@Paul6006, we can take some measures to prevent it. I have carbon monoxide detector in the guests room, well visible. I put nets on the windows, so it is inconvenient to smoke there. And as I’m smoker myself, I have place just in the patio with ashtray and small taburete. I’m smoking there and my guests do the same. Ah, and i put it in my house rules, that if someone smoke inside he will be charged for cleaning 50€ and kicked out immediately based on violating safety of the house.