that make fit impossible, like beards (those that touch the edge of the mask, blocking the skin contact seal):
Other risks besides improper fit:
- Touching the front of your respirator or mask, which you should always assume is contagious.
- Taking risks you wouldn't otherwise take without this protection. Face coverings are meant to lower your exposure risk, and even then, perhaps only slightly. Wearing one should remind you to be even more, not less cautious.
Taking off (doffing) a respirator
Removing a respirator is called doffing.
The process is similar for that of medical masks. See below.
Can you reuse masks?
Conventional wisdom and earlier CDC recommendations said no, masks and respirators are single-use disposable items. It's even recommended to discard medical masks after every patient interaction.
But in the current climate, we need to improvise and make supplies last.
A paper, on the protective efficiency and wearing time of medical protective masks gives us some ideas on how long masks can really last. N95 masks were found to be effective for much longer than advertised.
Smart Air conducted less formal tests which found only a 2% reduction in efficiency over 11 days.
The CDC agrees that the effective mask lifetime is longer than manufacturer guidance would suggest, but that
extended use is favored over reuse because it is expected to involve less touching of the respirator and therefore less risk of contact transmission
There are a few ways to clean and decontaminate respirators, all of which do some damage, eventually rendering the respirator unusable, either physically destroyed or no longer filtering efficiently. So the goal is to disinfect the respirator as effectively as possible while causing as little damage, especially to the filtering capacity, as possible.
While washing them with soap and water or alcohol will reduce the respirators filtering capacity, there are better ways.
The CDC outlines all of the known methods, here. Their list is more extensive than our summary.
High doses of sterilizing ultraviolet light were found effective in Effects of Ultraviolet Germicidal Irradiation (UVGI) on N95 Respirator Filtration Performance and Structural Integrity:
We found that UVGI exposure led to a small increase in particle penetration (up to 1.25%) and had little effect on the flow resistance but
At the higher UVGI doses, the strength of the layers of respirator material was substantially reduced (in some cases, by >90%)
Holland's RIVM conducted a pilot study which found that
FFP2 face masks retained their shape and were able to retain particles in a ‘quick’ test after sterilizing once and twice with a short hydrogen peroxide process. In times of scarcity, FFP2 masks can be used three times when sterilized twice with hydrogen peroxide in between use.
This use of VHP is also being employed in the United States.
Heat from an oven
Here's a Reddit thread where people are testing this method.
Steam (water vapor)
The CDC reports that:
Steam treatment may be a suitable approach for decontaminating FFRs. The limited number of studies for steam report minimal effect on FFR filtration and fit performance and a minimum 99.9% reduction in H1N1 and bacteriophage MS2.. used microwave steam bags, designed for disinfecting infant feeding equipment, to decontaminate six FFR models and achieved 99.9% inactivation of MS2 bacteriophage. Filtration performance of all tested FFRs scored above NIOSH certification requirements... Using microwaves to produce steam to decontaminate FFRs is not without limitations. Not all microwaves are constructed the same and some are more powerful than others. The effect of higher power microwaves on FFRs is unknown. Furthermore, the metal nosebands of FFRs may cause arcing, sparks inside the microwave oven, during exposure to microwaves.
EOW/ECA can, in theory, be used to disinfect respirators.
The same Stanford research as above says:
Electrolyzed water is produced by the electrolysis of water containing dissolved sodium chloride (salt). This electrolysis produces a slightly corrosive solution of hypochlorous acid and sodium hydroxide. The resulting water can be used as a disinfectant. It can kill some viruses in 5 seconds if used immediately and within 5 minutes if used within 48 hours of production. One challenge is that it weakens when it is in contact with proteins such as body fluids, like blood, mucous, stool or vomit. The other challenge is that at least one manufacturer will void the warranty if electrolysed water is used on their equipment due to corrosive activity. Electrolyzed water loses its potency over time (> 48 hours) and needs frequent monitoring to maintain correct potency.
The CDC also lists moist heat incubation, liquid hydrogen peroxide, and ethylene oxide as alternatives.
A minor problem faced by medical facilities is how to actually store respirators and masks and the theoretical risk of them breeding bacteria. A 3M bulletin explains:
These studies found there were surviving organisms immediately after loading and that they survived for varying lengths of time depending on the storage conditions of the study. Usually storage under high-humidity conditions was the most favorable for long term survival. However, these storage conditions are not typical of respirator storage practices in most respirator programs. Storing filtering facepieces used against bioaerosols in resealable plastic bags may be inappropriate, since the filters may be moist from use, and storage in plastic will keep the humidity level high.
These concerns have prompted some to state that a traditional filter without a nanoparticle coating of a biocide would turn into a breeding ground for a virus or bacterial agent. The studies mentioned above do not support this claim.
One investigation of respirators incorporating antimicrobial-treated filter media found that there was non-detectable or no effect on the viability of penetrating particles. Another study found an insignificant difference in the fractions of surviving organisms captured on untreated filters and those filters treated with iodine and similar environmental conditions.
How are disposable respirators made?
The respirators most in demand by medical professionals, those produced under the N95 standard and its competitors, aren't made from off-the-shelf materials like cotton (but it's also a lot less complicated than it sounds). They require rolls of melt-blown, nonwoven polymer nanofibers. A polymer melt (polystyrene, for instance, familiar as the material used for packing foam) is extruded through small nozzles, while blowing gas deposits the fibers in a random pattern.
The majority of the material is manufactured in China.
In 2019, China's polypropylene (one of the raw ingredients used to make melt-blown fiber) production capacity was 52 million tons. In 2020, China's newly expanded propylene production capacity reached 7.63 million tons per year. According to estimates, one ton of melt-blown fabric can produce 300,000 N95 masks.
We plan on making a post specifically on mask manufacturing soon, with original videos and photos from one of the many micro-factories emerging in China to meet demand.
Are the masks sterile?
Disposable medical masks belong to Class II medical devices, and must be produced in a purification workshop of class 100,000 or above...
Before entering the production workshop, the staff took the reporter to a special area where they wore hats, masks, shoe covers, changed protective clothing and disinfected their hands...
"You can't directly enter the workshop in this way." Feng Yuexiang, director of the mask production line, said that everyone had to go through a special dust removal room separately. After the door is closed, the airflow in the room will "wind shower" 360 degrees to the human body, blowing away the dust attached to the surface of the body and clothing...
The entire production process of medical masks should be completely sterile, and every part of the machine must be disinfected before production in the workshop every day. This is also the biggest difference from ordinary civilian masks.
To curb the spread of COVID-19 in the general population, all of these standards are functionally identical. Their principle benefit for this case over simpler surgical or cloth masks with particulate filtration is tighter fit.
Because SARS-CoV-2 is primarily transmitted via air droplets (it can be an aerosolized) - even speech
generates thousands of droplets that are otherwise hidden to the naked eye - the superior capabilities of N95 and similar mask standards at blocking small airborne particles, like pollution or toxic paint fumes, are not needed.
N95, the US standard
These masks are not resistant to oil aerosols - that's the n in N95. The typical application for N95 masks is in mining, construction, and painting, where oil resistance is a necessity. For our purposes, it's not relevant.
The standard requires the mask to filter at least 95% of airborne particles of 0.3μm in size, more than two hundred times smaller than the width of a human hair.
These are the common type of respirator worn by medical professionals in the US (between N95 respirators, powered air purifying respirators (PAPR), or half- or full-facepiece respirators):
In this analysis, 1,904 (18%) healthcare respondents reported wearing respiratory protection and were classified as respirator users. The proportion of respirator users was highest for aerosolized medications (ribavirin, 79%; pentamidine, 56%; and antibiotics, 26%) and lowest for chemical sterilants (0%). The proportion of respirator users caring for patients with ILI symptoms was 21%, and less than 11% for all other hazards. Use of an N95 respirator was reported the most often (93%) of the respirator types. The most common reasons for not using a respirator when potentially exposed to the hazardous chemicals were “not part of our protocol” and “exposure was minimal.” Only 25% of respirator users reported that they had been fit tested.
Surgical N95 respirators
A surgical N95 respirator is a NIOSH-approved N95 respirator that has also been cleared by the FDA as a surgical mask. The main distinction is that surgical masks are required to protect the wearer from direct splashes and sprays of infectious blood or body fluids. NIOSH provides a list of all such approved models. Approved vendors are:
|Supplier/Manufacturer||Model/Product||Approval Number||User Donning Instructions|
|84A-0006||[PDF – 72 KB]|
|1870||84A-3844||[PDF – 190 KB]|
|1805||84A-5469||[PDF – 330 KB]|
|1805S||84A-5470||[PDF – 330 KB]|
|1870+||84A-5726||[PDF – 285 KB]|
|1804||84A-7789||[PDF – 298 KB]|
|1804S||84A-7790||[PDF – 298 KB]|
|Aero Pro Company, LTD
|AP0018 N95||84A-4049||[PDF – 63 KB]|
|AP0028||84A-4175||[PDF – 65 KB]|
|Alpha Pro Tech
|MAS 695||84A-0457||[PDF – 211 KB]|
|Ammex Corporation [*E]
|N95F||84A-4541||[PDF – 5.6 MB]|
|N95CMA||84A-5411||[PDF – 538 KB]|
|Cardinal Health [*E]
|N95-ML||84A-3323||[PDF – 842 KB]|
|N95-S||84A-4107||[PDF – 5.6 MB]|
|N95A-ML||84A-5411||[PDF – 538 KB]|
|N95A-S||84A-5463||[PDF – 538 KB]|
|Cardinal Health [*Z]
|USA-N95-R||84A-5527||[PDF – 3.2 MB]|
|CVS Pharmacy, Inc. [*E]
|399575||84A-5411||[PDF – 586 KB]|
|Dentec Safety [*E]
|AD4N95||84A-4541||[PDF – 581 KB]|
|AD2N95A||84A-5411||[PDF – 538 KB]|
|Dynarex Corporation [*E]
|2295||84A-3323||[PDF – 842 KB]|
|2296A||84A-4541||[PDF – 5.5 MB]|
|2295A||84A-5411||[PDF – 538 KB]|
|Emerald Medical Inc. [*E]
|N9500||84A-3323||[PDF – 2.7 MB]|
|Emerald Professional Products [*E]
|N9500A||84A-5411||[PDF – 586 KB]|
|First Aid Direct [*E]
|305050A||84A-5411||[PDF – 538 KB]|
|GlaxoSmithCline Consumer Healthcare [*C]
|ActiProtect UF||84A-5163||[PDF – 59 KB]|
|Halyard Health, Inc.
|46827 (FDA)||84A-7518||[PDF – 561 KB]|
|46828 (FDA)||84A-7519||[PDF – 561 KB]|
|46867 (FDA)||84A-7520||[PDF – 561 KB]|
|46727 (FDA)||84A-7521||[PDF – 561 KB]|
|46728 (FDA)||84A-7522||[PDF – 561 KB]|
|46767 (FDA)||84A-7523||[PDF – 561 KB]|
|42355 (FDA)||84A-7524||[PDF – 561 KB]|
|42126 (FDA)||84A-7525||[PDF – 561 KB]|
|Inovel, LLC [*J]
|84A-0013||[PDF – 375 KB]|
|84A-0005||[PDF – 1.07 MB]|
|84A-0010||[PDF – 1.07 MB]|
|Livingstone International Pty, Ltd. [*E]
|FMN95RN||84A-5411||[PDF – 538 KB]|
|Louis M. Gerson Company, Inc.
|1730||84A-0160||[PDF – 122 KB]|
|84A-4123||[PDF – 122 KB]|
|Magid Glove and Safety, LLC [*E]
|910-N95||84A-3323||[PDF – 842 KB]|
|Makrite Industries, Inc.
|910-N95||84A-3323||[PDF – 842 KB]|
|910-N95S||84A-4107||[PDF – 5.6 MB]|
|910-N95FMX||84A-4541||[PDF – 5.6 MB]|
|9500-N95||84A-5411||[PDF – 538 KB]|
|9500-N95S||84A-5463||[PDF – 615 KB]|
|Medline Industries, Inc. [*E]
|NON24506||84A-3323||[PDF – 842 KB]|
|NON24507||84A-4107||[PDF – 5.6 MB]|
|NON24506A||84A-5411||[PDF – 538 KB]|
|NON24507A||84A-5463||[PDF – 538 KB]|
|84A-0013||[PDF – 375 KB]|
|84A-4339||[PDF – 375 KB]|
|1510||84A-5171||[PDF – 375 KB]|
|84A-5172||[PDF – 375 KB]|
|1712||84A-5227||[PDF – 127 KB]|
|Pasture Pharma Pte. Ltd. [*E]
|PM10||84A-4541||[PDF – 85 KB]|
|PM15||84A-5411||[PDF – 112 KB]|
|Pasture Pharma Pte. Ltd. [*Q]
|84A-7453||[PDF – 1.3 MB]
[PDF – 1.3 MB]
|84A-7454||[PDF – 3.1 MB]
[PDF – 3.3 MB]
|Precept Medical Products [*E]
|65-3395||84A-3323||[PDF – 182 KB]|
|65-3395S||84A-4107||[PDF – 182 KB]|
|65-3395||84A-5411||[PDF – 586 KB]|
|65-3395S||84A-5463||[PDF – 586 KB]|
|RP88020||84A-5216||[PDF – 256 KB]|
|USA-N95-S||84A-5229||[PDF – 1.8 MB]|
|RP88010||84A-5475||[PDF – 187 KB]|
|USA-N95-R||84A-5527||[PDF – 3.2 MB]|
|Protective Industrial Products. [*E]
|270 3000||84A-4541||[PDF – 5.6 MB]|
|270-2000A||84A-5411||[PDF – 538 KB]|
|Pyramex Safety Products [*E]
|RM10||84A-5411||[PDF – 586 KB]|
|Safety Zone, LLC. [*E]
|RS-900-N95A||84A-5411||[PDF – 538 KB]|
|San-M Package Company, Ltd.
|84A-3348||[PDF – 61 KB]|
|Shanghai Dasheng Health Products Manufacture Company, Ltd.
|DTC3M-1||84A-4331||[PDF – 247 KB]|
|DTC3B||84A-4336||[PDF – 247 KB]|
|Shanghai Gangkai Purifying Products Company, Ltd.
|GIKO 1400||84A-4282||[PDF – 78 KB]|
|Sperian Respiratory Protection USA, LLC [*C]
|HC-NB095||84A-4357||[PDF – 33 KB]|
|HC-NB295F||84A-4371||[PDF – 33 KB]|
|HC-NB095F||84A-4372||[PDF – 33 KB]|
|HC-NB295FP||84A-5667||[PDF – 70 KB]|
|1730||84A-0160||[PDF – 122 KB]|
|Zhou Medical Solutions, LLC [*E]
|84A-5411||[PDF – 587 KB]|
|84A-5463||[PDF – 587 KB]|
| A – Private label of Louis M. Gerson Company, Inc. (800-225-8623)
AA – Private label of Foss Manufacturing Company (603-929-6000)
C – Private label of Honeywell Safety Products (800-430-5490)
E – Private label of Makrite Industries, Inc. (617-964-1365)
J – Private label of Moldex-Metric, Inc. (800-421-0668)
Q – Private label of Champak Enterprise Company, Ltd. (886-3-3808818)
Z – Private Label of Prestige Ameritech (817-427-2700)
How N95 masks filter out small particles
But a SARS-CoV-2 virion ranges from 80–120 nm in diameter (or 60 to 140 nm, depending on who you ask), which is 0.08 to 0.12 μm (or 0.06-0.14 μm), a significantly smaller particle than the 0.3 μm particles the N95 standard is rated to filter out. So how can these masks possibly be effective?
The virus doesn't travel in an ambient cloud with you, pollinating everyone around it with its glow. It's a respiratory illness, largely transmitted through droplets you expel by coughing - at upwards of 60mph (100km/h) - or even speaking.
Regarding the transmission of IRD, there are many widely accepted facts: a) IRD are transmitted via droplets originating in the respiratory system of an infected individual, b) non-infected individuals could be infected via direct, indirect and/or airborne route, c) infection could occur at a very short distance but also at a very long distance, and d) cough is the most representative source of droplets expelled as aerosol. Consequently, viruses such as Influenza A H1N1 are transmitted when an infected patient expels droplets of different sizes loaded with pathogenic microorganisms, to the surrounding environment as an aerosol during coughing
Which raises the question: what size are these droplets?
An experiment was carried out:
They determined that
97% of droplets, expelled in one second during coughing, are smaller than 1 μm, 2.7% of droplets are between 1 - 10 μm.
Of these very small particles that make up the majority of what we cough out, a substantial portion fall into the range masks are rated to filter:
But there's still something strange about this. Why 0.3 μm? Why not test with smaller particles? Because it's paradoxically easier to filter out smaller particles. The 0.3 μm test is the more challenging one.
How could this be?
Since filters are tested against the most penetrating particle size of 0.3 μm an APR with a P100 classification would be at least 99.97% efficient at removing particles of this size. Particles with a size both less than and greater than 0.3 μm are filtered at an efficiency greater than 99.97%. Although it is counter-intuitive that particle sizes of less than 0.3 μm are filtered with a greater efficiency, the forces which have the greatest impact on the effectiveness of filtration (aerosol impaction, interception, and diffusion) are weakest at this size for filters tested by NIOSH.
Once we reach the nanoscale, these very small particles aren't captured by a filter, but by a process known as Brownian diffusion (or Brownian motion),
the characteristic random wiggling motion of small airborne particles in still air, resulting from constant bombardment by surrounding gas molecules. It is
the dominant particle deposition mechanism for small particles ( 0.1 μm) over short distances.
At low air speeds, the randomly moving particles in the air collide into one another, and are pushed into the mask's interwoven fibers. This is how filters act more efficiently at smaller particle sizes, while 0.3 marks the barrier zone that's the most challenging.
The CDC explains the multiple ways in which particle filters operate. And for more, check out the smart guys at Smart Air who demonstrated the ability of masks to filter these tiny particulates.
If you're infected (often unknowingly), you can reduce the distance these water droplets carrying the contagion travel and how long they linger in the air for (see our extensive explanation of this process in our Case for Masks article) with the mask barrier.
If an infected person's droplets linger in the air and you pass through them while blocking access of water-droplet sized particles into your respiratory system, you're less likely to be infected. Even reducing the quantity of the virus - the aggregate viral load - is sufficient in many cases to avoid infection, as your immune system fights off smaller quantities of the virus.
You can see the size and distribution of droplets dispersed from coughing, below. Without a mask (left), there's a far greater density of droplets, up to 6000 droplets/cm3. With masks (right), that falls to under 600 droplets/cm3.
Read on for empirical validation of mask efficacy.
There is an additional layer of certification for those devices which are both approved by NIOSH as an N95 respirator and also cleared by the Food and Drug Administration (FDA) as a surgical mask. These products are referred to as Surgical N95 Respirators.
Though stringent, the N95 standard is by no means the best - it's simply the American one, which means that hospitals in the US are mandated to use it. In such heavily regulated environments, other, equally stringent standards from abroad may as well not exist.
|Type||0.3μm particle penetration|
|N95, R95, P95||Blocks >= 95%|
|N99, R99, P99||Blocks >= 99%|
|N100, R100, P100||Blocks >= 99.97%|
Note that the N95 standard says nothing about the shape of the mask per se:
How long can I use an N95 mask for?
FFP2, the European standard
The European counterpart to the American N95 standard, FFP is certified by the European Union. The requirements test for "inward leakage", penetration of filter material, flammability, breathing resistance and more.
|Type||Max inward leakage||Aerosol penetration||Band color|
|FFP1||22-25%||Blocks 80%||Yellow elastic bands|
|FFP2||8-11%||Blocks 94%||White or blue elastic bands|
|FFP3||2-5%||Blocks 99%||Red elastic bands|
FFP2 is the recommended standard for the current situation. FFP3 masks are both less common and much less breathable, usually requiring an exhaust valve, which makes them useless for protecting others from your exhaled water droplets.
Here's the design of the test chamber for this standard:
KN95, the Chinese standard
KN95 is equivalent to N95 and specified by the Chinese standards body in GB2626-2019, "Technical requirements for medical protective masks." The protection specified in the standard include various types of particulate matter, including dust, smoke, mist and microorganisms. Dust resistance rate, breathing resistance, detection methods, product identification, packaging, etc. have strict requirements. The standard divides the mask into three grades: 90 (KN90, KP90), 95 (KN95, KP95), and 100 (KN100, KP100) according to the filtering efficiency of the mask.
As explained in more detail below, they're functionally nearly identical to N95 masks.
Some telltale characteristics of KN* masks:
- Usually folds in middle vertically (half-cup shape)
- Some are 3 layers, some are 4 layers, some are 5 layers
The US FDA has provided a list of Authorized Imported, Non-NIOSH Approved Respirators Manufactured in China.
P2, Australian/New Zealand standard
P2 is the equivalent in Australian/New Zealand Standards, defined in AS/NZS 1716.
- Class P1
- Intended for use against mechanically generated particulates of sizes most commonly encountered in industry. Has a low to medium absorption capacity filter
- Class P2
- Intended for use against both mechanically and thermally generated particulates. Has a medium absorption capacity filter
- Class P3
- Intended for use against all particulates including highly toxic materials. Has a high absorption capacity filter. Nevertheless this can only be achieved in a full face respirator
Korea 1st class, aka KF94
The Korean counterpart of N95 is Korea 1st class, defined in KMOEL-2017-64, commonly referred to as KF94.
- Often folds horizontally (duckbill/cup shape)
- Duckbill-shaped masks does not touch the lips and is a bit easier to speak while wearing.
Other national standards
- The Japanese equivalent is DS/DL2, DS/DL3, defined in JMHLW-2000.
- Mexico also refers to its respirators as N95, P95, R95, N99, P99, R99, N100, P100, R100, defined by their standard, NOM-116-2009.
- Brazil and others have defined their own standards as well.
The FDA has released a list of overseas manufacturers of non-NIOSH approved respirators.
N95 vs. FFP2 vs. KN95 vs. P2 vs. KF94 vs. DS
Now that we're familiar with all of the standards, let's see how they compare.
Respirator-makers 3M published a thorough side-by-side analysis of rival national mask standards.
Based solely on these standards, you can expect N95s to be slightly more breathable, and KN95s to have a different fit. Korean KF94's have a more comfortable (and frankly, stylish) fit, a style recommended by the WHO (for what that's worth):
Use a fluid-resistant medical or surgical mask with a structured design that does not collapse against the mouth (e.g. duckbill or cup shaped).
||≥ 95%||≥ 94%||≥ 95%|
|Test agent||NaCl||NaCl and paraffin oil||NaCl|
|Flow rate (of test agent)||85 L/min||95 L/min||85 L/min|
|TIL||N/A||≤ 8% leakage||≤ 8% leakage|
|Inhalation resistance – max pressure drop||≤ 343 Pa||
≤ 70 Pa (at 30 L/min)
≤ 240 Pa (at 95 L/min)
≤ 500 Pa (clogging)
|≤ 350 Pa|
|Flow rate (inhalation)||85 L/min||Varied||85 L/min|
|Exhalation resistance - max pressure drop||≤ 245 Pa||≤ 300 Pa||≤ 250 Pa|
|Flow rate (exhalation)||85 L/min||160 L/min||85 L/min|
|Exhalation valve leakage requirement||Leak rate ≤ 30 mL/min||N/A||Depressurization to 0 Pa ≥ 20 sec|
|Force applied||-245 Pa||N/A||-1180 Pa|
|CO2 clearance requirement||N/A||≤ 1%||≤ 1%|
Conclusion: after spending a lot of time building this table, it turns out, they're basically all the same. You'll do well with any masks adhering to these standards.
Some US regulatory agencies have reached the same conclusion. They're now treating foreign standards as largely interchangeable with N95:
Medical masks are often made out of layers of breathable, paper-like synthetic fabric that is cut into a rectangular shape and has pleats to help it expand and fit more snugly around your face. They are disposable and designed to be used just once... While they can protect you from large droplets and splatter, their looser fit is partly what makes them less effective than N95s.
This type of mask fall into two categories, Procedure and Surgical. For our purposes, the difference is immaterial, so skip to the next section if you're not interested in this minutiae.
The US CDC recommends they be discarded after each patient encounter.
Standards for medical masks are less stringing with regards to filtration ability:
Manufacturers of surgical masks... must demonstrate that their product is at least as good as a mask already on the market to obtain “clearance” for marketing. Manufacturers may choose from filter tests using a biological organism aerosol at an airflow of 28 L/min (bacterial filtration efficiency) or an aerosol of 0.1 µm latex spheres and a velocity ranging from 0.5 to 25 cm/sec (particulate filtration efficiency). It is important to note that the Food and Drug Administration specifies that the latex sphere aerosol must not be charge-neutralized.
Procedure masks vs. surgical masks
|Procedure mask||Surgical mask|
|Three or four layers of construction||Three or four layers of construction|
|Two ear loops secure mask to face||Two straps secure mask to face|
|Not suitable for OR||Primarily used by OR staff|
|Used on hospital floors, isolation, sterile, patient bedside visits, labor and delivery, ER and ICU||Intended for a high risk of fluid exposure|
The design of both is often nearly identical, though four layer construction is more often found in surgical masks than the slightly less protective procedure masks. The difference is that surgical masks are designed for sterile environments, like an operating theater, and procedure masks are designed for merely clean environments.
Given that this distinction is subtle and unimportant for our purposes (reducing Coronavirus transmission, not avoiding blood splatter from a ruptured aorta), we'll refer to these masks interchangeably as "surgical masks" or "medical masks."
Many sources online have been spreading a popular myth, that surgical masks are meant to protect the patient, not the doctor. A common example says something like,
The truth is, that unlike N95 masks that filter in both directions, a surgical mask is designed to filter the air during exhalation and not inhalation. But this is not the truth. That theory isn't borne out in the research.
Another myth is that surgical masks are effective for only a few hours and must be immediately disposed of after. In fact, they last much longer and their safe life can be extended through sanitizing.
How to don (put on) a medical mask
How doff (take off) a medical mask
Never touch the front of the mask as this should be treated as contaminated after use. There is even a recommended process for disposal. Because of the enormous volume of masks being used and disposed of during this pandemic, proper waste management of potentially contagious materials shouldn't be taken lightly.
It is best to prepare a plastic bag for unused masks. Bare masks that contact clothing or other things will increase the risk of transmission.
When removing the mask, try not to touch the outside of the mask. Remember to wash your hands immediately after removing the mask. Try not to reuse disposable medical masks. Once the mask is broken or dirty, it needs to be replaced with a new one immediately.
“In the hospital, these masks will be put into a clean, airtight bag and thrown into a yellow trash can.” Tao Xiaoqing, director of the Department of Interventional Vascular Medicine at Weihai Haida Hospital, said that medical waste is generally divided into black, yellow and red… Among them, black plastic bags contain domestic waste, yellow plastic bags contain medical waste (including infectious waste), and red plastic contains radioactive waste and other special medical waste.
Du Huanzheng, a professor at the School of Environment and Sustainable Development of Tongji University, said in an exclusive interview with the reporter of Science and Technology Daily that wearing masks is to prevent virus infection. Masks that have not been exposed to the new coronavirus are treated as general solid waste and domestic waste. However, judging from the current situation, ordinary citizens can’t judge whether they have been exposed to the new coronavirus when they go out. In accordance with the principle of caution, from the perspective of maximizing the protection of the life and health of the public, it is recommended to put used masks in sealed bags such as plastic bags. In cities such as Shanghai where garbage classification is implemented, put sealed bags in “harmful garbage” bins.
What if some cities do not have stricter rules for garbage separation and disposal? Tao Xiaoqing suggested that the mask waste at home should be placed in a bag to avoid secondary pollution. High temperature and medical 75% alcohol can kill the new coronavirus. It is recommended to use alcohol spray to sterilize the bag and seal it before discarding it.
Medical mask standards
There are many standards of this style of mask. Again, for our purposes, if the mask is suitable for a hospital environment, whether fully sterile or not, it's suitable for casual use.
SmartAir has a concise summary of the different mask standards:
But if you'd like more details, read below.
A mask made to any of these specifications will suffice:
ASTM, a US standard
ASTM International (formerly known as the American Society for Testing and Materials) is an
international standards organization that develops and publishes voluntary consensus technical standards. They've released many of their standards relevant to COVID-19 for free.
Cardinal Health found that 57% of face mask units sold in 2016 did not have or claim an ASTM rating and 48% of OR staff are unaware of ASTM standards and different levels of protection and that the ASTM standards aren't popular either as tattoos or children's names.
ASTM masks have three tiers based on the level of protection needed:
- ASTM Level 1
- Low barrier protection for general use for low-risk, nonsurgical procedures and exams that do not involve aerosols, sprays and fluids. An ear loop mask is a level 1 mask. ASTM level 1 masks are the general standard for both surgical and procedural use.
- ASTM Level 2
- Moderate barrier protection for low-to-moderate levels of aerosols, sprays and fluids.
- ASTM Level 3
- Maximum barrier protection for any situation that has the potential for exposure to heavy levels of aerosols, sprays and fluids.
The ASTM standard also specifies BFE (BFE95/BFE99), which shows the percentage of actual Staphylococcus aureus bacteria blocked, with a mean particle size of 3 μm (+/- 0.3 μm).
Adherence to the ASTM F2100 standard also requires testing for PFE, which measures the penetration fo very small particles, down to 0.1 μm. Although 3M notes that the
PFE test is a quality indicator for healthcare surgical/procedure masks. The PFE test is not an indicator of respirator protection performance.
NF EN 14683, the European standard
This standard was developed by the French national organization for standardization, AFNOR.
They define surgical masks as
intended to prevent the outward projection of droplets produced by the person wearing the mask. It also protects the wearer against the projection of droplets produced by another person. On the other hand, it does not protect against inhalation of very small particles suspended in the air.
The standard defines three categories of surgical masks:
- Type I
- Bacteria filtering effectiveness > 95%.
- Type II
- Bacteria filtering effectiveness > 98%
- Type IIR
- Bacteria filtering effectiveness > 98% and splash-resistant.
There are at least two standards for medical protective masks in China. YY0469-2011 "Medical surgical masks" and GB19083-2010 "Technical requirements for medical protective masks." Further specifications are provided in the documentation for each specific mask type.
- YY/T 0969-2013
- Single-use medical face mask. If a listings includes this standard, it means the mask is allowed to be used in Chinese hospitals. Used in general medical environments to block the pollutants exhaled and sprayed from the mouth and nose. Not suitable for surgery.
- YY 0469-2011
- Medical surgical mask for operating theaters -
The Chinese standards of GB 19083, YY 0469-2011 and YY/T 0969-2013 basically cover the classification and requirements described in the European standards, EN 14683.
You can find pictures of both styles, above.
You can also find mask accredited by ANSI or AORN, among others. They're also perfectly fine.
There are many styles of reusable masks, which we believe are the most practical solution for everyday wear, provided they're cleaned and disinfected. We'll cover only the realistic options - not belt mounted, fan-driven space suits with cartridges. As a consumer, you don't need a full or half-face powered respirator. There are cheaper and far more pragmatic solutions out there.
Note that you're unlikely to studies on the efficacy of many reusable masks, since besides the impractical "blower" setups, they're not used in medical environments.
Disinfection and cleaning are made easier with removable filters, where you need to only clean the outer shell, but you should take extra precautions as the masks will be worn and potentially exposed to the virus for far longer than disposable models.
The key is to own the most effective masks that you'll actually wear, since mask wearing should be a daily routine.
What to look for in a reusable mask
- Removable filters for easier cleaning and disinfection.
- Quality and comfort of the outer shell and ear loop material, a weak point that might stretch out or fall apart over time.
- The filter material, its grade and certifications or tests, if any.
- The layers of filtration.
- How the filter is installed. Does it allow for a complete seal? For example, some models have a silicone layer around nose which can be optionally folded up for an extra tight seal.
- Look for an optional neck-clip to not cause too much stress on the ears from daily wear. These devices are usually heavier than the disposable ones.
Styles of reusable masks
This is not an exhaustive list. Reusable masks come in many shapes, sizes and materials. The filters themselves also come in a myriad of specifications. These are all less regulated than medical masks and respirators. Most are designed with protection against dust, pollution, or pollen in mind. In the pollution masks, you'll often see PM 2.5 filtration capabilities advertised. With a strong seal, this should offer comparable filtration to N95-class respirators or HEPA filters.
Half-face unpowered respirators
Japanese Pitta mask
Small, powered masks
Homemade masks have been covered extensively online, and they are indeed much better than nothing:
Less sanguine is this study, which concluded that
a homemade mask should only be considered as a last resort to prevent droplet transmission from infected individuals, but it would be better than no protection.
Many are experimenting with DIY filter inserts:
But be aware:
We're unsure of the potential toxicity of inhaling air filtered through a coffee filter, or worse, repurposed vacuum cleaner bags. The effects of HEPA filters are also unknown.
- Can I use a vacuum cleaner bag or coffee filter as a filtering material?
- Despite their good filtering capacity, these filter types do not meet the requirement of section 5.1.8 “Harmlessness of inhaled air”. This is because these materials are likely to release irritant substances into the inhaled air that can give rise to allergy risk (in particular serious asthma attacks) and/or toxicity. In addition, these filter types do not give good results in terms of breathability. This idea perhaps does not enable production of a washable mask due to the envisaged materials.
As an alternative to makeshift filters, you can buy mask gaskets, filter inserts for any style of mask, made from nonwoven material. Though they're made from the same material as respirators, there is no certification or testing for these. But better than breathing through a coffee filter.
Sources of DIY instructions:
- The CDC: Use of Cloth Face Coverings to Help Slow the Spread of COVID-19 (sewn and non-sewn methods)
- MakerMask, which is a
source for science-based mask designs for community makers to combat the spread of COVID-19.
- The Open Source COVID19 Medical Supply Guide
- Free sewing patterns
- Advice on suitable materials
Surgical masks vs. N95-equivalent
It turns out that surgical masks are nearly as effective as N95-equivalent masks at reducing Coronavirus transmission.
One of the most robust studies to date, a multi-year investigation concluded that
surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals.
Notably they tested only surgical masks ( 62356, Kimberly-Clark), which proved remarkably effective:
We detected coronavirus in respiratory droplets and aerosols in 3 of 10 (30%) and 4 of 10 (40%) of the samples collected without face masks, respectively, but did not detect any virus in respiratory droplets or aerosols collected from participants wearing face masks, this difference was significant in aerosols and showed a trend toward reduced detection in respiratory droplets
Another study compared surgical masks and N95, clinical trial involving 2862 health care personnel, concluding that surgical masks work just as well at preventing influenza (though we should note that the flu is not a Coronavirus):
...there was no significant difference in the incidence of laboratory-confirmed influenza among health care personnel with the use of N95 respirators (8.2%) vs medical masks (7.2%).
Finally, models provide some corroboration:
A great summary of these findings, which reached the same conclusion as we have - both mask types are effective - was published by SmartAir.
Counterpoint and further reading
It's important to point out that not all researchers agree that 1. masks are effective at reducing viral transmission and 2. if they do believe masks are effective, they argue that they're less effective than expected or that surgical masks are far worse than N95s.
Do N95 respirators provide 95% protection level against airborne viruses, and how adequate are surgical masks? says that N95 masks performed worse than expected, allowing in more than 5% of viral particles, and surgical masks much worse, showing penetration levels of 20.5% and 84.5%...
at an inhalation flow rate of 85 L/min, which is about the rate advertised by many air compressors that can inflate a car tire in two minutes. The masks were tested against
aerosolized infectious agents in the size range of 10 to 80 nm.
Another dissenting, but very small study (with an N of only 4): "Effectiveness of Surgical and Cotton Masks in Blocking SARS–CoV-2: A Controlled Comparison in 4 Patients." The researchers found that
swabs from the outer mask surfaces of the masks were positive for SARS–CoV-2 with all of the four patients and concluded that
Neither surgical nor cotton masks effectively filtered SARS–CoV-2 during coughs by infected patients.
But they failed to consider two vital details: first, no N95 masks were tested. Second, they never verified whether the masks reduced the distance droplets traveled, arguably the most important feature of masks when worn universally.
This experiment did not include N95 masks and does not reflect the actual transmission of infection from patients with COVID-19 wearing different types of masks. We do not know whether masks shorten the travel distance of droplets during coughing.
See also, a nuanced review of other relevant studies on masks and thoughtful commentary at Slate Star Codex, who references these studies:
- Effectiveness of precautions against droplets and contact in prevention of nosocomial transmission of SARS
- Surgical mask filter and fit performance
- Respiratory performance offered by N95 respirators
- Do N95 respirators provide 95% protection level against airborne viruses, and how adequate are surgical masks?
- Rapid awareness and transmission of severe acute respiratory syndrome in Hanoi French Hospital, Vietnam.
Any mask is better than none. Tight-fitting masks are better than looser ones.
A study on SARS-CoV found that wearing a N95 mask is a 91% effective intervention to prevent SARS infection (beats 10x daily hand-washing)
Even with non-professional fitting, masks are not a perfect solution, but a very good one. Masks don't give you immunity while walking outside any more than seatbelts make you invincible in a car.