People tend to walk 10-15 mins at either end when taking the tube, and will walk more through the day. My understanding is this activity is net positive despite the extra particulates while on the tube. I might be misremembering this with a study I read on cycling in road pollution though

Almost certainly some asbestos dust still in there from prior decades as well, the tube is absolutely filthy and never cleaned properly

Just one more reason to wear an N95 on the Tube.