C.S.
Let them know the consequences for smoking in the house. It's not just you who will be losing a place to live but them as well. I had this problem. My Mother doesn't believe in second hand smoke and everyone in my house smoked except for me. She said that if I didn't want to deal with the smoke then I could go outside. Since moving out of my parent's house it's a rule no smoking in the house.
If you rented first and then brought them in then you could tell them the new rule. No smoking in the house.
HOpe this helps.
Cami
Great Falls, Montana