What Research Is Relevant?

Covid-19 is a brand-new disease, which means scientists are still scrambling to learn the basics, including how to best prevent its transmission. This lack of data is why we must look at other, closely related examples for guidance.

Although Covid-19 is a novel disease, it is caused by the SARS-CoV-2 virus, which is closely related to the virus that caused the SARS epidemic 17 years ago. However, since Covid-19’s true genetic identity was not widely publicized until recently, influenza research probably shaped doctors’ initial opinions about face masks instead. What does the actual science say about SARS?

Can face masks offer protection against SARS?

According to research on the SARS epidemic, face masks were the most consistently effective intervention for reducing the contraction and spread of SARS. In a Cochrane Review on the subject, six out of seven studies showed that face masks (surgical and N95) offered significant protection against SARS. Hand washing was also very effective, supported by four out of seven studies in a multivariate analysis.

Although most of the studies in the Cochrane Review were on medical workers in a hospital setting, one study followed community transmission of SARS in Beijing. It found that consistently wearing a mask in public was associated with a 70% reduction in the risk of catching SARS. Additionally, the authors of the paper noted that most people in the community wore simple surgical masks, not N95 respirators.

This may be why public health agencies in many other countries encourage — or even require — the general public to wear face masks. It may also be why the CDC currently recommends that we wear face masks while in the same room as a sick family member who is not wearing a mask.

Of course, one of the most important takeaways from this data is that our hospital workers are in extreme danger given the current face mask shortage. Additionally, it is essential that we learn how to properly wear a mask, since another review on SARS health care workers emphasizes that face masks offer the most protection when they are used consistently and worn properly.

Can face masks offer protection against the flu?

In general, influenza reviews are more in line with current advice about prioritizing hand washing. Whereas most influenza studies are consistent about handwashing being good, they are somewhat inconsistent about face masks being helpful. Most studies in the above-linked influenza review found face masks to be useful protection against influenza, although the following two studies had contrasting results.

One study showed that children who improperly used masks caught H1N1 2% more often than children who didn’t wear masks (not statistically significant, if you are wondering). However, since the children in the study who wore face masks correctly cut their risks in half, the researchers suggested they should all be taught proper mask technique. The other study found an increased risk associated with wearing face masks, but based its conclusions on only 14 cases of H1N1.

Another frequently referenced review on face masks and influenza states the familiar warning that masks could increase your risk of transmission. However, their cited evidence is not a research study, but a WHO info sheet about face masks that makes the same claim — without any citations to back up the claim.

Aren’t viruses so small that they can pass through a face mask?

If Covid-19 was floating in the air, free of fluid, it would be around 100 nanometers in size. N95 respirators can only filter out 95% of particles sized 100–300 nanometers. Surgical masks offer even less protection. So the answer is yes — viruses can technically pass through both types of face masks.

But will they? That is less certain. When a person coughs, the droplets are typically somewhere between 8,000–100,000 nanometers in diameter. *Aerosols, the smallest respiratory droplets, are only estimated to comprise 0.00024% of the total volume of liquid expelled during a cough. Most droplets, especially the large ones, will quickly settle and end up on surfaces. The smaller droplets stay in the air longer — but are still large enough that a properly-fitted N95 mask could filter them out, since N95 masks are 99.5% effective against particles sized 750 nanometers or bigger.

*Influenza research finds that simple surgical masks can reduce the number of exhaled viruses in large droplets by 96%, and in aerosols by 64%. Another recent paper also confirms that simple surgical masks can reduce the detection of seasonal coronavirus in both aerosols and larger droplets. (For a deeper dive on aerosols and droplets, read my article about airborne viruses.)

Since most virus-containing fluid will end up on surfaces, it remains important to wash our hands regularly and avoid touching our faces when in public. One of the strongest arguments in favor of both surgical and N95 masks is that they help prevent us from touching our faces with dirty hands when used correctly.

Face masks might offer some protection from airborne viruses in droplets, especially if the droplets that carry them remain a large-enough size. But if you are in a dry climate and the liquid evaporates before the droplets can settle, Covid-19 might end up in a droplet that is too small to be filtered by a face mask. Also, researchers have warned that SARS could be aerosolized (made into a very tiny droplet) by nebulizer therapy and CPAP machines, which would help it penetrate face masks. This warning might be important for hospitals and nursing homes.

Are N95 respirators more effective than surgical masks?

There is an ongoing scientific debate as to how much benefit comes from wearing N95 respirators instead of surgical masks. When used properly, N95 might offer more protection since it filters out smaller particles (discussed above) and also creates a tighter seal against the wearer’s face than a surgical mask. On the other hand, N95 masks are less comfortable, are frequently misused, and are taken off more often than a surgical mask.

Theoretically, N95 respirators might be a better choice if you are taking care of someone with Covid-19. When used properly in a medical setting, N95 respirators were more effective against SARS than surgical masks in three different studies.

However, when moved out of a hospital setting and into everyday life, those differences could even out. If we look at the data on influenza, reviews typically conclude that N95 respirators are not significantly more effective than surgical masks. This is likely why the public is being advised against wearing N95 masks.

*Recent concerns have been raised about N95 respirators and other particulate masks with exhalation valves. These valves are designed to make the mask more comfortable for the wearer, but offer no protection to others if the wearer is infected. It’s currently recommended to either not wear masks with valves in public — or to wear a fabric mask over valved masks.

What about fabric masks?

Of course, stores are sold out of both surgical and N95 masks, which means that homemade masks are the only option remaining for many of us. Even hospitals are using and requesting homemade fabric masks — that’s how bad this shortage is. Although none of the above research applies to fabric masks, there is a little bit of research on this topic.

An important thing to keep in mind is that every cloth will have a different weave, which allows larger or smaller particles to pass through. As you might imagine, researchers find that N95 respirators are more effective than cloth at preventing small particles from passing through. However, fabric masks might be comparable to some surgical masks.

*To my knowledge, only one research article has tested fabric masks against surgical masks in a clinical setting. They found that nurses and doctors wearing surgical masks got sick less often than those wearing fabric masks. Although I do not question this article’s data and agree that the surgical masks outperformed the fabric masks used in this experiment, I do want to point out:

The authors’ financial ties to 3M

99.6% of the “control” population wore masks (cloth, surgical or N95) when appropriate

Medical workers wearing cloth masks treated up to 661 patients during the experiment, while the “control” group maxed out at 199 patients — which means the most active cloth mask wearers were exposed to 3 times the risk of their “control” counterparts

The cloth chosen for this experiment had a much worse filtration efficiency than the material used in the surgical masks

85% of the confirmed illnesses in this study were due to the common cold (rhinovirus) which, according to recent research, penetrates surgical masks much more effectively than influenza or coronavirus

The authors of the study have issued a public statement emphasizing that all health workers should wear respiratory protection, and that a cloth mask might offer protection when disposable masks are unavailable

If you do wear a fabric mask, treat it as you would a disposable mask and don’t reuse it without ensuring it has been sterilized. Fabric mask disinfection is not an active research topic, but the standard wisdom of running it through a long, hot wash is supported by research showing that the SARS virus can be inactivated by high heat and detergent. UV light can also be used against SARS, but since the light needs to reach the virus in order to inactivate it, UV might not effectively sterilize porous fabric.

What are the dangers of wearing a face mask

Most importantly, we don’t want our nation’s supply of face masks to be depleted while we’re still in the early stages of the pandemic and our risks are relatively low. Maintaining the health of our medical workers is essential at this stage.

While wearing a mask, you might end up touching your face more often and transferring the virus from your hands to your face. In order to avoid this, wash your hands both before and after adjusting or removing your face mask.

If your mask becomes moist against your skin, viruses could diffuse across the mask and reach your face. If you are a medical worker or taking care of someone who is sick, this is very important. However, since viruses are not alive and cannot replicate outside of a living cell, there is no risk that a face mask will somehow increase the quantity of viruses you are potentially exposed to. Any quantity of viruses that diffuse across a wet face mask will still be fewer compared to if you weren’t wearing a face mask.

Used face masks need to be sterilized or disposed of properly. Hospitals are currently figuring out ways to sterilize and reuse disposable face masks. If you are disinfecting multiple face masks, make sure that the wearers’ names are written on the masks and never used by anyone else. Ethylene oxide or heat treatment at 70C/160F for 30 minutes might be the best options for sterilizing porous face masks.

*For a more historical perspective on the pitfalls and dangers of masks, read my article about why gauze masks failed in 1918.