Running with a Cotton mask.

I've been running with a cotton mask almost 3-5 times a week.

I wanted to increase my performance with Hypoxic Training primarily. This is 80% of the reason. See Strange benefits.
Second, at about 10% of the reason was that I had a mild panic attack wearing a mask about a couple of months ago when I went out and had a mask for an extended period of time. Since we get everything delivered and I can work remotely I've never needed to wear a mask unless I go outside. So to my surprise wearing a mask since it was an Asian practice to be considerate spreading germs which I admired from HK, Taiwan, and Japan.

Strange Benefits.

Nasal Breathing even at 10kph. So one of the hardest things I learned was forcing myself to slow my breath under heavy exertion. I learned the following things.

I have to concentrate and make sure all my "breathing muscles" are activated when breathing to make sure as much air as possible is replenished.
When I fail in concentration my heart has an Offbeat that doesn't rise with my breathing rhythm. I think it feels a bit like starving for oxygen and I could feel a wave of weakness as my body is looking for air that isn't there.
When I run breathing through my mouth - my dehydration speeds up and my mucus gets thick and sticky in my throat. This causes me to cough and "knocks the air" out of me.

Nasal Breathing with a mask at 8kph.
Learning that my Exhaustion is based on what Speed I can breathe in a way that doesn't lead me to cough and lead to "Hitting the Wall" or glycogen exhaustion.