# Where Does the Smoke Go?

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 (m$^3$), and the mass is uniformly dispersed over the bedroom’s air volume, then the resulting PM$_{2.5}$ concentration in the room is calculated by dividing the total amount emitted (14 mg) by the total room volume (41 m$^3$):

Maximum Concentration = (14 mg)/(41 m$^3$) = 0.341 mg/m$^3$

Normally, pollutant concentrations are expressed in micrograms per cubic meter (μg/m$^3$), and since 1 mg = 1,000 micrograms, the maximum PM$_{2.5}$ concentration associated with a single smoked cigarette will be (0.341 mg/m$^3$)(1,000 micrograms/mg) = 341 μg/m$^3$, 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/m$^3$. 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 time$^5$. 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.

To gain insight into this phenomenon from measurements in a real home, a single Marlboro regular filter cigarette was smoked in a 41 m$^3$ bedroom with the door closed$^5$, and the particle concentration was measured over the next 3 h with an instrument that measures the particle mass concentration:

This graph shows that a maximum particle concentration over 320 μg/m$^3$ occurred almost immediately after the cigarette ended at 2:38 PM, which is close to the the predicted concentration using the equation above. For the next hour and a half, the concentration in the room gradually decreased, whereupon the bedroom door was opened by one of the occupants, causing the bedroom’s particle concentration to decay more rapidly as the pollutant passed through the door into the rest of the house.

To summarize, while the bedroom door was closed, the smoke was gradually removed from the air in the bedroom both by deposition (sticking) on surfaces and by exchange with external air (infiltration), but the removal process took several hours after the cigarette was smoked for the smoke to fully clear. Many toxic air pollutants were released in the emissions of the cigarette, some in gaseous and some in particulate form. The particles in particulate matter are themselves composed of many toxic compounds, such as polycyclic aromatic hydrocarbons (PAHs).

The horizontal dashed line in the figure above shows the US federal National Ambient Air Quality Standard for particulate matter (PM$_{2.5}$), which specifies that outdoor air concentrations in the U.S. should not exceed 35 μg/m$^3$ averaged over 24 h. These regulatory standards are based on extensive research on the effects of daily variations in ambient concentrations on human health and on other health-related studies, and this research is summarized in EPA’s research compendium on particulate matter6. For example, a study of women in Seattle$^7$ found that deaths from heart attacks, coronary disease, strokes, and clogged arteries were 24% more likely for every rise in the ambient PM$_{2.5}$ concentrations of 10 μg/m$^3$.

Even though high concentrations will persist in the bedroom for hours after the cigarette ends, just one cigarette will not necessarily cause the federal air quality standard of 35 μg/m$^3$ to be exceeded in this bedroom, since the EPA standard specifies a 24-h average. However, in this bedroom with its typical size, it turns out that 2 or more cigarettes smoked can cause the health-based ambient 24-h average standard to be exceeded indoors, depending on the ventilation rate in the room (see next section).