Good question Hobbes. This website does a decent job of breaking down what are the risks of smoking. However, there is little to no information on the scenario you described (i.e. the weekend smoker who has one cigarette at a party for example). Even extrapolating from second hand smoke is difficult, because even there, the issue is second-hand smoke exposure on a daily basis (like the wife of a chain smoker). I tried to find some dose-response data, but failed (in my cursory attempt). Sorry.