But the opposite concern also exists: egress, or transmission of particles from the wearer to the outside world. Historically, much less research has been conducted on egress, but controlling it—also known as “source control”—is crucial to stopping the person-to-person spread of a disease. Obviously, society-wide source control becomes very important during a pandemic. Unfortunately, many articles in the lay press—and even some in the scientific press—don’t properly distinguish between ingress and egress, thereby adding to the confusion.

Read: Everyone thinks they’re right about masks

The good news is that preventing transmission to others through egress is relatively easy. It’s like stopping gushing water from a hose right at the source, by turning off the faucet, compared with the difficulty of trying to catch all the drops of water after we’ve pointed the hose up and they’ve flown everywhere. Research shows that even a cotton mask dramatically reduces the number of virus particles emitted from our mouths—by as much as 99 percent. This reduction provides two huge benefits. Fewer virus particles mean that people have a better chance of avoiding infection, and if they are infected, the lower viral-exposure load may give them a better chance of contracting only a mild illness.

COVID-19 has been hard to control partly because people can infect others before they themselves display any symptoms—and even if they never develop any illness. Three recent studies show that nearly half of patients are infected by people who aren’t coughing or sneezing yet. Many people have no awareness of the risk they pose to others, because they don’t feel sick themselves, and many may never become overtly ill.

Read: Why we’re running out of masks

Think of the coronavirus pandemic as a fire ravaging our cities and towns that is spread by infected people breathing out invisible embers every time they speak, cough, or sneeze. Sneezing is the most dangerous—it spreads embers farthest—coughing second, and speaking least, though it still can spread the embers. These invisible sparks cause others to catch fire and in turn breathe out embers until we truly catch fire—and get sick. That’s when we call in the firefighters—our medical workers. The people who run into these raging blazes to put them out need special heat-resistant suits and gloves, helmets, and oxygen tanks so they can keep breathing in the fire—all that PPE, with proper fit too.

If we could just keep our embers from being sent out every time we spoke or coughed, many fewer people would catch fire. Masks help us do that. And because we don’t know for sure who’s sick, the only solution is for everyone to wear masks. This eventually benefits the wearer because fewer fires mean we’re all less likely to be burned. My mask protects you; your masks protect me. Plus, our firefighters would no longer be overwhelmed, and we could more easily go back to work and the rest of our public lives.