In light of Covid-19 we’ll look at the difference between respirator filtering standards such as N95 and FFP2/FFP3…

Masks vs Respirators

Before we go any further, let’s just clarify on a technical difference between a “mask” and a “respirator”. In day to day language we often say mask, when referring to what are technically called respirators.

Uses for Masks:

  • Masks are loose fitting, covering the nose and mouth
  • Designed for one way protection, to capture bodily fluid leaving the wearer
  • Example – worn during surgery to prevent coughing, sneezing, etc on the vulnerable patient
  • Contrary to belief, masks are NOT designed to protect the wearer
  • The vast majority of masks do not have a safety rating assigned to them (e.g. NIOSH or EN)

Uses for Respirators:

  • Respirators are tight fitting masks, designed to create a facial seal
  • Non-valved respirators provide good two way protection, by filtering both inflow and outflow of air
  • These are designed protect the wearer (when worn properly), up to the safety rating of the mask
  • Available as disposable, half face or full face

Respirator Standards

Whilst surgical style masks are not redundant by any means (discussed more below), they aren’t designed to protect the wearer, whilst respirators are.

The US Center for Disease Control (CDC) cites the N95 respirator standard as part of the advised protective equipment in their Covid-19 FAQ and their SARS guidance (SARS being a similar type of Corona virus). Which suggests that an N95 or better respirator is acceptable.

N95 vs FFP3 & FFP2

The most commonly discussed respirator type is N95. This is an American standard managed by NIOSH – part of the Center for Disease Control (CDC).

Europe uses two different standards. The “filtering face piece” score (FFP) comes from EN standard 149:2001. Then EN 143 standard covers P1/P2/P3 ratings. Both standards are maintained by CEN (European Committee for Standardization).

Let’s see how all the different standards compare:

Respirator StandardFilter Capacity (removes x% of of all particles that are 0.3 microns in diameter or larger)
FFP1 & P1At least 80%
FFP2 & P2At least 94%
N95At least 95%
N99 & FFP3At least 99%
P3At least 99.95%
N100At least 99.97%

As you can see, the closest European equivalent to N95 are FFP2 / P2 rated respirators, which are rated at 94%, compared to the 95% of N95.

Similarly, the closest to N100 are P3 rated respirators – with FFP3 following closely behind.

You could approximate things to say:

KN95 vs N95

According to 3M (source), the Chinese KN95 standard has an equivalent specification to N95/FFP2 respirators . To quote:

“It is reasonable to consider China KN95, AS/NZ P2, Korea 1st Class, and Japan DS FFRs as equivalent to US NIOSH N95 and European FFP2 respirators”

In practice the issue is more complex, and I wouldn’t take for granted that all KN95 respirators are up to the same standard.

Things to watch out for:

  • Typically KN95 respirators are held in place by over-ear elastic loops, rather than behind the head elastics. This can result in a weaker seal. Fortunately there are methods for tightening – see this YouTube video for ideas. Products called “ear savers” can also aid with tightening, and can be found on eBay or you can 3D print them.
  • There’s no guarantee that all KN95 respirators actually meet the Chinese KN95 standard. However, with the current respirator shortage, unfortunately the same goes for N95/FFP also.

The KN95 specification is referred to as GB 2626-2006 (preview here) – so you will generally see that written on the KN95 respirators. From July 1st 2020, it’ll be replaced by GB 2626-2019, an updated specification (preview here).

I’ve linked below to a supplier of KN95 respirators in the USA – in case it’s of use to some readers.

Valve vs Non-Valved Respirators

✅Valved respirators make it easier to exhale air. This makes them more comfortable to wear, and leads to less moisture build-up inside the respirator. Ideal for things like DIY/construction work.

❌The problem with valved respirators is that they do not filter the wearer’s exhalation, only the inhale. This one-way protection puts others around the wearer at risk, in a situation like Covid-19. It’s for this reason that hospitals and other medical practices do not use valved respirators.

One hack to protect (and respect) others when wearing a valved respirator is to put a surgical mask or “cloth face covering” over the valved respirator, to (partially) filter the out breath.

How big is the Coronavirus, and can respirators filter it?

TL;DR – yes, respirators with high efficiency at 0.3 micron particle size (N95/FFP2 or better) can in theory filter particles down to the size of the coronavirus (which is around 0.1 microns). What that doesn’t tell us is how much protection respirators will provide against coronavirus when in use – we will need to wait for future studies to confirm.

Read on to learn more…

A recent paper shows that the coronavirus ranges from between 0.06 and 0.14 microns in size. Note that the paper refers to the coronavirus particle as 2019-nCoV, which was it’s old name. The virus is currently called SARS-CoV-2, and the illness it presents in people is called Covid-19.

Respirator’s are measured by their efficiency at filtering particles of 0.3 microns and bigger (noting that the coronavirus is smaller than that).

The reason for the focus on 0.3 microns is because it is the “most penetrating particle size” (MPPS). Particles above this size move in ways we might anticipate, and will get trapped in a filter with gaps smaller than the particle size. Particles smaller than 0.3 microns exhibit what’s called brownian motion – which makes them easier to filter. Brownian motion refers to a phenomenon whereby the particle’s mass is small enough that it no longer travels unimpeded through the air. Instead it interacts with the molecules in the air (nitrogen, oxygen, etc), causing it to pinball between them, moving in an erratic pattern.

According to researchers this point between “normal” motion and brownian motion is the hardest particle size for filters to capture.

What we can take away from this, is that high filter efficiency at 0.3 micron size will generally translate to high filter efficiency below this size also.

For more discussion and details on the subject of respirator filters and brownian motion – see this great post at

Now lets look at specific research that measures the filter efficiency at 0.3 microns and below (coronavirus territory)…

  • This article by 3M discusses research showing that all 6 of the N95 respirators they tested can efficiently filter lower than 0.1 micron size with approximately 94% efficiency or higher. The graph below is from that article, and illustrates this:

  • Additionally, have a great article on this subject, citing research showing that the respirators tested could filter down to 0.007 microns (much smaller than Covid-19). For example the 3M 8812 respirator (FFP1 rated) was able to filter 96.6% of particles 0.007 microns or larger. Suggesting FFP2 or FFP3 would achieve even greater filtration.

The below image (click it to expand) shows the size of the coronavirus, relative to other small molecules like a red blood cell, or the often talked about PM2.5 particle size.

Image of coronavirus vs other particles – from

N vs P respirators? (Oil Resistance)

The CDC explains that in the USA there are 3 ratings for protection against oils; N, R or P:

  • N = Not resistant to oil
  • R = somewhat Resistant to oil
  • P = strongly oil Proof

What this means in practice, is for industrial settings, where the air might contain a lot of oil particles, if the mask isn't P rated, then over time the oil may degrade and reduce the filter performance.

For the vast majority of people trying to reduce exposure to Covid-19, it won't be necessary to protect against oils - this is primarily designed for industrial use settings.


Surgical vs Non-Surgical Respirators?

Alongside "regular" respirators, there are also what are often referred to as "surgical" or "surgically approved" respirators. These carry the aforementioned ratings such as N95/FFP2, but are also approved for fluid resistance. A qualification governed by ASTM F1862 - which covers the edge case in which an artery is punctured, and high pressure blood is sprayed directly at the respirator. To pass the test, the mask has to withstand this spray without leaking fluid inside the mask.

You can see why this type of mask if important for surgery, but it's not clear outside of that setting how much extra benefit it would provide. Regular N95/FFP2 masks will block things like coughs and sneezes.

The comparison table below shows how a regular N95 mask (8210), stacks up against 2 surgical N95 masks (1860 and 1870+).

Example of Surgical vs Non-Surgical Respirators

See this comparison table below for the key differences (source: 3M website):

  N95 Respirator 3M Model 8210 Surgical N95 Respirator 3M Model 1860 Surgical N95 Respirator 3M Model 1870+
Designed to help protect the wearer from exposure to airborne particles (e.g. Dust, mist, fumes, fibers, and bioaerosols, such viruses and bacteria)
Designed to fit tightly to the face and create a seal between the user’s face and the respirator
Meets NIOSH 42 CFR 84 N95 requirements for a minimum 95% filtration efficiency against solid and liquid aerosols that do not contain oil
Cleared for sale by the U.S. FDA as a surgical mask
Fluid Resistant - Meets ASTM Test Method F1862 “Resistance of Medical Face Masks to Penetration by Synthetic Blood” which determines the mask’s resistance to synthetic blood directed at it under varying high pressures[1] ✅ 120 mm Hg ✅ 160 mm Hg

According to the 3M wesbite:

[1] "ASTM F1862 is a standard test method for resistance of medical facemasks to penetration by synthetic blood. This test is required because during certain medical procedures, a blood vessel may occasionally be punctured, resulting in a high-velocity stream of blood impacting a protective medical facemask. The test procedure specifies that a mask or respirator is conditioned in a high-humidity environment to simulate human use and is placed on a test holder. Synthetic blood (2cc) is shot horizontally at the mask at a distance of 30 cm (12 inches).

Surgical masks and respirators are tested on a pass/fail basis at three velocities corresponding to the range of human blood pressure (80, 120, and 160 mmHg). The inside of the mask is then inspected to see if any synthetic blood has penetrated to the inside of the facemask. Fluid resistance according to this test method is when the device passes at any level."

In essence, all 3 of these masks should be adequate, as per CDC guidance on 2019-nCoV and SARS. As mentioned above, where the 1860 and 1870+ are superior to the 8210 is when faced with high velocity liquid spray - which is possible during surgery (e.g. punctured artery), but unlikely in day-to-day usage.

Risks With Using Respirators

There are a number of possible risks with respirators that it’s worth being aware of, so that you can avoid making them.

  1. Not fitting and wearing respirators correctly – A respirator can’t fully protect you if it doesn’t fit your face. See OSHA guidance on fit testing and fit checking for more info.
  2. Touching the front of the respirator (which catches viruses etc) and then transferring that to other objects, which could eventually lead back to your mouth and nose.
  3. Taking unnecessary exposure risks because you’re wearing a respirator. Don’t let it give you false confidence. The safest thing is maintaining social distance.

For further discussion on these 3 points, see the expandable box below:

1. Not fitting and wearing respirators correctly

It's important to make sure that the respirators we use form a tight fit on our face, such that all air is being filtered and not passing in through the sides. Under ideal situations, one would try on a number of respirators to find one that fits perfectly. After that, you would "fit test" the respirator by putting it on tightly, then checking if you can smell or taste a chemical that you hold nearby. If you can, the seal may not be adequate. If you can't, that suggests you passed the fit test. See more about this process on the OSHA website - including details on their list of approved chemicals for fit testing. However, currently we're under pandemic conditions with shortages on both respirators and the chemicals used for fit testing them. Therefore we have to make-do as best we can with what we've got available. However, a focus on correctly fitting respirators is crucial.

2. Touching the front of the respirator

The front of the respirator can be thought of like a net - catching and filtering viruses and bacteria as we breathe. The problem then occurs if we touch the front of the mask and then touch our faces. In essence we need to treat the front of the mask like it's a hazardous material, and always wash hands carefully after touching it. Also avoiding touching the outside, and then the inside of the mask, because the inside has to make tight contact with your face, and is difficult to clean.

3. Taking unnecessary exposure risks

Don't let wearing a respirator give you confidence to take unnecessary risks. The efficacy of respirators is less than 100%, unfortunately. Both due to their filtering capacity limits (<100%) and due to the 2 points discussed above. So for example, don't go to an event with lots of people (especially if it's indoors), and think it's safe because you're wearing a respirator. The safest thing you can do is practice social distancing.

Reliable Brands?

Examples of well known, reliable manufacturers of respirators include:

  • 3M
  • Moldex
  • GVS

For more options, the CDC maintains a list of N95 approved respirators.

Surgical Masks

Surgical masks are generally speaking a 3-ply (three layer) design, with 2 sheets of “non-woven” fabric sandwiching a “melt-blown” layer in the middle. It’s the melt-blown layer that provides the filtering capability. A melt-blown material is also used in respirators, and thus you can imagine it’s more expensive and hard to come by recently, due to demand.

Image of the melt-blown filaments under microscope come from

The melt-blown fabric is made by melting a plastic, then blowing it from either side at high velocity onto a rotating barrel. Done right, this results in a fabric composed of tiny filaments. For a more technical (!) explanation of the process – see here.

Diagram of melt-blown machinery (left) comes from Erdem Ramazan’s book, and the image of melt-blowing in progress (right) comes from 4FFF on wikipedia

Not all melt blown fabric has the same filtering capability, some are better than others. Unfortunately we can’t test the filtering capability of the melt-blown layer without specialized knowledge and equipment. What we can do, however, is at least check that the melt-blown layer is present.

Below I show an example of a surgical mask (left) that came without the melt-blown layer. You can imagine that, given the extra cost and current scarcity of melt-blown fabrics, manufacturers might cut corners with this layer, so it’s worth keeping an eye on.

Choosing surgical masks that have been tested according to a set of standardized test methods (ASTM F2100, EN 14683, or equivalent) will help avoid low quality products. The ASTM standard for surgical masks (particularly levels 2 & 3) are primarily focused around fluid resistance during surgery. These higher levels don’t offer much extra in the way of protection from Covid-19 under non-surgical conditions.

If you see reference to BFE95/BFE99 - BFE = "Bacterial Filtration Efficiency" - and the score = % of particles blocked, with a mean particle size of 3 microns (+/- 0.3microns) - source.

Similarly, PFE = Particle Filtration Efficacy. ASTM F2100 measures the PFE down to 0.1 microns (source).

This table by Nelson Labs gives more examples of surgical mask specifications, including BFE/PFE.


Can Surgical Masks Filter the Coronavirus?

Whilst FFP2/FFP3 or N95/N100 are the gold standard as far as face protection goes, what about surgical masks, do they provide any protection?

Strictly speaking, surgical masks are primarily designed to protect vulnerable patients from medical professionals. Stopping the wearer (e.g. surgeon) from spreading their germs when coughing/sneezing/speaking. So they’re designed to protect patients, not to protect the wearer.

An obvious flaw with surgical masks compared to respirators is their lack of a tight face fit, which leaves gaps around the edges.

There isn’t currently research available on the efficacy of surgical masks (or even respirators), for protecting wearers against the coronavirus. Although this isn’t totally surprising given how new the virus is.

In lieu of that, the below looks at research around the use of surgical masks and N95 masks in the context of influenza, looking specifically at the protection given to the wearers. Influenza may be a good virus particle to compare it to, as they are both transmissible through droplets and aerosol, both cause respiratory infection, and both are similar in particle size.

N.B. Please don’t conflate the comparison to the influenza particle as suggestion that they are comparable illnesses – current data suggests that the coronavirus may have a higher mortality rate.

Source for coronavirus (SARS-CoV-2) size is this paper, whilst sources for Influenza size are this paper (eventually published in Vaccine), and a Frontiers in Microbiology paper.

In the first study we will look at, 2,862 US health care personnel were split into 2 groups, those wearing N95 masks and those wearing surgical masks1. There were 207 lab confirmed influenza events in the respirator wearing group, compared to 193 in the mask wearing group – a difference that was not statistically significant.

In the next study, Canadian nurses were split into 2 groups, those wearing N95 masks and those wearing surgical masks. There were 50 cases of influenza in the surgical mask group, compared to 48 in the N95 respirator group2. Again, no significant difference.

So where does this leave us? Those 2 studies suggest that surgical masks are approximately comparable to N95 masks when it comes to preventing influenza illness in close contact clinical settings. What this doesn’t tell us, is whether they’re better than wearing nothing on our faces.

To find that out, we need a study that has a control group that doesn’t use any facial protection. Due to ethical considerations, those studies aren’t abundant, but we do have at least one.

In this Australian study, they looked at 286 adults in 143 households who had children with influenza-like illness3. For clarity, influenza-like illness is not the same as laboratory confirmed influenza. It’s diagnosed by symptoms like fever, dry cough and feeling sick, which could mean influenza, but could also be caused by the common cold or other viruses. They found that adults who wore masks in the home were 4 times less likely than non-wearers to be infected by children in the household with a respiratory infection. There is nice analysis of the study here by Imperial College London.

It’s definitely fair to note that this Australian study was very small, and could not be considered definitive by any means. That being said, we’ve got to work with what he have, and this at least gives us some data points:

  • Wearing a surgical mask or N95 (FFP2) respirator was better (in the study) at protecting against influenza-like illnesses than wearing nothing at all
  • Whilst we can anticipate surgical masks to be inferior to respirators, the studies above suggest they are not as inferior as one might assume. For example the first two studies didn’t find a significant difference between surgical masks and N95 respirators, when protecting wearers against influenza.
  • Important to note that we’ve used influenza protection as a proxy for SARS-CoV-2 (coronavirus). This is done because SARS-CoV-2 is new and there are no comparable studies on it. But of course the drawback is that it still leaves a lot of uncertainty, as SARS-CoV-2 may act quite differently in terms of transmission.

In a lab setting, with artificial conditions, we find that surgical masks are able to block 80% of particles down to 0.007 microns. Compared to the 3M 8812 respirator in this study which blocked 96% (FFP1 rated). This generally aligns with our discussion above.

In conclusion: we don’t know how much protection surgical masks provide against the novel coronavirus. However, the above at least suggests that a surgical mask may provide more than zero protection – and that’s worth being aware of. It makes sense to only wear them for protection as a method of last resort – with respirators being the primary choice.

It is much safer to avoid the company of people who are sick or potentially sick, and to reduce social contact overall, especially to large groups of people (see the social distancing section below). To repeat, the use or surgical masks would have to be a last resort – and wearing one should not encourage anyone to take unnecessary exposure risks.

If we are in the presence of someone sick, who has/might have the coronavirus, it makes sense for them to wear a mask or respirator to reduce their ability to spread the disease.

DIY / Homemade Masks

Early on in the pandemic, the CDC announced guidance to American citizens that “cloth face coverings” should be used in public settings where social distancing measures are difficult to maintain. At the time, there was quite a shortage of masks, so people were making their own. I’ve left this section of the article up, as it briefly covers the efficacy of household materials.

Image from the DIY mask project

So how does one make their own mask?

Firstly, it’s worth noting how various household items compare in terms of filter efficacy and breath-ability. For that, we can refer to a Cambridge University study (link), which revealed “the pillowcase and the 100% cotton t-shirt were found to be the most suitable household materials for an improvised face mask”.

Image via SmartAirFilters

Interestingly, other items such as vacuum cleaner bags and dish towels showed greater filtration capacity, so why didn’t the study pick them? Unfortunately those items performed badly in the breathability tests. A mask is little good if you can’t breathe out of it. See this write-up of the study for more details (and nice graphs!)

Of course, it goes without saying that generally the protection these DIY masks is below that of surgical masks and respirators.

What are respirators protecting us against?

A primary reason for wearing a respirator is to protect from droplets. For example if a sick person coughs or sneezes when in close proximity to us, the respirator forms a barrier to prevent their bodily fluids reaching our face.

Droplets are generally large, and gravity drags them down to land on objects, rather than staying in the air. So they don’t travel very long distances. There is however research into micro droplets, which get ejected even during talking. This Vimeo video made by Japanese researchers, captures micro droplets on video using high speed cameras. We know large droplets play a role in transmission, but it’s not yet clear what role micro droplets play.

Image from Sui Huang’s blog post on the need for mask usage

What may remain in the air for some time are aerosolized virus particles. So for example, you could imagine someone creating two issues when sneezing, the first are ejected droplets, which travel a short distance, then second, aerosolized virus particles that stay in the air for longer.

Currently there is debate and uncertainty around how long Covid-19 can remain aerosolized, and how much of a risk that vector is compared to others.

What we can do is be aware of what research currently says, and err on the side of caution until its been confirmed.

Scientists at the National Institute of Allergy and Infectious Diseases (NIAID) published a study in NEJM (link) on what can happen under controlled lab conditions. They used a nebulizer, which creates an aerosol from liquids, and tested how long the virus remains measurable in the air whilst aerosolized. They also tested how long the virus was measurable on other surfaces. Their results showed the virus remained measurable for the full duration of the aerosolization experiment; 3 hours.

See the graph below for more details:

Image via NIAID pre-print

Dr John Campbell has a YouTube video discussing the NIAID paper in more detail.

Mouth & Nose
Then lastly, whilst the respirator covers our face, it makes it very hard for us to touch an object with the virus and transfer it to our mouth and nose. This is a kind secondary benefit, in addition to the two mentioned above. We just need to make sure we wash our hands carefully as soon as we take the respirator off.

How risky are contaminated objects (fomites)?

Early on in the pandemic we didn’t have a lot of data (or experience) to go on. As a result, myself and others bought into research that suggested the coronavirus could survive for very long periods of time on surfaces. See the image below as illustration of previous estimates:

Image via NIAID pre-print

This lead to lots and lots of hand washing! However, as time goes on, and our knowledge grows, it appears the risk of transmission via virus contaminated objects (also known as fomites) is lower than we feared.

We have seen this shift similarly with the risk of outdoor transmission, whereby coming into close contact with people briefly (joggers, walkers) appears to be lower risk than we first feared.

Emanuel Goldman argues in his Lancet comment that the studies suggesting SARS-CoV-2 persists for multiple days on surfaces are based on starting with a very large number of viral particles on a small surface area, which doesn’t reflect real world situations. For example, these studies used 104, 106 and 107 viral particles, whereas he estimates (based on this influenza study) actual human droplets from a cough or a sneeze contain closer to 101 to 102 (10 to 100) virus particles, which is vastly less.

Then Emanuel points to a study of SARS (which is similar to SARS-CoV-2), where they swabbed surfaces in 2 hospitals, finding 26 samples of SARS RNA. However, none of the viruses showed any growth when cultured. Meaning that even though the virus was present, it wouldn’t pose a threat to humans if they came into contact with it (it was effectively “dead”). This is relevant because viral particles can be present, but non-infectious.

Emanuel concludes his article by saying that he thinks the chance of transmission through inanimate surfaces is small, and limited to instances in which an infected person coughs or sneezes on a surface – that someone then touches within 1 to 2 hours. However if objects have not been in contact with an infected carrier for many hours, they are unlikely to pose a risk.

Assuming Emanuel is right, and as always, more research is needed to verify, it suggests we’re most at risk via airborne transmission (droplets or aerosols), rather than through touching infected objects (fomites). And we should triage our risk strategy to fit this.

Is Eye Protection Necessary?

Whilst the coronavirus can’t penetrate skin, it can penetrate all exposed mucous membranes, which includes the eyes.

This is why you often see medical professionals wearing eye masks when in contact with infected patients.

That said, eyes are presumably a lower risk as a route of entrance, compared to the mouth, which is constantly breathing air directly into the lungs.

It seems reasonable that eye protection be prioritized in high risk situations – such as in a hospital with infected patients.

Additional Subjects of Importance

Shave! (When Wearing a Respirator)

When wearing a disposable respirator, it is important the wearer has no facial hair around the seal. Bad news guys! 🙁 This 2010 literature review found that “in the presence of facial hair, face seal leakage increases from 20 times to 1000 times“. However, hair under the mask (moustache, goaty, etc) doesn’t cause a problem. See this illustration from the US CDC showing all the permutations of facial hair that are issues and non-issues. The alternative for those who want to keep their beard is to wear a full-face respirator, for which facial hair wouldn’t typically cause an issue.

Important Hygiene Measures

Regular Hand Washing

– The CDC recommend regular hand washing with soap and water for at least 20 seconds.
– Prioritize washing prior to eating and after being out.
– Regular hand washing dries the hands, which at an extreme, may make them vulnerable to infection. To mitigate this, regularly use a glycerin based moisturizer with pump or squeeze mechanism. Those that you scoop are less hygienic.
– A study showed that we touch our face on average 15x per hour. That behaviour may be difficult to change, but if we keep our hands clean, it’s less detrimental.

Image showing the differences in effect between types of hand washing.

How to maintain a healthy immune system?

We don’t currently have a vaccine, or robust anti-viral medications to tackle Covid-19. In the meantime, we’re reliant upon our immune system to fight the virus. Below we’ll look at steps we can take to maintain it, and put ourselves in the best position, should the “worst case” happen:


Get adequate, high quality sleep. For most people ‘adequate’ means 7-8 hours. It’s no coincidence that “burning the candle at both ends” increases risk of illness. A 2004 literature review concluded that “sleep deprivation has a considerable impact on the immune response” and “should be considered a vital part of the immune system”4


Exercise regularly, but don’t overdo it. To quote a 2007 study on exercise and the immune system – “moderate exercise seems to exert a protective effect, whereas repeated bouts of strenuous exercise can result in immune dysfunction”5.

Vitamin D

Prior to the Covid-19 outbreak, there was evidence to suggest that:

  • Vitamin D plays a key role in immune function, and, being deficient in vitamin D can make you more susceptible to infection6
  • Vitamin D supplementation protects against acute-respiratory tract infections – as seen in this BMJ meta-analysis covering 25 randomized controlled trials (11,321 participants).

Now that Covid-19 has been around for a few months, we’re starting to see research about vitamin D status as it relates to Covid-19. The two studies below point to vitamin D levels affecting “severity of outcome”, i.e. if someone has low levels of circulating vitamin D, they’re more likely to have a severe illness. As of yet, I haven’t seen any data on vitamin D actually preventing illness. More research needed. For now, let’s look at the research we do have:

  1. An April 9 paper from Mark Alipio in the Philippines (link) that retrospectively analyzed 212 cases of laboratory confirmed Covid-19, and found that with each standard deviation increase in Vitamin D levels, the odds of having a mild clinical outcome, rather than severe was approximately 7.94 times. And the odds of having a mild outcome, rather than critical, was 19.61 times. This paper was discussed in the British Medical Journal here.
  2. An April 30 paper from Indonesian researchers (link) retrospectively analyzed 780 cases with laboratory confirmed Covid-19. They extracted vitamin D status from their medical records, and when controlling for age, sex and co-morbidities, vitamin D status strongly correlated with risk of death from Covid-19. Specifically they said that, compared to normal, those with insufficient vitamin D were approximately 12.55 times more likely to die. Then those who are deficient (less than insufficient) in vitamin D were 19.12 times more likely to die.

Both studies used the same definitions for “normal”, “insufficient” and “deficient” levels of vitamin D:

CategorizationBlood levels of Vitamin D - 25(OH)D
Normal30 ng/ml or higher (75 nmol/L or higher)
Insufficient21 to 29 ng/ml (52.5 - 72.5 nmol/L)
DeficientUnder 20 ng/ml (Under 50 nmol/L)

Whilst we still need more research to confirm these results, it seems the potential upside of maintaining normal (but not excessive) vitamin D levels is high, and the downside is low to zero.

So how do we get enough vitamin D?

Whilst we can get vitamin D from some foods in our diet (for example oily fish, liver, egg yolks), it’s often hard to get enough with these alone. The major source of vitamin D for humans is sunlight (specifically UV-B rays). Without sufficient sun, it’s common to run a deficit on vitamin D. For example, in winter months in the UK, up to 40% of the population are severely deficient (<10ng/ml / <25 nmol/L)7.

If you’re concerned you’re not getting enough sunlight, then supplementing vitamin D is a way to mitigate this.

What is an adequate amount of vitamin D? The National Institutes of Health (NIH) suggest getting 600iu (15mcg) from all sources, per day, for adults. Similarly the National Institute for Health and Care Excellence (NICE) suggest a supplement containing 400iu (10 micrograms) taken daily.

Dr John Campbell has a great video on vitamin D and the immune system . He cites the NICE guidelines of supplementing 400iu per day, but says he personally takes a vitamin D supplement containing 1,000iu daily.

When looking for a supplement, there is evidence to suggest (link) that vitamin D3 raises levels of vitamin D with 1.7x greater efficiency than D2. Examples of NSF certified manufacturers selling vitamin D3 are Life Extension – 1,000iu, Thorne Research – 1,000iu and Pure Encapsulations – 1,000iu.


Whilst the evidence around vitamin D status and Covid-19 risk is becoming substantial, I would say that the evidence around Selenium status is still in its infancy. However, I think it’s worth looking briefly. Prior to the outbreak there was already evidence that selenium status plays an important role in immune function. For example:

  • Selenium deficiency increased virulence of RNA viruses such as coxsackievirus B3 and influenza A89
  • Selenium status also mediated effects of HIV10, “epidemic hemorrhagic fever”11 and hepatitis B12

As recently as April 28, an international team of researchers led by Surrey University’s Professor Margaret Rayma identified a potential link between Covid-19 cure rate and regional selenium status in China13. The graph below summarizes the findings quite succinctly, suggesting that higher levels of selenium resulted in higher cure rates from Covid-19. China is a particularly good country to analyze this in because across the country they have both some of the highest, and lowest, levels of selenium intake globally.

The authors note that there are significant limitations to their study, including:

  • Most of their data on selenium status is from 2011 – 9 years ago.
  • Lack of data around age and co-morbidities for cities. Such that demographics make-up will vary, and this hasn’t been controlled for.
  • Lack of info around regional variation in treatment protocols / capacity – which again hasn’t been controlled for.

So whilst the data isn’t robust enough to say that selenium status definitely plays a key role in Covid-19 mortality rate, it’s at least an indication more research should be done.

For a longer review on the paper, see this Surrey University post.

So how do we get enough selenium?

Unlike with vitamin D, where it’s hard to get adequate amounts from our diet alone – with selenium this should be possible. Foods high in selenium include brazil nuts, tuna, sardines, ham, shrimp and more. See Table 2 on the NIH site for a list of food sources. A simple dietary modification could be to add extra brazil nuts to your weekly food intake. Another easy source of selenium are multivitamin supplements that contain selenium. For reference, the NIH Recommended Daily Allowance is set at 55mcg for adults.

Indoors vs Outdoors Risk

Early on during some national lockdowns, there was a conflation of risk between indoor and outdoor settings – assuming both were as bad as each other. This led to some countries prohibiting outdoor access altogether, save for essential trips such as groceries. Not allowing people to get exercise and sun exposure (vitamin D) is likely to weaken their bodies and immune system. Therefore it’s important we update our views as more evidence emerges

Whilst intuitively we might assume risk of transmission indoors is higher, due to poor ventilation, it’s important we combine this theoretical idea with actual studies. Below we’ll look at some of the current evidence:


Studies have found high transmission rates indoors, including on cruise ships (National Instatitute of Infectious Disease, 2020), in churches (CDC, May 22) and during indoor choir practice (CDC May 15). The cruise ship example (Diamond Princess) was particularly unfortunate, whereby after detecting a single initial Covid-19 case, and then quarantining everyone aboard the ship, it spread to 712 of the 3,711 people board (19%) (source).

Whilst locations such as cruise ships and churches may be more easily avoided, offices and public transport may not.

A Korean analysis of a call-center outbreak (Emerging Infectious Diseases, April 2020) found that transmission was relatively localized in the office. Despite workers interacting with people in the elevators and lobby, the spread was limited almost exclusively to people who worked on the same floor. Indicating duration of contact was a key facilitator for spread.

Schematic of Korean call center outbreak – blue indicates seating location of infected people


Finding examples of outdoor spread has been less easier than indoor spread. Of course, absence of evidence does not equal evidence of absence.

A Chinese pre-print study (medRxiv, April 7) analyzed 318 Chinese outbreaks involving 3+ people, covering 1,245 confirmed cases in 120 cities. They divided the locations in which the outbreaks occurred into 6 categories: homes, transport, food, entertainment, shopping, and miscellaneous.

It found just 1 outbreak originated whilst people were outdoors. With homes and transport being 2 locations with the most outbreaks.

The analysis was done during winter however, whilst people were spending less time outdoors. A similar study done during warmer months is likely to find a higher incidence of outdoor transmission. Particularly if we bear in mind duration of contact is a key aspect also.


Figure from the Chinese study showing location of infections over time

As things stand, we’re still in the early days of research into indoor vs outdoors transmission. As it accumulates, hopefully we can use it to make better risk-based decisions around our behavior.


Hopefully if you’ve stumbled across this article, and you were confused about the difference between N95, KN95 and FFP2/FFP3 masks, this has cleared things up for you.

For Spanish speaking friends who might find this article useful, it’s translated here.

If you have any further questions, please leave them below in the comments.

Post Change Log

Given that the situation with Covid-19 is evolving rapidly, I’ve decided to add a Change Log for this post. It will list changes I’ve made from May 2020 onwards.

Further Reading

If you enjoyed this post, other popular posts on the blog include:


  1. N95 Respirators vs Medical Masks for Preventing Influenza Among Health Care Personnel – A Randomized Clinical Trial – Lewis J. Radonovich Jr, MD et al. – JAMA – Sept 2019
  2. Surgical Mask vs N95 Respirator for Preventing Influenza Among Health Care Workers – A Randomized Trial – Mark Loeb et al. – JAMA – Nov 2009
  3. Face Mask Use and Control of Respiratory Virus Transmission in Households – MacIntyre et al. – Emerging Infectious Diseases Journal – Feb 2009
  4. Sick and tired: does sleep have a vital role in the immune system? – Bryant et al. (2004)
  5. Exercise and the Immune System – Brolinson (2007)
  6. Vitamin D and the Immune System – Cynthia Aranow (2011)
  7. SACN – Vitamin D & Health – UK Government Report (2016)
  8. The influence of selenium on immune responses – Hoffmann and Berry (2008)
  9. Host nutritional status: the neglected virulence factor – Beck et al. (2004)
  10. High risk of HIV-related mortality is associated with selenium deficiency – Baum et al. (1997)
  11. Inhibitory effect of selenite and other antioxidants on complement-mediated tissue injury in patients with epidemic hemorrhagic fever – Hou (1997)
  12. Protective role of selenium against hepatitis B virus and primary liver cancer in Qidong – Shu et al. (1997)
  13. Association between regional selenium status and reported outcome of COVID-19 cases in China – Rayman et al. (2020)

Posted by John Alexander

Hi, I'm John, a researcher and writer.

With a keen interest in health and longevity.

Note: not an MD or PhD.

Hope you enjoy the site. If you've suggestions for content you'd like to see - let me know.

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1 hour ago

This is a very useful and helpful article! Thank you so much. Have you done any research of the effects of continually wearing masks (all different kinds)? I am a Health and Safety Coordinator in a steel manufacturing facility and have had continuous complaints from customers as well as employees of: headaches, migraines, dizziness, drowsiness, fogginess, nausea, vomiting, nosebleeds, chest and nasal congestion, instances of pneumonia (bacterial and regular), numerous cases of upper respiratory infection.

Reply to  John Alexander
4 minutes ago

Thank you, I will continue on my journey of finding more information and documenting such instances

Peter Grandics
Peter Grandics
5 months ago

The disclaimer on surgical masks states that they do not protect the wearer against infections. I believe this is truthful. So why is it still mandated to wear them?

Mosharraf Hossen
Mosharraf Hossen
5 months ago

Excellent..all in one..

Mohammad Adi
Mohammad Adi
10 months ago

thanks so much. i am an MD internist, your information is really good. all what i need in one place. thanks bro..

Kapil Mittal
Kapil Mittal
10 months ago

So much valuable info at one place. Thanks for that.

11 months ago

Excellent article! Thank you for being so thorough, citing resources, and presenting the facts not personal opinion. I am a Physician Assistant in New York. I have read numerous medical journal articles, CDC updates, and NIH studies. This article is by far the the most concise and practical explanation of masks and respirators.

Carolyn Smith-Evans
Carolyn Smith-Evans
1 year ago

I am in the hazardous air quality of the West Coast fires in Oregon and am trying to find masks that will help with the smoke etc. when no alternative to being outside or driving a car. Do you have information about that?

Very helpful article. Thanks!

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