Previous research has found that exposure to indoor air pollution becomes elevated when one is close to the source of pollution. This phenomenon has been called the "proximity effect". We observed the same effect in our study of outdoor air pollution near smokers, with high levels occurring near active smoking and low levels occurring further away (beyond about 2 meters, or 6 feet, from a single smoker). Note that the more smokers present, the further away you would likely have to be to avoid any exposure.