Here's a cut and paste of the basic (as nobody seems to follow links)
Can face masks help prevent the spread of the coronavirus that causes COVID-19? Yes, face masks combined with other preventive measures, such as frequent hand-washing and social distancing, help slow the spread of the virus.
So why weren't face masks recommended at the start of the pandemic? At that time, experts didn't yet know the extent to which people with COVID-19 could spread the virus before symptoms appeared. Nor was it known that some people have COVID-19 but don't have any symptoms. Both groups can unknowingly spread the virus to others.
These discoveries led the U.S. Centers for Disease Control and Prevention (CDC) to do an about-face on face masks. The CDC updated its guidance to recommend widespread use of simple cloth face coverings to help prevent transmission of the virus by people who have COVID-19 but don't know it.
Some public health groups argue that masks should be reserved for health care providers and point to the critical shortage of surgical masks and N95 masks. The CDC acknowledged this concern when it recommended cloth masks for the public and not the surgical and N95 masks needed by health care providers.
How do the different types of masks work?
Surgical masks
Also called a medical mask, a surgical mask is a loose-fitting disposable mask that protects the wearer's nose and mouth from contact with droplets, splashes and sprays that may contain germs. A surgical mask also filters out large particles in the air. Surgical masks may protect others by reducing exposure to the saliva and respiratory secretions of the mask wearer.
At this time, the U.S. Food and Drug Administration has not approved any type of surgical mask specifically for protection against the coronavirus, but these masks may provide some protection when N95 masks are not available.
N95 masks
Actually a type of respirator, an N95 mask offers more protection than a surgical mask does because it can filter out both large and small particles when the wearer inhales. As the name indicates, the mask is designed to block 95% of very small particles. Some N95 masks have valves that make them easier to breathe through. With this type of mask, unfiltered air is released when the wearer exhales.
Health care providers must be trained and pass a fit test to confirm a proper seal before using an N95 respirator in the workplace. Like surgical masks, N95 masks are intended to be disposable. However, researchers are testing ways to disinfect N95 masks so they can be reused.
Some N95 masks, and even some cloth masks, have one-way valves that make them easier to breathe through. But because the valve releases unfiltered air when the wearer breathes out, this type of mask doesn't prevent the wearer from spreading the virus. For this reason, some places have banned them.
Cloth masks
While surgical and N95 masks are in short supply and must be reserved for health care providers, cloth masks are easy to find and can be washed and reused.
Asking everyone to wear cloth masks can help reduce the spread of the coronavirus by people who have COVID-19 but don't realize it. And countries that required face masks, testing, isolation and social distancing early in the pandemic seem to have had some success slowing the spread of the virus.
Cloth masks are cheap and simple to make. Instructions are easy to find online. Masks can be made from common materials, such as sheets made of tightly woven cotton. The CDC website even includes directions for no-sew masks made from bandannas and T-shirts. Cloth masks should include multiple layers of fabric.
How to wear a cloth face mask
Cloth face masks should be worn in public settings where social distancing measures are difficult to maintain, such as in grocery stores, especially in areas of significant community-based transmission.
Here are a few pointers for putting on and taking off a cloth mask:
Place your mask over your mouth and nose.
Tie it behind your head or use ear loops and make sure it's snug.
Don't touch your mask while wearing it.
If you accidentally touch your mask, wash or sanitize your hands.
Remove the mask by untying it or lifting off the ear loops without touching the front of the mask or your face.
Wash your hands immediately after removing your mask.
Regularly wash your mask with soap and water in the washing machine. It's fine to launder it with other clothes.
Finally, here are a few face mask precautions:
Don't put masks on anyone who has trouble breathing, or is unconscious or otherwise unable to remove the mask without help.
Don't put masks on children under 2 years of age.
Don't use face masks as a substitute for social distancing.
www.mayoclinic.org/diseases-conditions/coronavirus/in-depth/coronavirus-mask/art-20485449
Many dentists are now going back to work wearing two masks, an N95 type to protect themselves and a surgical mask to protect their patients.
More light reading en.wikipedia.org/wiki/Face_masks_during_the_COVID-19_pandemic#Mask_use_and_policies_by_country_and_territory
There are also an awful lot of freshly peer reviewed papers and even more previews that can be hunted down on Google Scholar.
Many people who are infected with coronavirus have no clue as they are either asymptomatic or have such mild symptoms it is indistinguishable from lockdown malaise. This is an unusual problem and has added to the swift spread of this virus.
A surgical mask or fabric face covering prevents egress. We all leak droplets when we breath, talk and laugh as well as the obvious coughs and sneezes. Keeping those droplets off other people and other things, could pretty much halt the progress of this virus. All we need is 75%+ of people to wear face coverings on public transport, shops and other places where we might get close.