thanks conley; i wanted to make sure you weren't too young before I gave my opinion. it sounds like your parents vacate the building when they smoke, which means you probably won't get 2nd hand smoke.
it's understandable that you would worry about them, considering pot is illegal and all. i've happened to smoke with a lot of old timers, and for the majority of them they have their lives in order. do your parents lead happy lives? are they able to function on a daily basis? if so, then i would consider their habit like drinking a glass of wine each night.
however, pot is highly carcinogenic, and is harming their lungs. one thing i've learned here at SR is you cannot make people change unless they want to. have you tried having a conversation with them about this?