But aren’t masks supposed to protect other people? Not the wearer? I’m not that sick!

First off, even if you don’t have symptoms, you might actually still be sick or infected!

Secondly, that’s also not true. Masks still have an effect on slowing down the rate of infection even if you’re not sick!

We intuitively assume that masks only work in one direction. However, they work in both directions. Both inhaling and exhaling the air we breathe; the air that potentially might COVID-19 viral particles in them. So when everyone’s wearing masks, there are actually two masks between any two people.

Let’s say, for simplicity, that masks are equally effective stopping the infection of COVID-19 from both inhaling and exhaling air (basically from yourself and from other people). 2I know that not all masks work this way, in actuality the effectiveness could depend on a myriad of factors from the material its made to how well it fits, but stick around to find out what happens if you change how much a mask is effective!

one person coughing onto other person who's wearing a mask, showing that the mask is reducing the rate of transmission by 50%

As you can see here, the chance of infection between two people is 50% when one of them wears a mask that is 50% effective against stopping the spread of the virus. 3Yes, I know that masks are not always 50% effective against reducing the transmission of disease. This is just for simplicity’s sake. It’s the same effectiveness whether the infected person’s wearing the mask, or the non-infected but susceptible person’s wearing the mask.

However, if both people are wearing a mask…

one person with a mask coughing onto other person who's also wearing a mask, showing that the two masks are reducing the rate of transmission by 75%

The chance of infection is reduced by 75% instead of 50%, because the masks halved the chance of infection twice. The chance of getting infected with COVID-19 is even more reduced!

Here’s a simulation where you can change the overall effectiveness of masks, both through inhalation and exhalation (also includes an additional route where both people are *not* wearing masks, just to drive the point further!). Try to make the mask effectiveness on either “inhale” or “exhale” have a really small number, and the other one having a ridiculously large number.

See how the drop in disease transmission between both people wearing masks is always higher than only one person wearing a mask? Even if only one side of the mask is incredibly effective against filtering out air particles? And, a mask will always at least be a little bit effective in reducing disease transmission; whether that be through inhale or exhale. 4Also, obviously, without wearing a mask to block air particles, disease will be transmitted without being impeded, therefore it will always be at 0%.

However, this is only representative of mask wearing between two individuals. How about the entire population?

Here, we can see all four possible routes of disease transmission, but this time with a slider for the percentage of people who wear masks in the population.

Legend: Drop In Disease Transmission, Chance of This Route Happening, Drop In Disease Transmission to Non-Mask Wearers, Drop In Disease Transmission to Mask Wearers, Average Drop In Disease Transmission

In reality, not everyone will wear masks. Since any amount of people will be wearing masks at any time, the four possible routes of disease transmission will also vary. That’s why the simulation also shows a chance of each route happening. How likely each route would happen depends on the number of people in a population who are wearing masks.

Finally, we can calculate the average drop in disease transmission across an entire population (calculated using the average across all relative drops in disease transmission!). Go ahead, see what happens when people wear 60% effective masks, but only 30% of people wear masks. 75% of people wearing masks that are only 25% effective? Watch how the average drop in disease transmission changes! This simulation is intended for you to test your own assumptions and theories. 5This is a pretty simple simulation: obviously not everyone will wear masks that are 50% effective. But it’s okay because this is just a simplified simulation for educational purposes. It’s so that we’re able to see a simplified roadmap on why masks work!

I’ve also included an average drop across both mask wearers and non-mask wearers, just to see how much of a benefit masks can offer if a lot of people wear them. See how the average drop in disease transmission for non-mask wearers *increases* when mask usage goes up? Even non-mask wearers get a marginal benefit! 6 Although don’t use this as an excuse not to wear masks, we need people to wear masks to protect everyone in the first place!