The exhaled mainstream smoke and the sidestream smoke enter the air surrounding the smoker. If the physical volume of the location in which smoking occurs is relatively small, as in a car or a bedroom, then the concentrations of gases and particulate pollutants in this volume will become extremely high. For example, if the particulate matter generated from smoking a single cigarette is emitted into a bedroom with a physical volume of 41 cubic meters (m3), and the mass is uniformly dispersed over the bedroom’s air volume, then the resulting PM2.5 concentration in the room is calculated by dividing the total amount emitted (14 mg) by the total room volume (41 m3):Maximum Concentration = (14 mg)/(41 m3) = 0.341 mg/m3Normally, pollutant concentrations are expressed in micrograms per cubic meter (μg/m3), and since 1 mg = 1,000 micrograms, the maximum PM2.5 concentration associated with a single smoked cigarette will be (0.341 mg/m3)(1,000 micrograms/mg) = 341 μg/m3, a relatively high pollutant concentration.Thus, because of the large amount of smoke particle mass emitted by a single cigarette, the cigarette in a bedroom will cause a very high initial concentration of 341 μg/m3. This initial high concentration occurs soon after the cigarette stops burning, which then is followed by a slow decrease in the concentration in the room with time5. This slow decrease in concentration happens during the pollutant decay period, during which fresh air infiltrates into the room through cracks, gaps, and windows, gradually replacing the room’s air while at the same time some particles from the smoke deposit on the walls and furniture.