In all the questions I’ve been directly asked or encountered online during the COVID pandemic, the effectiveness of masks is undoubtedly the most frequent. And rightfully so, when recommendations have swung so far from one side to the other it’s hard to know what to do. Added to the confusion are research studies that show masks are effective and others that show that they aren’t, and it becomes nearly impossible to make sense of it all. So what’s the truth? Are masks really effective?

Like any good scientific query, the answer to the question depends upon how specific the question is. Are masks effective? is not really a good question because it is too broad — are they effective at what, for whom, and when? Let’s break this down to two basic questions that differ based on what side of the mask we’re evaluating.

Are masks effective at preventing COVID-19 for the mask wearer?

The answer to this question depends on the type of mask we’re talking about (see, even this question is too broad). Medical N95 masks block 95% of a challenge aerosol (typically sodium chloride, which is 0.3 microns in size) in a tightly controlled environment. SARS-CoV-2 is about half the size of the test aerosol at 0.12 microns, but is present in far less concentrated amounts than the challenge test (even when caring for someone with COVID-19). So N95 masks are reasonably effective at preventing the virus from reaching the nose and mouth of the mask wearer, provided they are properly fitted. Notably, N95 is not the only medical-grade respirator (some are better and some are worse at the challenge test) though it is the most common. In contrast, homemade cloth masks are much more porous and much less effective at preventing tiny virus particles from getting through, but it also depends on the material, how many layers, and whether a filter is also included. The more material (layers or filter), the better the mask will be at protecting the wearer. In general though, cloth masks are not as good as medical-grade masks at preventing infection for the mask wearer.

Early in the pandemic masks were not recommended for the general public for two primary reasons. The first was that there weren’t enough cases to warrant it and second, there was reasonable assumption that SARS-CoV-2 was transmitted in a similar fashion as it’s cousins SARS-CoV-1 and MERS-CoV (back to that in a moment). America’s Doctor-in-Chief Anthony Fauci was quoted in late February saying “There’s no reason to be walking around with a mask”. And take a look at this Tweet from the U.S. Surgeon General Dr. Jerome Adams.

In retrospect, these were major missteps that provided ammunition to what would become an anti-mask establishment. What’s important to remember is that both of these statements were referring to medical-grade masks for the general public. At a time when the U.S. was preparing for the surge in infections and hospitals reaching capacity, the last thing we needed was for the public to buy all the medical masks and leave healthcare workers exposed. Secondly, the U.S. had very few cases of COVID-19 at the time — 62 known cases on February 29 — so a recommendation to wear a mask in public at that time would have seemed like an absurd overreaction. So let’s ask the reverse question by looking at the other side of the mask.

Are masks effective at preventing coronavirus spread in a community?

If homemade cloth masks aren’t very effective at blocking virus particles from reaching the face of the mask wearer, why would they be effective for everyone else? It’s a valid question and one I’ve seen in various iterations on social media. The answer lies in how the virus is transmitted from one person to another: respiratory droplets. When you cough or sneeze (and honestly even when you talk), droplets containing all sorts of microorganisms including viruses are expelled into the air in front of you. A barrier, even a relatively porous cloth one, close to your face prevents these droplets and most of the microbes within them from getting through, thereby preventing the spread of your germs to other people.

I mentioned above that early in the pandemic we thought that SARS-CoV-2 was transmitted like SARS-CoV-1 and MERS-CoV. This was a reasonable hypothesis since most respiratory infections spread person-to-person when an infected individual coughs or sneezes, expelling the greatest concentration of viruses. When this is the case, it’s relatively straightforward to contain an outbreak since sick people can be easily isolated (the SARS outbreak in 2002-2003 was limited to less than 10,000 people because of this). What we didn’t know at the time was that SARS-CoV-2 had a trick up it’s sleeve — it can spread from asymptomatic and pre-symptomatic people. That is, it can be spread from healthy individuals out in our communities.

Putting together the rise in cases in the U.S. and evidence that SARS-CoV-2 could be spread by healthy-appearing people, the recommendation to wear a mask in public was a no brainer. And just to be clear, Dr. Fauci and Dr. Adams didn’t just flippantly ‘change their minds’ about masks. They followed the research, tracked the data, and updated their recommendations–this is how science works. Studies have now shown that public health interventions including mandated mask wearing have reduced COVID spread in multiple community settings.

TAKE HOME MESSAGE: Medical-grade masks are effective at preventing infection for the mask wearer, but should be reserved for healthcare workers because of close and prolonged contact with infectious patients. Homemade cloth masks are much less effective for preventing infection for the individual wearing the mask, but relatively good at keeping microbes to oneself. Homemade cloth masks stifle community spread when the majority of people are wearing them.

When presented with conflicting or evolving instructions, people will often revert to their inner beliefs whether they align with scientific data or not. They will find the viewpoint and studies that support what they already believe and stick to it because it is undoubtedly more comforting to be validated than to change your opinion. Most of the studies cited by anti-maskers refer to those showing that cloth masks are not effective for the mask wearer, but fail to realize that that’s not the point. In the middle of a pandemic that has swept through the world claiming at least half a million lives, anti-mask sentiment is a dangerous view point.

And yet, I get it. It’s hard to change behavior. It’s annoying to bring masks with you everywhere. It’s difficult to breathe in the mask. It’s uncomfortable to sweat in the mask. I would be embarrassed by a mask tan (or burn as my skin prefers to do). No one in their right mind wants to cover their face with a mask. But this we must do.

I wear my seatbelt in the car and I put on a helmet when riding a bike, but these analogies do not suffice. They are about me. I do these things to protect myself. We are being asked to change our behavior for the good of others and our selfishness shows. Generations before us were forced to risk their lives for the freedom of others. We are being asked to cover our faces to fight in this battle. This we must do.

Read more from the CDC on cloth face masks