The CDC has found that COVID-19 is primarily spread from person-to-person contact, rather than from surfaces. While it is true that touching a contaminated surface and then touching your face can potentially transmit the virus to you, this is not the primary mode of transmission. Instead, you are more likely to catch it from another person — whether or not they appear to have any symptoms at all. The virus is able to move through the air on small water droplets, (dispersed by coughing, sneezing, or even talking) which can then land on someone else’s face, or be inhaled into their lungs.  This method of transmission is much more dangerous than surface spread, but it can be limited by social distancing, and, importantly, masks.
Face masks work by capturing respiratory droplets before they can disperse into the air. The fibers in these masks are spaced out enough that smaller molecules like carbon dioxide and oxygen can pass through uninhibited. But respiratory droplets are huge in comparison; ranging from about 20 to 500 micrometers, they are potentially hundreds of thousands of times larger than a molecule of oxygen. In most cases, this means they are too large to escape between the fibers of a face covering, which in turn means that any viral passengers which might be riding on those droplets are unable to escape! In general, two or more layers of fabric will absorb nearly all of them.
It is very important to remember, however, that while some masks DO reduce your risk of catching the virus, the most significant effect they have is reducing your risk of transmitting it. Which means that by wearing a mask, you are protecting the people around you, and their masks are there to protect you! Sometimes wearing a mask may seem unnecessary — after all, if you have the virus, shouldn’t you just stay at home and avoid being around other people in the first place? But the insidious issue is most people are contagious for up to two days before showing any kind of symptoms. While you may not feel sick with COVID-19, you might just not feel sick yet, and in those two days you could expose many other people without ever knowing it!
In general, the CDC recommends that you should wear a mask any time you are around people you don’t live with, when you can’t stay at least 6 feet away from everyone at all times. Masks should cover both the nose and mouth to be effective, should fit snugly to minimize gaps, and should be secure under the chin.
Different Types of Masks
The sudden onset of this pandemic, the conflicting information on face coverings, and the wide variety of options available have created a good deal of confusion about what exactly makes a mask “good enough.” Luckily, researchers have been hard at work determining how and why certain masks work — and which masks don’t. This is an ongoing area of research, and new findings may change the consensus, but that’s just good science! The best we can do is follow the most up to date guidance for now, and if better research comes out, update our response. At the time of this writing, here is what we know: the goal is to minimize the spread of respiratory droplets, and prevent them from contacting the mouth, nose, or eyes. Any protective covering we choose should be evaluated by that standard.
You have probably seen images of doctors and nurses treating covid patients wearing large plastic face shields. They span from the forehead to below the chin, wrap around the sides somewhat, and seem to provide a good covering without really affecting anyone’s ability to breathe. That seems pretty good, but it’s not the whole story. While it is true that these face masks do stop any droplets coming toward your face, they are woefully inadequate on their own. In those same aforementioned images, the hospital staff are wearing much, much more protection than just a face shield. Typically they are wearing at least one type of mouth and nose covering as well. The face shield is mostly useful to shield the eyes of people who are at high risk of exposure, and are useful mainly in conjunction with, rather than instead of, nose and mouth coverings.
Nose and Mouth Coverings
There are several types of coverings currently in use, and it’s hard to know which is most effective. Here are a few common types, roughly ordered from most to least effective. 
N95 and N99 Masks
These masks offer the greatest protection. They seal tightly to the face, and their construction is exceptionally good at capturing particles. The problem is, these masks are in short supply. As such, they should be reserved for people who are in constant contact with infected individuals, such as front-line medical staff. While using one of these might slightly decrease your risk of infection, it deprives someone who desperately needs it of their mask, which risks making the pandemic worse overall. Remember: wearing a mask is mostly about protecting the people around you, and using up the supply of N95s and N99s does the opposite!
Less effective than N95 and N99, but still very effective at reducing transmission. Similar to N95 and N99 masks, however, these should be reserved for medical personnel, or people experiencing clear COVID-19 symptoms.
Cheap, accessible, and easy to produce, these are the ideal option for most people. A good fabric mask should have at least three layers:
- Inner layer, made of cotton or a cotton blend.
- Intermediate filtering layer, made of polypropylene interfacing or tightly-woven cotton.
- Protective outer layer, preferably of a hydrophobic (water-repelling) material if possible.
This chart from the WHO compiles information about filtering effectiveness for homemade mask materials.  Some studies have also suggested that mixing fabric types (e.g. using a mix of cotton and silk layers) can improve mask effectiveness. In general, though, any mask is better than nothing, as long as it fits closely, is made of a non-stretchy material, and your breath passes through the fabric, rather than leaking out of the sides of the mask. 
These have become very popular, as they are easy to pull over the face when needed, and can hang comfortably around the neck when not in use. The problem is that many neck gaiters are made of a single layer of fabric, or of a very stretchy fabric, which, when pulled, can create openings large enough for some droplets to escape. Wearing a gaiter is still better than nothing, and if you do decide to use one, select one with multiple layers of fabric!
Masks with Vents
While these masks may be more comfortable, the very existence of a one-way exhalation vent defeats the primary purpose of wearing a mask. Masks like these are optimized to allow air (and the respiratory droplets it carries) to escape more easily, and are therefore not effective for preventing the spread of COVID-19.
With all of the new developments and conflicting information swirling as this pandemic continues, it can be hard to know exactly what the right thing is to do. But at least one thing does seem to be unchanged from the very start. Wearing a mask over your nose and mouth significantly reduces your chance of spreading the virus. Not all masks are created equal, but a mask with at least 3 layers of cotton, which covers your mouth and your nose, fits close to your face, and is secure under your chin, should provide most of the protection you need. If any air can escape before passing through fabric, it will not stop potentially-infectious droplets from escaping. This includes masks which let unfiltered air out through a valve, around the edges), or which, like a face shield, provide no filtering at all.
There may be debate where you are about how much social activity is safe, about whether restaurants and other public spaces should be allowed to reopen, and about whether or not things are ready to return to normal. In some places, cautious reopenings may be appropriate, if everyone maintains social distance, washes their hands, and wears masks which prevent unfiltered breath from releasing droplets into the air. If we are careful, we can continue to limit the spread of COVID-19, and take steps toward the end of this global crisis!
Citations and Further Reading
- CDC | How COVID-19 Spreads
- MIT | Covid-19 Updates: Do cloth masks actually work?
- Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering
- CDC | How to Select, Wear, and Clean Your Mask
- CDC | COVID-19: Considerations for Wearing Masks
- The Simple Science Behind Why Masks Work
- COVID-19 and use of non-traditional masks: how do various materials compare in reducing the risk of infection for mask wearers?
- WHO | Q&A: Masks and COVID-19
- WHO | Advice on the use of masks in the community, during home care and in healthcare settings in the context of the novel coronavirus (COVID-19) outbreak
- MIT | Covid-19 Updates: How do I choose a cloth face mask?