And wear a respirator (N95 or better with P100 filters), not just a cheap dust mask. Asbestos is an inhalation hazard. Lead particles can be an inhalation as well as an ingestion and absorption hazard. Asbestos fibers can remain airborne for weeks or months, while lead particles are fairly heavy and will settle out quickly. Lead poisoning effects are fairly immediate, while asbestos related illnesses will take years to show up.I'd worry a lot more about the lead paint covering the plaster - or flaking off of it. Your plaster may or may not have asbestos - but you MOST ASSUREDLY have lead paint.
BTW - I won't get all trooly on you about the lead paint. Just avoid dry sanding, change your shoes and clothes before you leave the work area, and ABOVE ALL ELSE wash your hands. That's just good general advice anyway.