This is an answer I provided on Quora. I duplicate it here because I think many people can benefit from reading it.

“Since the Corona virus has a limited life cycle outside of the human body (up to 3 days on different surfaces), can the masks left unattended be reused?”

While sterilization of masks is an open issue, necessitated primarily by the shortage of masks, this questions expresses the primary misunderstanding about used masks.

Masks have two purposes. First, prevent infected people from filling the air with virus, whether through coughing or just talking. Second is to reduce, preferably eliminate, breathing-in of contaminated air. A third, rather minor, purpose is to keep people from touching their noses and mouths.

Here is how masks work, and how that impacts the second usage.

Suppose you make a mask out of solid plastic. You can’t seal it to the face or you will suffocate. If you don’t seal it to the face, air will come in around the mask, and that air will be just about as contaminated as breathing without the mask. A circuitous air passage does not remove much contamination, and the circuitousness around a face mask is minimal.

Instead, you use a filtering material. Just about any good filtering material will work for the first usage, since the existence of the mask will keep most viruses from leaving the mask, and the ones that do escape will lose most of the kinetic energy they need to travel very far.

For the second usage, the filtering material, to be effective, must at least filter down to the size of small water droplets, and in the case of the best, down to virus size. This is very small, and generally makes breathing harder if the mask is properly fitted. A good N95 mask has this quality. Medical people know how to fit them so air doesn’t go around the mask (no beards!), and they learn to put up with the extra energy it takes to breathe while wearing one.

The little mentioned truth is that N95 masks become progressively harder to breathe through because the tiny air passages tend to fill up with crap in the air. Most of this crap has nothing to do with Covid-19, including cigarette smoke particulates and diesel engine particulates. Medical people normally deal with this increased breathing effort by replacing the mask.

When untrained people, typically non-medical, notice difficulty breathing in an N95 mask, they almost always loosen it, allowing air to come in around the edges. When that happens, the mask is no better than a normal (what I call) sawdust mask. The first usage is still benefiting, but with a much more expensive mask. Sadly, people who already have breathing difficulty almost never properly fit an N95 mask because it interferes with their already labored breathing.

The worldwide shortage of N95 masks and other protective gear for medical personnel prompted WHO and other public health organizations to downplay the second usage of masks (self protection) so that N95 masks would be available to the people who need them most and know how to use them. This was taken to mean that masks aren’t effective against Covid-19: it was meant to save lives.

The primary problem with the second usage of masks (protecting self) is that it doesn’t scale to the general public, or in most cases even work.

Now we’re realizing that because anyone can be a carrier without knowing it, everyone should use a mask for the first usage (protect others). In many Asian countries, there is a culture of wearing masks for both usages. They are used extensively in Korea, and undoubtedly contribute to Korea’s relative success in dealing with Covid-19. (Though Korea’s “trace and test” methodology is the primary reason for their success.)

A mask for the first purpose doesn’t have to be a medical mask, and certainly the valuable N95 masks aren’t needed, or even useful after a short time. Multiple layers of cloth will suppress most emission of virus by coughing or breathing.

The biggest barrier to widespread use of masks in the Western world is the perceived benefit of wearing them. The downside of wearing a mask is obvious to most people, so they aren’t going to wear one without a good reason. The problem is that the first usage (protecting others) only makes sense to a Westerners if they have some suspicion that they are infected, and since most of us are in denial, it isn’t a compelling reason. The second usage (protecting self) is much more compelling, but the downsides of obtaining, properly using, and being seen wearing N95 masks make the second usage unattractive to most of us.

The original question refers to reuse of masks. When medical institutions consider this, they are acting out of desperation. They need masks for the second usage (protecting self), and they need it on a scale that most of us can’t imagine. Their biggest interest is in cleaning the masks of contaminants so providers can breathe sufficiently. Simultaneously, they need to ensure that infectious agents, including but not limited to Covid-19, are removed, since masks will be reused by random others.

Leaving the masks unattended accomplishes very little in the grand scheme of things, even if we could reliably predict how long to leave them.

The nice thing about accepting that most of us can only expect to benefit from the first usage (protecting others) is that effective masks for that purpose can be made from layers of cloth, and cloth can be washed with soap, and exposure to soapy water kills all coronavirus.