I remember from my days working for London Underground Ltd that whenever I'd blow my nose after a shift, my snot would have a concerning amount of black matter

I remember from my days working for London Underground Ltd that whenever I'd blow my nose after a shift, my snot would have a concerning amount of black matter in it. The days I was off and not entering the subway, my snot didn't have this black particulate matter. I like to think it was my nose doing its job and keeping all that from entering my lungs, but I'll never know.