But keep in mind cigarette studies use the metric "pack-years" where 1 pack-year means smoking 20 cigarettes a day, for a year. "Heavy" smokers are generally considered as 2+ pack-year smokers. And it's not like _every_ heavy smoker gets lung cancer. In fact "only" 25% of heavy smokers get lung cancer, 5% of "former smokers", and 0.5% of non-smokers get lung cancer. And 40 cigarettes a day is a LOT!
So you can do an extrapolation with the numbers above to estimate a conclusion. I'd be hard-pressed to believe that 1 cigarette per WEEK is even as harmful as living in a big city like NYC or SF.