If you’re cutting old cast iron, stop for a second. That pipe is probably held together with lead and oakum—cancerous old-school stuff. Wear a respirator, not a dust mask. Trust me, you don’t want that grey dust in your lungs.
Also, cast iron is brittle. It can crack unpredictably, sending a piece flying. I once had a chunk hit my safety glasses. I now wear them every time. You should too, unless you enjoy blinking out of one eye.
