How effective are face masks?

Published on: April 9, 2020

In this article, we’re going to evaluate how well a surgical face mask and an N95 protects you from viruses like SARS-CoV-2.  I’m also going to explain why a cloth mask over your N95 mask is a good thing, and I’m going to walk through some common disinfectant techniques – including which ones to avoid for masks.

But, to start with, let’s talk about the particle size of a virus.

Particle Size of SARS-CoV-2 (the virus that causes COVID-19)

SARS-CoV-2, that insidious coronavirus that causes COVID-19, ranges in size from 50 to 200 nanometers (nm).  Converting that over to microns (micrometers, typically abbreviated µm) that’s 0.05 to 0.2 µm.  

Okay, so let’s talk about the particle size that a mask can filter.  Hang on just a second. 

Respiratory Droplets

It’s still widely accepted that SARS-CoV-2 spreads from one infected person to healthy people via respiratory droplets.  And, many viral particles are suspended within those droplets.  How many viral particles are suspended depends of course on the contagious person’s viral load.  So, to evaluate a mask’s effectiveness against SARS-CoV-2, we need to look at the particle size of respiratory droplets.

Believe it or not, there have been studies conducted to look at the particle size distribution of respiratory droplets when people cough, sneeze, and talk.

Researchers have found a particle size distribution generally between 1 µm and 2000 µm (microns or micrometers).  1 µm is about five times larger than the biggest SARS-CoV-2 virus particle. 

Unfortunately, we aren’t quite finished yet.  Evaporation of respiratory droplets starts when they leave the mouth or nose of a person.  The relative humidity, temperature, and ventilation (closed room vs. room with a fan or proximity to an air vent) impacts just how fast that respiratory droplet evaporates.  And, believe it or not, researchers have actually looked at how quickly respiratory droplets decrease in particle size once they leave a person.

I’m not going into all the physics and all the findings here, I just mention it because the size of respiratory droplets change quickly enough to be measure over pretty short distances of less than a foot.  If you want to read the full article, it’s linked in the paragraph above.

Is SARS-CoV-2 Infectious as an Aerosol?

New research suggests SARS-CoV-2 may be spread as an aerosol.  That means the virus is viable in the air even after the respiratory droplet has fully evaporated around it.  It may settle on surfaces, but it can be re-suspended in the air, kind of like dust or pollen and breathed in.  The reason it is believed it can be transmitted this way is because the virus doesn’t die immediately once it’s expelled from its host.  It can survive in air for up to 3 hours. 

Studies also show it can survive on copper for 4 hours, cardboard for 24 hours and plastic and stainless for up to 72 hours

While it’s a more unlikely route of infection – you have to inhale enough of the viral particles to actually cause infection – it’s not improbable. 

And, epidemiologists believe this route of transmission may be why so many members of a Washington choir group contracted COVID-19.  Over the course of their practice, the air around them literally became filled with viral particles.

Two potential pathways for respiratory transmission

Because SARS-CoV-2 has a relatively long survival time in air and on surfaces, I’m going to mention the particle size of both the virus itself (ranging from 0.05 µm to 0.2 µm) and respiratory droplets (typically reported as 1 to 1,000 µm) when evaluating the efficacy of masks.

How does an N95 Mask work?

First, why is it called N95?

NIOSH – the US National Institute for Safety & Health made up a classification system for respiratory filters like the N95 face mask.

The letter N means the filter material is not resistant to oil.  And, the 95 means the filter is capable of filtering 95% of particles that are at least 0.3 µm in size or larger.  Now, if you look at the size of the virus itself, I know you’re thinking, “What’s the point? We’re all doomed!” well, don’t take your face mask off just yet.

There’s a reason why 0.3 µm (that’s 300 nanometers, nm) is used as the metric for assessing performance of N95 masks.  It’s the most penetrating particle size (you may see this written as MPPS in more scientific papers). 

How can 300 nanometers (0.3 microns, µm) be more penetrating than smaller particles?

Three types of ways N95 masks filter air

An N95 mask relies on three different methods for filtering particles from the air you breathe.

  • Inertial impact
  • Diffusion
  • Electrostatic attraction

Let’s talk about these one by one.

Inertial impact – when a particle hits the mask, it either bounces off the mask or it sticks to the surface of the mask.

To make its way into our respiratory tract, that particle still has to get through the mask somehow.  Enter diffusion and electrostatic attraction.

Diffusion – The N95 mask itself is made of non-woven polypropylene fibers more than one layer thick.  The non-woven nature of the fibers results in a tortuous path through which any particle has to traverse to make it to the inside of the mask where it can be inhaled.

Electrostatic attraction – During the N95 manufacturing process, the polypropylene fibers assume a surface charge.  This use of electrostatic charge (you may see this referenced as electret fibrous filter media) enables low resistance to air flow while allowing the mask to filter out ultrafine particles.

Impaction and diffusion are the two forces that have the greatest filtration capacity, and these forces just happen to be weakest at around 300 nanometers (0.3 microns, µm).

collection efficiency of N95 filters based on particle size from Wikipedia article

Sterilizing N95 Masks at Home

There’s no good way to sterilize your disposable N95 mask at home.

Isopropyl alcohol (aka rubbing alcohol aka IPA aka isopropanol) eliminates the electrostatic charge of the mask which doesn’t render it ineffective but it definitely reduces its efficacy and puts you more at risk of exposure. 

Before you reach for the Vodka, ethanol is in the same category so go ahead and drink that Vodka instead of dousing your mask with it. 

A soap spray is also prone to discharge the electrostatic charge because of its surfactant nature.  I found one report that said a mixture of anionic and cationic surfactants could restore the electrostatic potential of the mask, but unfortunately, those folks were not kind enough to publish that ratio or the surfactants they used.

Hydrogen peroxide is risky business because while it can disinfect, I’m just not sure spraying if you spray your mask daily after work that it will be thoroughly dry for you the next day, and it’s not good for you to breathe hydrogen peroxide.  So goes bleach, Lysol, and other common disinfectants.

Also, while there is ample literature that UVC sterilization works, these folks apparently didn’t bother to study whether UVA or UVB sterilization works, and of course UVC is entirely blocked by the atmosphere.

One more thing – as you wear an unprotected N95 mask around people with COVID-19, the number of viral particles captured in your mask starts to build, and those particles have an opposite electrostatic charge than the mask.  Over time, collection of viral particles can diminish the electrostatic charge of the mask also. 

I don’t know what that looks like once the viral particles “die” but my guess is just because they are no longer capable of causing infection doesn’t mean that protein spike and viral particle just disappears… those “dead” viral particles are still going to negate the electrostatic charge of the mask because the whole reason electrostatic attraction works is because the charge on the mask is opposite that on the outside of the virus.

However, the silver lining is that these trapped viral particles also make it more difficult for new viral particles to penetrate the mask because they become part of the tortuous path new particles have to traverse to make it into the stream of air you inhale.

The dark side is that they also make it harder to breathe because they block the path of airflow through the mask, and at some point, you have to toss that N95 because it’s too hard to pull air in through it.

So… what can you do to extend the life of your N95 mask?

Wear a cloth mask over your N95 mask

A cloth mask protects the N95 from exposure to the virus.  The cloth mask however only relies on two of those filtration methods listed above:  inertial impaction and diffusion. 

A double layer (or thicker) cloth mask with insert is definitely better than a single layer to help with diffusion – the more layers, the longer that diffusive pathway. 

And, most cloth masks out there are also made of woven fabric instead of non-woven fabric, so diffusion happens more rapidly through these masks than through an N95 mask because the diffusion pathway is not tortuous through each layer – it’s straightforward through each layer.

cloth face mask and materials needed to create face masks to protect against COVID-19

This is why it’s important to swap out your cloth masks often (and of course use good donning & doffing practices when switching it out to avoid contaminating your N95).

Logistics of switching out cloth masks safely

If you’re an essential worker and providing your own cloth masks, then carry two plastic sealable totes – like a Sterilite tote with a lid – with you to work each day.  One of those totes (please color code and label – the days are long and you will thank yourself when you’re tired) will contain your clean cloth masks and the second will contain your dirty cloth masks. 

For the “dirty” bin, go ahead and place an open lingerie or delicates bag inside – most of these cloth masks need some protection in the laundry to keep from getting ripped up by the washing machine (I’m speaking from personal experience here). 

Also, fill the bin about a third to half full with some soapy water before you head to work.  You can also spray 70% isopropyl alcohol in the “dirty” bin (just a few spritzes is fine, it will partially vaporize inside the closed container and help to decontaminate your masks). 

Then, when you get home, dump the entire contents of the dirty bin (including the soapy water or alcohol residue) into the washing machine and close the lid.  If you’re too exhausted to do a wash that day, don’t worry about it, but at least this way, you’re minimizing your exposure to potentially contaminated masks and doing it in a way that’s sustainable for the duration… however long that is.

If you need some places to order extra cloth masks from, you can find those linked below in references.  The same goes for your laundry bags.  If you’re in the healthcare field, I know you have your own protocols in place for how often to swap out your cloth masks. 

If you’re another essential worker, please have enough cloth masks on hand to change once per hour at a minimum and a few spares to change out if you feel you may have been near a person with COVID-19. 

Of course, this frequency is not based on anything scientific - just my own comfort level with a fairly thick cloth mask, so please take that suggestion with a grain of salt.  If it's easier for you to swap out less often and do it safely (because your hands are clean and you're able to isolate for a few minutes to safely swap), do that, if it's easier to swap out more frequently and safely, do that.

I know that was quite a tangent, but I’m trying my very best to help every essential worker out there with legit information and practical tips while you’re likely struggling for mental (and emotional) bandwidth. 

I also know this post is nearly a month into this declared pandemic, and I apologize for that – research on my end has been slow going (and frustrating to navigate).

Surgical Masks

If you don’t have an N95 and are not in healthcare, a surgical mask still offers some protection.  In fact, one study shows a surgical mask blocks about 80% of particles (compared with a handkerchief that blocks less than 30%). 

The folks over at Smart Air Filters really put surgical masks to the test evaluating how well surgical masks block virus sized particles when they’re being worn – the answer, 63% effective. 

So, the theory that surgical masks are ineffective because they fit poorly is unfounded – they definitely help!  

Now, would I wear a surgical mask alone before intubating a COVID-19 patient? 

No way in hell. Why? 

Because intubation raises a very real risk of producing aerosol.  Because the likelihood that any one of us contracts COVID-19 depends on the viral load we're exposed to and intubation is a time when viral load can be very high because the patient is very sick and the HCP is in very close proximity to the patient (social distancing doesn't apply when healthcare workers are trying to save somebody's life). Besides, I don't know how to intubate anyways :)

But, I’d go to the grocery store with a surgical mask (and a cloth mask over the top of it) while also practicing social distancing. 

Here’s the other thing – if we ALL wear masks, then we increase that efficacy rate and reduce the potential of spreading this deadly virus even more. 

Surface Disinfecting & Contact Times

It’s impossible to talk about disinfectants without also talking about contact times.  Why is everybody saying “Wash your hands for at least 20 seconds” instead of just “Wash your hands”?  Because duration matters!

Here’s a list of easy to make disinfectant solutions at home published by the CDC and the EPA.

Homemade Solution

How to Make

Required Contact Time (minutes)

Bleach Solution

1/3 cup bleach in 1 gallon of water


4 teaspoons bleach per quart of water


70% Isopropyl Alcohol

Rubbing Alcohol – make sure it’s 70%


ECRI published a table listing different disinfectants
and the necessary contact times to kill novel coronavirus, SARS-CoV-2, the virus that causes COVID-19.  I’ve copied part of their table below, and if you’re interested in the whole table, just visit the link.

Product Name

Required Contact Time (minutes)

Pine-Sol Spray 19054


Lysol Brand Heavy Duty Cleaner Disinfectant Concentrate


Lysol Brand Disinfectant Bleach Plus


Lysol Brand All Purpose Cleaner


Lysol Laundry Sanitizer


If you want a great illustration of how soap destroys novel coronavirus, click this link and scroll down to the last illustration (which is also a video).

The really great thing is – laundry detergent is all you need when you’re washing clothes too. 

Here’s a list of some places to check for face masks:

I haven't tried ANY of these yet although I have two of the three on order.  Also, please keep in mind that to my knowledge, none of these companies are testing these masks for any particle assurance, and the masks should be worn over either an N95 mask or a surgical mask to offer maximum protection against coronavirus.

There's one company in LA that's testing masks with a particle counter, but I cannot find that they're offering those masks for sale yet, so they're not on the list.  I'll update if and when they start offering those.

Janery - I love Janery's masks because Jane has put some serious thought into the design.  They are shaped more like a surgical mask than an N95, they have a metal piece to create a tighter seal around your nose, and they have a pocket to add your own filter.  Plus, Janery donates a mask for every one purchased.

Be sure to sign up for Janery's newsletter to stay in touch when these are released.

Headcovers Unlimited - I love Headcovers Unlimited because when there's not a pandemic, they make headgear for cancer patients.  Since the pandemic started, I've bought headcoverings for myself from this company to avoid having to wash my hair every time I make a trip into a public place (yes, I'm paranoid).

While this company sells masks, I eschew them in favor of Janery's masks which come with an adjustable metal nose piece and just are better for fully fitting over either a surgical mask or an N95.

Reusable Face Masks with Disposable Activated Carbon Inserts - The beauty of these types of masks is an activated carbon layer that provides an immense surface area of tortuous pathways to trap viruses. 

The only warning - activated carbon is not good for you to breathe, so it's necessary to be particular about which one of these masks you purchase.  I like the one linked above because the activated carbon is embedded onto the filter substrate.

Full Disclosure: Other activated carbon masks are available where the activated carbon is loose and held within a pouch insert.  Those particular ones make me very nervous because activated carbon is extremely fine, and if those pouch inserts release dust onto your hand when you hold them or as you're trying to insert them, then I worry about breathing in that dust. 

I know we all have one huge worry right now, but miner's lung is a real condition, and I'd opt for a cloth mask without the carbon insert or one where the activated carbon is embedded onto the filter substrate like the one linked above instead of one with loose carbon held inside a pouch insert. 

The really great thing about these masks is that the activated carbon layer is removable & replaceable, and even without that insert, there are four layers of spun fiber in addition to that carbon layer, so these are a great choice for longer wear times (though keep in mind, there still is not sufficient data supporting wear time and protection against SARS-CoV-2).

And, where you can purchase extra laundry bags:

I have two large washable mesh bags that I love, and alas, I cannot remember where on earth I bought them from.

Because of COVID, I'm ordering more of these laundry bags for keeping cloth masks safe during wash, and these are the two companies I'm trying.

The Laundress offers a washable bag bundle - one large bag and one small bag for $15.

Dropp also offers washable mesh bags - 2 large for $15.  These look remarkably similar to the ones I currently have.

If you're feeling isolated or overwhelmed or need some mental relief right now, please check out any of these:

Ways to keep company while sheltered in place

A hiker's guide to keeping sane while sheltered in place

Why COVID-19 is such a big deal (and how to keep your chin up)

Books to soothe your adventurous spirit during shelter in place

If you feel like you’re starting to get sick or just want some extra immune support, I encourage you to check out How to improve lymph flow

Finally, if you're having trouble sleeping, be sure to sign up for the newsletter so you get reminded of this upcoming post:  How to get a good night’s sleep

That’s all I can offer right now.  I know it’s not much, and I know the things you need most right now are in short supply. 

If there’s anything you can think of that I can help with, please leave a note in the comments below, get in touch with me by email, or drop me a line on IG.  My thoughts are with you all.

Be safe.  Be well. 


Respiratory Droplet Size of a Sneeze & Cough

Particle Size in Respiratory Droplets from Patients on Ventilators

Deep Exhalation and the release of particles in patients on ventilators

Effect of filter's electrostatic charge in presence of organic solvents like alcohol

Smart Air Filters:  Can Masks Capture Coronavirus?

Spread of Coronavirus via Aerosol

Choir group contracts COVID-19 en masse during rehearsal

Recommended handling of poultry potentially infected by H5N1 influenza

How N95 Masks Effectively Filter Air

How Alcohol impacts the electrostatic charge on filters

How N95 masks are impacted by alcohol

Characteristics of Respirators and Face Masks

What's a reliable way to disinfect an N95 mask without sacrificing function?

Evaluation of 5 Decon methods for filtering facepiece respirators

Aerosol transmission is an important mode of transmission of Influenza A




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