With the coronavirus (COVID-19) pandemic, you've likely by now encountered many different types of face masks on the market, whether cotton/fabric or surgical masks, or the more specialised N95 and FFP2/3 masks.
But what do they actually do and how do they work?
Donald Trump had at one point urged Americans at a press briefing to use their scarves as makeshift masks. So how good was his advice, and should you follow it?
If you want to know how masks work in the context of Covid-19, you first need to know two things:
- How SARS-CoV-2 is transmitted
- Particle Science
SARS-Cov-2 transmission
COVID-19 is spread primarily through droplets, which carry the virus in saliva and discharge from the nose.
When a person coughs, sneezes or exhales, these droplets can be deposited on surfaces and objects (fomites), which can then transmit the virus when a person touches this, and then touches their eyes, nose or mouth.
The debate is still ongoing about the route of transmission of Covid-19 and whether there is significant long-range airborne transmission. The World Health Organization (WHO) have noted that airborne transmission may happen in very particular circumstances, such as during Aerosol Generating Procedures (AGP) — specialised procedures carried out by medical staff, such as intubation.
Particle science
Droplets are a type of particle, and particle science is a complex discipline in itself. What makes it even more complex is that there is some variation in the literature on the classification of particle sizes.
Generally speaking, however, particles:
- <100 micrometres (microns) in size are considered inhalable - they can be breathed in.
- >20 microns are 'large droplets'.
- >5 microns are non-aerosol and termed 'respiratory droplets'. When exhaled, because of their larger size, like most other things, they cannot escape gravity, and generally fall downward within 2m, or evaporate.
- <5 microns - these can travel deep down into the lungs. They are light enough to float in the air, and can be blown and carried by the wind. A number of these particles suspended in the air are known more commonly as 'aerosols'. These can be generated to varying degrees when talking, coughing or sneezing.
- Droplet nuclei - to make things even more confusing, there are also 'droplet nuclei'. These are the dried-out residue of droplets once the water content has evaporated, and may carry virus particles inside.
What types of face masks are there?
Broadly speaking, they can be categorised into 'surgical masks' and 'respirators'.
Respirators can be further subdivided into disposable or reusable. Reusable includes half and full facepiece masks.
Surgical masks and respirators are governed by different standards across the US, Europe, Australia and New Zealand, and also China (see below):
What are the differences between surgical masks and respirators?
Surgical masks are generally:
- Disposable and single-use
- Loose-fitting
- Used mainly to prevent droplets from the wearer from being expelled into the environment
- In terms of particle size, surgical masks may only offer some protection from the largest of droplets falling onto it. For example, saliva globules from someone sneezing directly onto your face. It would not protect from the much smaller particles or aerosols we mentioned earlier from being inhaled, simply because there are too many areas of leak around the mask.
- Rarely filter particles <5 microns and their filtering efficiency can vary from 14%-99%
Whereas respirators:
- Are disposable, or have changeable filters
- Are tight-fitting
- Primarily protect the wearer from inhaling hazardous airborne particles
- Can protect from both large and very small airborne particles (we will go on shortly to look at just how small the particles they can filter)
Depending on which part of the world you're from, respirators are given different names (see below):
As you can see, N95 in the US is the rough equivalent to FFP2 in Europe, P2 in Australia and New Zealand, and the KN95 in China. The US N99 is the equivalent of FFP3 in Europe.
Respirators conforming to standards should reach a minimum filter efficiency. The higher the percentage number, the more efficient the respirator should be in filtering out airborne particles. For example, the N95 filters out at least 95% of particles at 0.3 microns in diameter, whereas the FFP3 filters at least 99%.
This raises the question: how big is the size of a SARS-CoV-2 virus particle?
Individual particle sizes may vary, but generally, they are in the region of 0.12 microns (approx. 120nm). Many virus particles, don't forget, are contained within the droplets we mentioned earlier.
But wait, you're probably thinking, wasn't it mentioned earlier that respirators filter out particle sizes of 0.3 micrometres? So how can SARS-Cov-2 particles be filtered out if they are smaller than that?
How does mask filtering work?
The exact science is complicated, but very interesting. Essentially, 0.3 micron sized particles are considered to be the hardest to trap.
Now imagine there is a net, which is the filter, and a marble is the 0.3 micron particle.
Marbles travelling towards the net will mostly get trapped, but a very small few will pass through, because they are the perfect size to do so.
Any particle bigger than the marble — let's say a tennis ball, for example — is easily caught in the net.
You would therefore logically conclude that any particle even smaller than the marble (i.e. <0.3 microns) would fit through the net even more easily. Interestingly, it's a bit more complicated than that.
When particles get that small, they start to behave very strangely. Because they have so little mass, they are bounced around like a pinball, even by gas particles around them. This is known as Brownian motion. As a result, they move in a random zigzag pattern that actually makes them easier to catch in the filter media.
Therefore, respirators are measured on their ability to trap the most difficult particle size of 0.3 microns, which is known as the Most Penetrating Particle Size (MPPS).
Taking the N95 mask as an example: if it can filter out 95% of particles at 0.3 microns, which are the hardest to capture, it can generally filter out smaller particles like Covid-19 with no problem.
If fitted correctly, respirators have generally been found to filter efficiently at particle sizes <0.1 microns, although the absolute lowest limit is difficult to pinpoint exactly.
Fit testing
Which brings us onto the next concept of fit-testing. Because people come in all different shapes and sizes, there is not a one-size-fits-all mask. For respirators to be truly effective, they must be fit tested so a tight seal can be formed. Surgical masks do not have the same effectiveness or purpose because they are loose fitting with gaps around the sides.
Fit testing is a process carried out by a trained professional, who ensures there is an adequate seal between the wearer's skin and the facepiece.
This is most commonly done with various 'taste' tests using different sprayed substances. If the wearer can taste or smell these while wearing the mask, then the fit test is deemed failed.
Some of the commonly used taste substances are:
- Isomyl acetate (banana smell)
- Saccharin (sweet taste)
- Bitrex (bitter taste)
Fit tests are an important point to bear in mind, particularly for members of the public who may wear inadequately fitted respirators and be given a false sense of security.
How do home-made masks compare?
As a rough comparison:
In one 2014 study, using what is known as particle penetration testing:
- Cotton masks were found to block about 30% of particles fired at them, while
- Cotton handkerchiefs blocked from 2% (1 layer) up to 13% (4 layers).
In another 2010 study, researchers looked at common fabric materials over 5 categories including sweatshirts, T-shirts, towels, scarves and cloth masks. On average,
- Sweatshirts ranged from 18% to 60%
- T-shirts <14%
- Towels 34 to 40%
- Scarves between 11% to 27%
- Cloth masks blocked from 10% to 26% of particles
What's interesting is that the US Centers for Disease Control had at one point recommended the wearing of cloth face coverings in public spaces where other social distancing measures were difficult to maintain.
Summary
Surgical masks are more useful to protect others from your secretions, otherwise known as 'source control'.
Respirators are more widely used in medical/industrial settings, where exposure to airborne particles would be a problem.
As an endnote, wearing masks alone is not a foolproof way to avoid infection and is one more layer of protection alongside effective infection control measures.
