Once a baseline measurement is obtained from those already smoking subjects/participants, we measure the correlation between avg. number of cigarettes smoked per weak and lung capacity. This way one doesn't have to randomize or control, unethically asking people to smoke if they don't already. We already have a hypothesis based on the prior that volume of cigarettes smoked has a strong positive correlation with lung damage, and so reducing the number of cigarettes smoked would improve lung functioning in smokers.

It was not clear from this description what exactly your design was. Is it the case that you find some smokers, and then track the relationship between lung capacity and how much they smoke per week (which varies due to [reasons])? Or do you artificially reduce the nicotine intake in smokers (which is an ethical intervention)? Or what?