My initial research on this subject did not go deep enough. I realize now my sources were entirely American who tell you not to wear a mask. My conclusion now?
COVER UP. Not so much for yourself. But for all of us. Remember, medical professionals need these more than you and there is a global shortage.
There are a few different kinds of masks that are effective for different things.
Always wear a surgical mask if you are sick and I'm not talking about just for Covid-19. I'm talking always. In some cultures, it's standard consideration to not spread your illness. It is the best way to cover your cough. While there is no specific evidence to support that wearing masks decreases instances of influenza, it is thought to be associated with a higher degree of hygiene.
As avid and oblivious face touchers, it can be difficult to unlearn these behaviors but it is possible to train yourself not to touch your face or create systems which reduce the likelihood of transmission. Wearing a mask or glove is a great way to remember not to touch your face.
The kind of mask you need for this is a surgical mask (global shortage). These masks are designed to prevent droplets that could be laced with viruses from leaving your mouth and going into the air.
Wearing a mask when you're not sick can be complicated and ineffective in protection. You need to be hyper aware to not use contaminated hands to touch it, touch your face accidentally, avoid adjusting it etc. The exterior of your mask may also be contaminated with particles it has trapped.
Expired masks are sometimes rendered ineffective because of an antiviral substances they are coated with or the seal being compromised.
Your best protection is a full face ventilator mask. Or check out this DIY version using a snorkle mask.
Your next best protection is a ventilator mask.
And your next best protection to that is an N95 (severe global shortage!) or something like it.
Check out these DIY washable versions.
With this one, you can look like a ninja instead of a pandemic polly.
Here. you can find a study on the efficacy of home made masks.
Bottom line. Masks are a lot more about protecting others. If everyone covered up, we'd all be at less risk.