The very simple reason why masks are safe to wear (and everything you’ve read which says they aren’t is wrong)

Masks are safe to wear. Thanks to analytics I’m aware that only about a third of readers of my posts actually read them in their entirety. So before you go I want you to know: masks are safe to wear. Whatever you may have read online, they are safe. If that’s all you take away that’s enough for me. My ego can take it. If you’d like to know more then please read on. It’s really very simple: size matters.

I’ve argued before that if you get 100 scientists in a room, broadly speaking, you’ll get consensus. The fact that scientists agree isn’t some grand Illuminati scheme; it’s because they follow a scientific method in the pursuit of replicable results. Conspiracy theorists do not. Get 100 conspiracy theorists together and you’re likely to get 100 different stories. They’ll agree that there’s a conspiracy but not on which one. It was the Mafia. No, it was the CIA. No, it was the military-industrial complex.

So it is with mask conspiracy theories. On the one hand, you may have seen that masks offer no protection against the SARS-CoV-2 virus because the holes in them are too big to block the virus. On the other, you may have read that the masks stop carbon dioxide from being breathed out by wearers result in the build-up of carbon dioxide, a lack of oxygen being breathed in. You may have heard of a supposed epidemic of people ill with hypercapnia (too much carbon dioxide) or hypoxia (lack of oxygen). Or you may have heard that they are causing dangerous infections due to build-up of bacteria.

It is nice of them to offer a choice. All are wrong. This is why.

An individual SARS-CoV-2 virus is tiny. Each particle is 100 nanometres across. Outside of the human body, these particles are called virions, biologically inert showing no signs of life. Inside a human body, the particles infect cells, hijacking the cells’ genetic material with their own. It’s this process which is called a virus. So, one conspiracy goes, 100 nanometres is too small for surgical masks to block so what’s the point?

However, SARS-CoV- 2 virions don’t exist in isolation. They are suspended in droplets of respiratory secretions much larger than the virions themselves. It’s these droplets which surgical masks are designed to block. And we know exactly what size droplets they can block thanks to research.

COVID-19 is not the 21st century’s first pandemic.

So-called ‘avian flu’ or H5N1 was the focus of the international community at the turn of the century. Here in the UK, this culminated in an outbreak at a turkey farm in early 2007. In the spring of 2009, a novel H1N1 influenza virus caused a pandemic known as ‘swine flu’ killing 151,700–575,400 people worldwide.

A surgical mark, Source: Shutterstock

It was against this backdrop that the Health and Safety Executive (HSE) in the UK investigated the protection that different masks afforded the wearer against droplets containing airborne virions. This included surgical masks.

They focused on the size of droplets a surgical mask could stop. The key distinction was droplets greater or less than 5 micrometres, or 5000 nanometres across. Droplets larger than 5000 nanometres tend to drop out of the air quickly due to their weight. Droplets smaller than 5000 nanometres are called aerosols and are more likely to remain airborne and represent a greater infection risk.

While they found that respirators could block 100% of aerosols they found only a 6 fold reduction by surgical masks. They concluded that surgical masks should provide “adequate protection against large droplets” they “might not sufficiently reduce the likelihood of transmission” from the smaller aerosols.

In other words, the holes in surgical masks were found to be too big to stop droplets smaller than 5000 nanometres across. This is why surgical masks are not recommended to be worn for aerosol-generating procedures such as intubation which make those smallest droplets.

Yet conspiracy theorists would have you believe that a mask which can’t stop droplets 5000 nanometres in diameter somehow does stop carbon dioxide molecules which are only 0.334 nanometres across. Or oxygen molecules which are only 0.299 nanometres from end to end. It is a physical impossibility.

An FFP3 mask, Source: Shutterstock

So that’s surgical masks but what about respirator masks? Surely the FFP3, worn by medical staff during aerosol-generating procedures must be a risk?

Of course not. True, the pores in respirator masks are smaller than in surgical masks. FFP3 masks are designed to filter 99% of all particles bigger than 600 nanometers. Small enough to stop aerosols, which is why they are worn by health workers for aerosol-generating procedures, but still much bigger than carbon dioxide and oxygen molecules.

Cloth masks, Source: Shutterstock

And what of cloth masks? The filtration effect of non-medical cloth masks is less than surgical masks or FFP3 masks. Therefore, for the reasons we’ve already looked at it would be impossible for cloth masks to cause hypercapnia or hypoxia. They are safe to wear.

However, a study in the British Medical Journal in 2015 advised that due to “moisture retention, reuse of cloth masks and poor filtration” cloth masks should not be used by healthcare workers. This is not surprising: cloth masks are not as good at filtering the air as surgical masks or FFP3 masks and so should not be worn by healthcare workers looking after patients in a high-risk environment.

For non-medical use such as going to the shops or on public transport however, a cloth mask is perfectly fine. The idea is that they trap the droplets spread by the wearer when they talk, cough or sneeze. This means they protect other people from the person wearing the mask. The CDC published a study in November 2020 which recommended cloth masks as suitable for community use. They advised that they should be washed regularly.

So everything you have read about masks causing hypoxia or hypercapnia is wrong. Size matters.

What about the conspiracy theory that masks can cause infections in the wearer through the build-up of bacteria? Again, wrong. There is no evidence of masks causing infections in the people wearing them. Why would there be? You’re wearing a mask. Unless you’re borrowing it (don’t) any saliva etc. on the inside is yours. If you have an infection and then put a mask on you’ll still develop symptoms but, of course, the mask didn’t cause that.

There is evidence of bacteria build-up on the outside, however. This makes sense. Surgical masks aren’t made up of some special anti-bacterial material. Latex gloves are the same. Although they’re a physical barrier they don’t actually repel bacteria. If you touch a surgical mask you’ll transfer any germs on your hands. It’s been shown that surgical masks can pick up significant bacterial growth on the outside correlating with bacteria in the environment. It’s been recommended that surgical masks should be changed every 2 hours for this reason. If you’re wearing a mask it needs to be regularly changed and any touching reduced to a minimum. Same as if you’re wearing latex gloves: they still need to be changed regularly or washed.

So there you have it. Masks combined with other social distancing measures have been shown to reduce the spread of SARS-CoV-2. As shown here masks are also completely safe. Whatever you may have read online, they are safe. You should wear one. Doctor and Educator. Podcast Blog on Medical Education and History of Medicine