You should not breathe the air from sewage pipes. You'd have to be at the end of your options to do this, if there's a fire and you can't leave the room via the door or window, you would want to cover the door with clothes/towels whatever to slow the ingress of smoke and open/smash the window calling for help. You should be crawling around if the room is filling with smoke, keep low to the floor as that's where the cleaner air is, the sewage pipe is not cleaner air.