Yet another reason to wear high quality N95 masks in the future when on public transit.
I worked on the tube for a while, in tunnels when trains were stopped etc. After work, I'd be filthy. I'd blow my nose and black gunk came out. God know what it does to your lungs.
This is pretty alarming. While I knew that it's bad in the subways just from the smell alone, I didn't know it was actively harming my health to the degree that it is. This is a good heads up, thanks.