The Definitive Guide to Masks

A thorough analysis of common types of respirator masks and how to fit and wear them.

Table of Contents

  1. Introduction
  2. Masks vs. respirators
  3. Respirator standards
  4. Medical masks
  5. Reusable masks
  6. DIY masks
  7. Surgical masks vs. N95-equivalent
  8. Conclusion
  9. Footnotes


3M 2091 filters

Modern respirators emerged from the nightmare of the 1910 Manchurian pneumonic plague, which killed nearly all infected victims within days. The Chinese Imperial Court brought in a young Hakka-Chinese doctor from Penang, Malaysia, named Wu Lien-teh (born Ngoh Lean Tuck), who developed a gauze mask which proved effective at preserving the lives of medical staff.

Here is the story, in his own words.

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:

Sara Little Turnbull
The untold origin story of the N95 mask (Fast Company)

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

Medical mask with ties
A medical (surgical) mask
An N95 respirator

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.

Battle on the USS COVID-19

We'll discuss the more practical options here, but do note that respirators also come in the form of PAPR, or "blowers." These offer the maximum level of protection, provided they're regularly cleaned and maintained. The filtration system can be face or belt mounted.

Powered respirators

Some models look like WWI-era gas masks (left) and others, like helmet system shown above, would do well as props in a low-budget space opera.

Common shapes and types

The WHO outlines forms of respirators with these technical specifications:

Particulate respirator (fluid-resistant) Particulate respirator (non-fluid-resistant)

Personal protective equipment for use in a filovirus disease outbreak: Rapid advice guideline (WHO)

Valve vs. non-valve

Respirator with valve

Some respirator models have an exhaust valve in the front, which is supposed to make them more breathable (this is debatable), at a cost of expelling potentially contagious droplets into the vicinity.

For this reason, we do not recommend respirators with valves. If you must wear one, you can tape the valve closed or add a second layer of surgical mask to cover it.

Are two masks better than one?

The only time two masks make sense is when a non-valved mask is used to cover a respirator with a valve to prevent contaminated exhaust from escaping. Otherwise, reports that:

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.

How to Perform a User Seal Check with an N95 Respirator

OSHA produced a good instructional video on this process.

Specific instructions from individual manufacturers can be found here.

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:

  1. Put on the N95 respirator.
  2. Hold the respirator in the palm of your hand with the straps facing the floor.
  3. Place the N95 respirator on your face covering your nose and mouth.
  4. Pull the bottom strap up and over top of your head, and put it behind your head below your ears.
  5. Take the upper strap and put it behind your head towards the crown of your head.
  6. Mold the nose piece of the respirator over the bridge of your nose to obtain a tight seal.
Donning PPE: Put on N95 Respirator

Always follow the manufacturer’s instructions for wearing a respirator. Perform a fit check to ensure there is a good seal against the skin.

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):

Facial Hairstyles and Filtering Facepiece Respirators
Unfortunately, beards preclude the possibility of a complete seal, so respirators won't work to their full capacity if you have facial hair. The CDC has provided a very detailed infographic.

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.

Study on the protective efficiency and wearing time of medical protective masks

After an hour of wear, the average filtration efficiency of N95 masks tested was 97.4%.

After the second day of use, filtration efficiency was measured at over 95%, six days, 90%, and after fourteen days, still over 80%.

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.

Can Facial Masks be Disinfected for Re-use?
Researchers at the Stanford University School of Medicine compared respirator disinfection methods. They found that using an autoclave, 160C dry heat, 70% isopropyl alcohol, and soap and water (20-min soak) caused significant degradation to filtration efficiency.

The CDC outlines all of the known methods, here. Their list is more extensive than our summary.

Ultraviolet irradiation

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%)

Hydrogen peroxide

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

Oven heat

Researchers at Stanford found that respirators could be disinfected with a simple method, leaving them in the oven for 30 mins at 70°C (158°F).

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.

Other methods

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.

Storing respirators

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.

Melt-blown fiber production

Most of this melt-blown fiber is produced in simple factories like this. You can find the resulting product on Alibaba.

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.

They expected that mask production capacity would be increased to 35-40 million pieces per day by the end of February. Production now is estimated at over 200 million pieces per day.

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.

Respirator standards

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.

Water droplets expelled, comparison of no mask and mask
Source unknown

N95, the US standard

This is the US-standard, approved by NIOSH (a branch of the US CDC). They're usually round-shaped, with no fold, and elastic bands that go behind the head.

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:

Surgical N95 Respirators
Supplier/Manufacturer Model/Product Approval Number User Donning Instructions
3M Company
888-3M HELPS
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]
Obsolete 08/2009
84A-0013 [PDF – 375 KB]
Kimberly-Clark Corporation
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]
Moldex-Metric, Inc.
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]
Pasture E520S
Pasture E520CS
84A-7453 [PDF – 1.3 MB]
[PDF – 1.3 MB]
Pasture A520S
Pasture A520CS
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]
Prestige Ameritech
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]
SteelPro [*A]
1730 84A-0160 [PDF – 122 KB]
Zhou Medical Solutions, LLC [*E]
86-21-5431-3117 (China)
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:

Laser and sensor arrangement for Cough Aerosol detection.
Cough aerosol in healthy participants: fundamental knowledge to optimize droplet-spread infectious respiratory disease management (2012)

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:

Quantities of measured droplets in size category < 1 μm per second.
Cough aerosol in healthy participants: fundamental knowledge to optimize droplet-spread infectious respiratory disease management (2012)

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.

On the Minimum Efficiency and the Most Penetrating Particle Size for Fibrous Filters
On the Minimum Efficiency and the Most Penetrating Particle Size for Fibrous Filters (1980)

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.

The size distribution of the coughing droplet without mask The size distribution of the coughing droplet with mask
The Size and Concentration of Droplets Generated by Coughing in Human Subjects

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:

NIOSH Approved N95 Particulate Respirator, High Fluid Resistance 160 mmHg, Filtration Efficiency PFE = 99.9% @ 0.1 micron, Breathability - Delta P > 5.0 mm H2O/cm2, Flame Spread Class 1
Via Guide to Face Mask Selection and Use

How long can I use an N95 mask for?

Under normal circumstances, N95 masks are meant to operate for 8 hours of continuous usage. They are single-use, disposable items. So says the standard, anyway.

But there are increasingly sophisticated ways to extend their life.

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:

Typical apparatus used in the determination of inward leakage using sodium chloride

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.

GB2626-2006 masks
口罩是什么级别的 (2020)

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
  • Earloop

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.

Korean mask shape

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).

Test N95 FFP2 KN95
Filter performance
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:

Understanding ASTM levels of protection is key
Respirators Approved Under Standards Used in Other Countries or Jurisdictions That Are Similar to NIOSH-Approved N95 Filtering Facepiece Respirators

Medical masks

Surgical mask

These are what we typically refer to as "surgical masks."

Though they may look like folded cotton, the interior filter layer is made of the same melt-blown, nonwoven fabric as N95s.

The New York Times explains:

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 to Wear a Face Mask Safely - Medical PPE Donning and Doffing

Hold the mask with the outer layer out and pleats folding down. The outer layer is usually colored and should be away from your face. Pull to open the pleats. Press the metal nosepiece to the bridge of your nose to shape the mask to your face. Pull the earloops over your ears. Press the nosepiece to your face one more time to ensure a secure fit. Via YouTube.

How doff (take off) a medical mask

How to Wear a Face Mask Safely - Medical PPE Donning and Doffing

When you are ready to remove your mask, make sure your hands are clean and pull the earloops back over your ears. Avoid touching other parts of the mask to prevent contamination. And discard appropriately. Via YouTube.

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:

SmartAir summary

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:

Understanding ASTM levels of protection is key
Choose the right mask: The right mask is an ASTM-rated mask. (Cardinal Health)
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.

Chinese standards

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 - Used by clinical medical staff in the process of invasive operation

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.

Other standards

You can also find mask accredited by ANSI or AORN, among others. They're also perfectly fine.

Reusable masks

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.

Biker mask

Biker mask

Biker mask with replaceable filter. Filter may not seal adequately for anti-viral purposes, as these are primarily dust musks used while riding.

Newer versions of this style have velcro that reaches around the head instead of an earloop.

Common style

Cambridge Mask, Vogmask, Style-Seal, Respro

You can find these masks made by Cambridge Mask, Vogmask, Style-Seal, Respro, and others. They're characterized by an earloop and a cotton outer shell, sandwiched with a layer of non-breathable material that has a piece of plastic behind it for the replaceable round filters, which adhere with velcro. These masks do leak somewhat. It's impossible to make a complete seal with this design.

Half-face unpowered respirators

3M 2091 filters

These are often used in industrial settings where protection from toxic air particles and breathability matter. Excellent seal. Be aware that like all face coverings, you'll need to clean and sanitize them after use. They take replaceable filters and are slightly more practical than powered respirators.

Because these are non-medical masks, they don't block or filter your exhaust, as with valved respirators.

Japanese Pitta mask

3M 2091 filters

Nice-looking masks, without a filter, primarily meant to block pollen. In their tests, SmartAir found that they captured an astounding 0% of 0.3-micron particles and only 64% of larger 2.5-micron particles. They may still protect others against your coughing.

Small, powered masks

3M 2091 filters

Advertised as pollution protection, there is a new class of mask with small fans, rechargeable batteries, and even advanced features, like air quality monitoring in an app. Here's a detailed unboxing of the Purely mask.

DIY masks

Homemade masks have been covered extensively online, and they are indeed much better than nothing:

The Case for Universal Cloth Mask Adoption & Policies to Increase the Supply of Medical Masks for Health Workers April 1 2020
...each cloth facemask generates thousands of dollars in value from reduced mortality risk. Each medical mask, when used by a healthcare worker, may generate millions of dollars in value The Case for Universal Cloth Mask Adoption & Policies to Increase the Supply of Medical Masks for Health Workers April 1 2020

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:

Dubious coffee filter mask insert

We used a coffee filter in this mask. Paper towels have also been tested. One experiment found that two layers of paper towels on their own blocked between 23 and 33 percent of 0.3-micron particles. Coronavirus: Which Mask Should You Wear? (New York Times)

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.

For instance, here's the European certification body, AFNOR, which is responsible for surgical mask standards:

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:

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

Efficacy of surgical face masks in reducing respiratory virus frequency of detection and viral shedding in respiratory droplets and aerosols of symptomatic individuals with coronavirus, influenza virus or rhinovirus infection

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:

Number of infected cases with no masks, surgical masks and N95s in a simulation
Figure A Number of infected cases with no masks, surgical masks and N95s in a simulation

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:


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.

Published on

Updated on


  1. Via NIOSH: N = Not resistant to oil, R = Somewhat resistant to oil, P = Strongly resistant to oil. Surgical N95 – A NIOSH-approved N95 respirator that has also been cleared by the Food and Drug Administration (FDA) as a surgical mask. N99 – Filters at least 99% of airborne particles. Not resistant to oil. N100 – Filters at least 99.97% of airborne particles. Not resistant to oil. R95 – Filters at least 95% of airborne particles. Somewhat resistant to oil. P95 – Filters at least 95% of airborne particles. Strongly resistant to oil. P99 – Filters at least 99% of airborne particles. Strongly resistant to oil. P100 – Filters at least 99.97% of airborne particles. Strongly resistant to oil.
  2. NIOSH recommendation: N-series filters generally should be used and reused subject only to considerations of hygiene, damage, and increased breathing resistance. However, for dirty workplaces that could result in high filter loading (i.e., 200 mg), service time for N-series filters should only be extended beyond eight hours of use (continuous or intermittent) by performing an evaluation in specific workplace settings that demonstrates: (a) that extended use will not degrade the filter efficiency below the efficiency level specified in 42 CFR 84, or (b) that the total mass loading of the filter(s) is less than 200 mg. These determinations would need to be repeated whenever conditions change or modifications are made to processes that could change the type of particulate generated in the user’s facility.
  3. The EN 149:2001+A1:2009 standard, which they made publicly available on March 20, 2020, defines three classes of filter efficiency for these masks, namely FFP1, FFP2 and FFP3
  4. Defined here:The laboratory tests shall indicate that the particle filtering half mask can be used by the wearer to protect with high probability against the potential hazard to be expected. The maximum range is for at least 46 out of the 50 individual exercise results (i.e. 10 subjects x 5 exercises). The lower end of the range represents the additional test these masks must pass, what at least 8 out of the 10 individual wearer arithmetic means for the total inward leakage shall be not greater than.
  5. The penetration of test aerosol puts the mask up against dry sodium chloride, 95 l/min, and aerosolized paraffin oil of 95 l/min.
  6. Must be ≥ X% efficient. The filter is evaluated to measure the reduction in concentrations of specific aerosols in air that passes through the filter.
  7. The aerosol that is generated during the filter performance test.
  8. "Respiratory protective equipment self-priming filter anti-particulate respirator" is a mandatory national standard in China, proposed by the State Administration of Work Safety, by the national individual protection Equipment Standardization Technical Committee (SAC / TC 112) is under the jurisdiction. GB 2626-2006 "Respiratory protective equipment self-priming filter type anti-particulate respirator" is about to be scrapped, replaced by its new standard GB 2626-2019 "Respiratory protective self-priming filter type anti-particulate respirator" It will be released to the whole society on December 31, 2019 as "People's Republic of China National Standards Announcement No. 17 of 2019", and will be officially implemented on July 1, 2020. The new standard is proposed and managed by the Ministry of Emergency Management.
  9. The amount of a specific aerosol that enters the tested respirator facepiece via both filter penetration and faceseal leakage, while a wearer performs a series of exercises in a test chamber. And IL is the amount of a specific aerosol that enters the tested respirator facepiece, while a wearer performs a normal breathing for 3 minutes in a test chamber. The test aerosol size (count median diameter) is about 0.5 micro meter. Tested on human subjects each performing exercises. Numbers on average (arithmetic mean), P2 is individual and arithmetic mean. Japan JMHLW-Notification 214 requires an Inward Leakage test rather than a TIL test.
  10. The resistance air is subjected to as it moves through a medium, such as a respirator filter.
  11. 3M notes that Inhalation resistance testing flow rates range from 40 to 160L/min. Exhalation resistance testing flow rates range from 30 to 95 L/min. Some countries require testing to be performed at multiple flow rates, others at only the high or low end of those ranges. Although this appears to suggest that the standards’ requirements for breathing resistance (also called “pressure drop”) differ from each other, it’s important to understand that pressure drop across any filter will naturally be higher at higher flow rates and lower at lower flow rates. Given typical pressure curves for respirator filters, the standards’ various pressure drop requirements are actually quite similar.
  12. According to data released by the National Development and Reform Commission, as of February 11th, the national mask production enterprises have a recovery rate of more than 76% and a capacity utilization rate of 94%, while the capacity utilization rate of medical N95 masks is as high as 128%... All mask manufacturing enterprises in Shanghai have resumed work. According to statistics, there are 17 manufacturing enterprises including masks and accessories in Shanghai. In the early stage of the outbreak, the normal daily output of Shanghai was about 400,000 to 500,000 per day, and most of it was exported. On January 27, the production capacity was restored to 800,000 / day. On January 28, it had increased to 1.1 million. On January 30, the city ’s output exceeded 1.4 million. On February 4, it exceeded 2.6 million. In order to meet the needs of epidemic prevention, from export to domestic sales... Guangdong enterprises resumed work and launched a robot mask production line. As of February 7, 92 of the 129 related medical supplies companies in Guangdong have started construction, and 37 mask manufacturing enterprises in the province have started construction of 35. On this basis, in order to improve the production efficiency of masks in the province, enterprises in Guangdong Province introduced a flexible AI vision automatic mask machine and began mass production of masks using the "Robot Mask Production Line" for 24 hours from February 10, Nissan exceeds 150,000 pieces... China's existing mask production capacity is more than 20 million pieces / day, superimposing the recent new production capacity initiatives, and it is expected that the mask production capacity will be increased to 35-40 million days / day by the end of February. After reaching production, the domestic mask supply will be guaranteed. Via China Export and Credit Insurance Corporation Issues Domestic Mask Supply and Demand Risk Analysis and Prospects
  13. GB refers to mandatory national standard. Currently active standard: GB2626-2019. The new standard, GB2626-2020, is effective July 1st, 2020.