Nicotine is absorbed very quickly by mucous membranes.
In a room where people are smoking there is some detectable nicotine in the air because not all of the smoke gets inhaled … some nicotine is emitted from the burning end of the cigarette. The smoke exhaled by the smokers contains almost no nicotine; it is absorbed by the smoker.
When the air is tested in a room where people are vaping there is virtually no nicotine in the air; very nearly all of the nicotine is absorbed by the vaper.
Somebody asked me one day how much eliquid I used in a day and I didn't know, so I devised an experiment. I put everything away, took two fresh, clean tanks and a 30ml bottle of 18mg juice and used only those things until the bottle was empty. That took 11 days. When I smoked, I smoked about 2.5 packs per day.
30 ml / 11 days = 2.73 ml of juice per day
2.73 ml * 18mg/ml = 49mg
So, 2.5 packs a day @ about 1mg per cigarette is about 50mg of nicotine per day. And my juice calculation comes up with 49mg per day. And yes, I repeated it … and got the same results.
So, forget counting puffs. We all puff differently. To compare your nicotine consumption between cigarettes and ecigs, you need to know how much you smoked, how much ejuice you use in a day, and the strength of the juice. Multiply the ml of juice you use in a day times the strength of the juice and the answer is the approximate equivalent number of cigarettes per day.
If you smoked unusually strong or unusually weak cigarettes, you may be able to find a more accurate figure than 1mg per cigarette for your brand, but 1mg per cig is a reasonable number for a rough ballpark estimate.