This was a pretty contentious part of the debate on the CDC's recommendations and has come up elsewhere as well: how effective are masks at preventing infection when worn by an uninfected individual? How about preventing spread when worn by an infected individual? Are there things you can do that make it more or less useful?

This is a thread for evidence to answer those questions and related ones. Obviously the more rigorous the better, but case studies like this one about one dude on a bus would be useful had they not been retracted (thanks to gwillen for pointing this out).

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Mar 11, 2020


The majority of studies either compare two different kinds of masks to each other, in a hospital setting, which is interesting but not the main thing we want, which is a comparison between mask and no mask. There are also a few studies looking at households where the housemates of a person with influenza-like illness are randomly assigned to use masks (or not); these studies find masks are not effective, but also show low compliance. In a household setting, most of the variance in whether a household member gets infected is likely explained by vaccination, other sources of prior immunity, and kitchen hygiene; whereas when considering infections of coronavirus occurring in public, none of these are factors. All studies had problems with noncompliance, and confounding between compliance and other precautions (ie people who comply with masks wash their hands at rates.)

This study is interesting in that it effectively has a "placebo mask" arm. There were three arms: a no-intervention arm in which health care workers continued wearing whatever masks they did before (less often than in either intervention arm, but still a significant amount), an intervention arm where hospital workers are given reusable cloth masks which do not work, and an intervention arm in which they're given disposable medical masks. The disposable medical mask arm did best, the cloth-mask arm did worst (worse than the no-intervention arm, due to a combination of not using other masks and taking fewer non-mask precautions). Presumably, this was able to get past an IRB because the study authors didn't know how bad cloth masks are.

Compliance (defined as "mask wearing more than 70% of working hours") was 57% in both the cloth mask and medical groups, and 24% in the no-intervention group.

The majority of infections in the study were from rhinovirus, which is transmitted via aerosol and contact droplets. These are the same modes of transmission as SARS-CoV2, but in different proportions; rhinovirus causes sneezing, so it generates a lot of aerosol, whereas SARS-CoV2 doesn't and aerosol is believed to be responsible for only a small portion of its transmissions. Other diagnosed infections in the study were from hMPV and influenza B. Rhinovirus has a smaller diameter than SARS-CoV2 (30nm), so it's unlikely that mask aerosol penetration of rhinovirus is higher than that of SARS-CoV2. In particle penetration tests, the cloth masks were almost completely ineffective, and the medical masks had some effectiveness but much less than N95.

Compared to cloth masks, medical masks reduced clinical respiratory illness from 7.6 to 4.8%, laboratory-confirmed viral infection from 5.4 to 3.3%, and influenza-like illness from 2.3 to 0.2%.


Mar 10, 2020


Simple Respiratory Protection—Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20–1000 nm Size Particles

Five major categories of fabric materials including sweatshirts, T-shirts, towels, scarves, and cloth masks were tested for polydisperse and monodisperse aerosols (20–1000 nm) at two different face velocities (5.5 and 16.5 cm s−1) and compared with the penetration levels for N95 respirator filter media. Average penetration levels for the three different cloth masks were between 74 and 90%, while N95 filter media controls showed 0.12%

Ben Pace

Mar 24, 2020


Scott Alexander has done a literature review called Face Masks: Much More Than You Wanted To Know

(If someone wrote an answer briefly summarising his conclusions and main evidences in 2-3 paragraphs, that would be a much better answer than this one.)


Mar 10, 2020


Surgical Mask vs N95 Respirator for Preventing Influenza Among Health Care Workers

Two groups of nurses were assigned to wear either medical masks or N95 masks when near patients with febrile respiratory illness (n=446). The study found no significant difference in the occurrence of flu.


Mar 10, 2020


Effectiveness of facemasks to reduce exposure hazards for airborne infections among general populations

Two mannequins were set up, one to simulate realistic coughing and the other realistic breathing. Transmission was tested with a variety of mask fits. Perfect fit did very well, normal fit did worst but was 40% effective.

Wrong link, should be

I'm curious whether "normal" is meant to simulate a proper OSHA-approved fit, or "how normal people wear masks". My impression was that the OSHA fit test standard for N95 masks was meant to achieve an extremely high-quality seal -- they test with volatile substances, and you fail if you can smell them. ( It seems like the "normal" setting here is probably a lower standard than what an OSHA fit is supposed

... (read more)
Fixed, thank you.


Mar 10, 2020


Face mask use and control of respiratory virus transmission in households

Among 286 exposed adults from 143 households with children sick with influenza-like illness, self-reported mask use correlated negatively with infection. Adherence was <50%.

I don't have a lot of faith in this one- parents who wear masks reliably are likely more conscientious at other preventative measures (e.g. handwashing) .


Mar 10, 2020


Effectiveness of surgical masks against influenza bioaerosols.

A dummy test head attached to a breathing simulator was used to test the performance of a variety of surgical masks against a viral challenge. Live influenza virus was measurable from the air behind all surgical masks tested. The data indicate that a surgical mask will reduce exposure to aerosolised infectious influenza virus; reductions ranged from 1.1- to 55-fold (average 6-fold), depending on the design of the mask


Mar 10, 2020


N95 Respirators vs Medical Masks for Preventing Influenza Among Health Care Personnel

Two groups of nurses were assigned to wear either medical masks or N95 masks when near patients with respiratory illness (n=2371). The study found no significant difference in the occurrence of flu or respiratory illness.


Mar 22, 2020


Some comments from Tara Mac Aulay on masks, in the context of donating to hospitals: 

Cross posting my comment from elsewhere re certification standards - KN95 is not sufficient in many settings, but still useful in some. Alibaba has a lot of masks available that are unsuitable, and not many masks that are. Finding the masks that are suitable and checking if the factory has been certified under the right standards is very useful

Alibaba suppliers commonly have KN95 masks in stock, but not surgical N95 masks.

In this time of shortage, many hospitals or healthcare workers will take what they can get, but there are some serious differences.

My understanding as is follows:

KN95 masks are tested for particle filtration to a similar standard as NIOSH rated N95 masks in the US. Surigical rated N95 masks certified by NIOSH have to pass a fit test and a fluid test in addition to the filtration tests standard for N95.

KN95 masks are not rated for use in many patient contact settings, they are predominantly designed for industrial use. KN95 masks provide limited protection to the wearer in this clinical setting, some versions also provide limited protection to patients.

In addition to passing the filtration tests, masks suitable for healthcare workers to use during Covid-19 care have additional requirements:

- masks that have a valve are not appropriate for treating suspected cases. Wearing a mask with a valve is much more comfortable, as the valve allows exhaled air to escape without restriction, meaning less force is required to exhale. However, masks with a valve do not protect patients from exposure from HCWs who are potentially infected. Thus masks with a valve should only be used with patients who are confirmed infected, and for some non-clinical staff who have had limited exposure. A mask with a valve is more comfortable for the general population, you'll be able to wear it for longer, and if you're mostly trying to protect yourself rather than others, this is ok. In some clinical settings, a mask with a valve is also ok. If you are buying masks for personal use to *prevent* yourself from becoming sick, get a mask with a valve. If you want to protect others when you are sick, use a mask without a valve.

- because in many settings you'll be using a mask without a valve to protect patients, the inside of the mask will get very damp. If the mask has not been tested for liquid resistance, then this can be a problem, as the fluid may seep through the mask over time, and then again risk infecting patients. If the masks are not tested for this, the filtration efficiency may also be impacted as the mask gets wet. Surgical N95 masks are tested for 2 way fluid resistance (to ensure no fluid from patients comes through the mask to wearer, and to ensure no fluid from wearer comes through to others)

- fit is very important. Surgical N95 masks are tested for fit and escaping/incoming airflow under normal conditions of use. Without this testing, and without a mask that fits well, HCWs and patients can be exposed.

Long story short, KN95 masks are still useful in many settings, and we need those too. But what we need even more is surgical rated N95 masks, ideally certified by NIOSH as such.

It's also likely that many of the masks that are rated as KN95 would pass liquid and fit tests if subjected, but without the testing and certification, I understand why many HCWs would demand certified masks to protect themselves and their families.

She also mentioned that she expects standards in hospitals to decline over time, and that they will eventually accept other mask types for donations, if they don't already.


Mar 10, 2020


Protecting healthcare workers from pandemic influenza: N95 or surgical masks?

A metareview of 21 studies and 25 lab-based publications. The majority of laboratory studies identified both mask types as having a range of filtration efficiency, yet N95 masks afford superior protection against particles of a similar size to influenza.

1 Related Questions

The 100% efficacy for a middle filter layer that's had a saltwater + surfactant sprayed onto it sounds really good; but I wonder how tight the filter material has to be, for that level of efficacy. I also wonder how much air resistance the salt coat adds. A HEPA filter + carbon would be less restrictive if the carbon part were salted than if the HEPA filter itself were salted, but that might not deactivate all of the virus.
Agreed. I'm just hoping that that they can give the OP some information about using HEPA filters. I've noticed that many N95 masks also have an exhaust valve.
4Bobette Bryan4y
I've read that Hepa vaccume filters can be dangerous as they can contain fiberglass particles. Is this true? Are any of them actually safe to breathe through?
1Tie Dye Queen Karen4y
Kirby says their HEPA vacuum bags do not contain fiberglass, nor chemical treatments. I called them & they emailed me back. Check with the manufacturer for the brand you want to use.
4Karen Kempton4y
Can you share the email from Kirby?
Answer by kathcdnApr 04, 2020120

Check out the research from Texaas A&M University Research Department - it should answer all of your questions, they tell you which MERV filter to use and how many layers you need depending on the rating.

This ended up answering the questions. Thanks! :)
7 comments, sorted by Click to highlight new comments since: Today at 11:38 PM

Note that (1) your second link is broken, but (2) that case study was retracted with no explanation yet given: (see the top of the article body, the headline was not updated): "Note: The study at the centre of this article on the transmission of the coronavirus was retracted on Tuesday by the journal Practical Preventive Medicine without giving a reason. The South China Morning Post has reached out to the paper's authors and will update the article."

Thank you, this is exactly the kind of help I was hoping to get from opening this thread.

Some info about how medical professionals use N-95 masks:

"Fit-testing" seems to be a requirement on an yearly basis. I'm not sure how rigorous or involved that is, but that seems to be an indication that it's not as simple as "civilians" just buying them and wearing them.

They're only intended to be used as one-time use masks. Apparently, healthcare workers are supposed to use a new one for every patient. Plus, they don't prevent viruses from infection via your eyes.

You can see the fit-testing on youtube. It's pretty involved and not something you can do at home--they put a hood over your head and spray in a bunch of super-sweet aerosol, and if you can taste it through the mask then the fit isn't good enough.

On the other hand, they do that once a year, just to make sure that model is capable of fitting your face shape. The masks are supposed to be one-size-fits most. On a daily basis, the fit procedure is to put it on and blow out and see if you can feel any air escaping around your nose. This seems very achievable and likely to give a decent fit for most people.

It seems like this should be it's own question.

are masks effective effective at preventing infection when worn by an uninfected individual?

That question seems ill-posed. It's not binary. The key question is how much risk reduction the produce.