I shared an office with a smoker back in the day -- one or two smoking visitors would leave a cloud hanging for an hour. About 10 years later (several years into the smoking ban in the U.S. workplace) went on a business trip to Japan where all the smokers at work were confined to a small "smoking lounge." You couldn't see their faces through the window due to the thick smoke! Nowadays I can easily smell someone smoking a block away. It's kinda the same reaction as when a vintage 60's car putts by trailing unburned hydrocarbons...