“My friend told me that masks only work half the time and that there’s no difference between wearing them and not wearing them.”

This was an unsolicited outburst from my 10-year-old daughter the other day. She was quoting a girl in her class. I was not entirely concerned with the opinion of the preadolescent but was concerned about where this gross misinformation was coming from. While I didn’t investigate, I assume that this child did not do an exhaustive search of the latest literature on the effectiveness of face coverings in preventing COVID-19. This would mean that she was parroting something she heard from an adult. While I again did not investigate, I assume that this adult did not do an exhaustive search of the latest literature on the effectiveness of face coverings in preventing COVID-19 either. As is the case for many things these days, lack of knowledge is not an obstacle to the vehement expression of opinion. You only need to read an online article to form a strong and unwavering opinion of anything.

But since the question is floating around out there I thought I would give an updated answer to the question that just doesn’t seem to want to die: Do masks work or not?

Early on in the pandemic masks were not recommended by the CDC for complicated reasons. Back in June I put forth the evidence for masks at that time including an explanation of the different types of face coverings and the reasons for the CDC’s early recommendation and reversal. Please read that post for the details of the types of masks and the evidence for them up to that point. In spite of all that evidence there is still a large population of people who do not believe that masks are helpful. The good news is that, since that time, more evidence has been published.

The evidence for masking will never be perfect. The only way to know if masks are effective with 100% certainty is to expose people who are healthy to COVID-19 in various scenarios with and without masks. This is obviously unethical. Even with imperfect sources, there is more than enough data from more than enough different sources supporting the fact that masks are beneficial to be convincing. So here is some more evidence.

Studies of the characteristics of the masks themselves.

One study measured how many aerosol particles were expelled by healthy humans doing various activities while wearing masks. They found a reduction in particle transmission by 90% with surgical masks during speaking and coughing. While these conditions were staged and not exactly what occurs in real life the benefits were clear.

Researchers at Duke University were able to devise a laser based optical measurement technique to measure droplets and evaluate the effectiveness of various types of masks and showed that multiple different types of face masks were very effective at reducing droplets when worn correctly. This was embarrassing for me personally because my laser based optical measurement techniques never work.

In a study published in the The Physics of Fluids, it was shown that an uncovered cough can travel up to 3 meters. In contrast, a simple disposable mask reduced the distance to 0.5 meters and an N95 mask limited the spread to just 0.1 and 0.25 meters. This just shows that The Physics of Fluids is far superior to The Physics of Gases or the prosaic Physics of Solids.

Studies of masks in the real world

A report of two infected hair stylists in Missouri who continued to work in spite of known infections was published by the CDC. Everyone in the salon was required to wear masks and chairs were spaced effectively apart. In spite of 139 people having significant exposures nobody contracted COVID-19. I do not have data on the quality of their highlights or the effects of COVID-19 on balayage or ombre.

A study of the spread of COVID-19 in NYC after implementation of various restrictions showed a decrease in the spread of infection by 60% when universal masking was implemented. Fuggedaboudit.

A study compared the case rates in different countries and evaluated multiple variable that could explain the differences. Mask wearing was one of the variables. They found that mask wearing was an independent negative predictor of COVID-19 deaths. In other words, in countries where mask wearing is more widespread and normalized there is a significantly lower death rate. The numbers were impressive. In countries without cultural norms or government policies supporting public mask-wearing the weekly increase in death rate was four times higher than in countries where people wear masks. Unfortunately, if you want to move to one of these countries, they probably won’t let Americans in.

Another study from Germany showed that, after the government required face masks, the number of new infections decreased between 15 and 75% over a 20-day period. Overall, the authors estimate that face masks reduced the daily growth rate of reported infections by around 47%.

A study of healthcare workers in Boston measured the number of healthcare workers who got infected before and after universal masking was mandated for all hospital employees. Before the mandate, masks were only required when coming into contact patients known to have COVID-19. Before the mandate more than 20% of exposed workers got infected. After masks became a requirement the infection rate cut in half in just 20 days. This was in spite of case numbers rising in the surrounding community. It is believed that the masks protected the workers from patients but also from infected coworkers. Wicked awesome!

Temporal Trend in Percentage Positivity of SARS-CoV-2 Testing Among HCWs

Why do I still need to quarantine if I was exposed wearing a mask?

The CDC says anyone who was within 6 ft of a person with COVID-19 for more than 15 minutes (or shorter if they the person was coughing, sneezing, or if there was a high risk interaction such as dancing). The quarantine recommendation holds even if the individuals were wearing masks.

I just brought you a pile of convincing data that masks are effective at preventing the spread of SARS-CoV-2. This would seem to mean that if you wear a mask you’re protected. This all seems contradictory and is the source of many questions I get which are all variations on:

Do masks work or not?

There are numerous memes, social media posts, and ranting about this issue. Some have asserted that “either masks work or they don’t” and questioned why lockdowns are necessary if masks supposedly work. Others asserted that “if masks work”, then there is no reason to restrict activities such as nursing home visits and in-person voting.

On this surface this sounds like a reasonable argument, I mean they sound so sure of themselves! The problem is that it is based on faulty reasoning. It is an oversimplification to claim that “either masks work or they don’t”. This reasoning assumes that, if the masks don’t work perfectly 100% of the time they don’t work at all.

These memes and arguments use two misleading rhetorical techniques. The first is known as “just asking questions” in which a person asks leading questions and posit baseless or disproven hypotheses without having to provide evidence. The second is a form of flawed reasoning known as the nirvana fallacy where a solution to a problem is rejected because it is imperfect even though a perfect solution is unrealistic.

In reality, the risks of getting infected and how severe the infection is are on a continuum. Even though masks are not 100% effective the evidence clearly has demonstrated that face masks reduce the transmission of COVID-19 a lot. Further, we know that, if someone is exposed to more viral particles they will get more severe disease. If masks decrease the amount of particles that are transmitted they will lead to less severe infection even if they don’t prevent the infection entirely. In fact, most experts think this is a major reason why there has been a decline in mortality over the past few month.

In other words, masks work but are imperfect in and of themselves and many people don’t wear them correctly. If you were exposed and wore a mask you are much less likely to have been infected and if you get infected it is more likely to be a mild case. In spite of that, there is still enough of a chance that you caught the bug and are contagious that we cannot not make any assumptions. That’s why you need to quarantine.

COVID-19 Risk Index

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