When a person smokes a cigarette, the part of the smoke that is inhaled directly into the lungs is called mainstream smoke. Pollutants inhaled in the mainstream smoke enter the lung directly and can be absorbed by the blood stream and body tissue. For example, inhaled carbon monoxide (CO) gas enters the blood stream where it ties to the human blood molecule (hemoglobin), thereby depriving the brain of oxygen as the blood enters the brain. Elevated CO in the blood may persist for many hours after the cigarette has been smoked, and it is possible to determine if a person has smoked a cigarette simply by measuring the elevated CO in a sample of the person’s blood. Some portion of this inhaled smoke is exhaled by the smoker as processed mainstream smoke. More than half the pollutants emitted by a cigarette come not from the smoked end of the cigarette but from its other end – the cigarette’s burning end – and are called sidestream smoke. Secondhand smoke, sometimes denoted as SHS, is a combination of both the exhaled mainstream smoke and the sidestream smoke emitted by the burning end of the cigarette, as well as any other smoke emitted from the end of the cigarette held by the smoker. Thus, secondhand smoke (SHS) is the total amount of pollution that leaves the immediate surroundings of the smoker.
Since each cigarette emits a large amount of fine particulate matter (7-23 mg) as SHS, and since this particulate matter comprises the visible part of the emitted smoke, some scientists believe the act of “smoking” should really be called "particling," a less romantic but scientifically accurate description of the activity of smoking. Tobacco company advertisements would be less exciting to young people and less likely to cause people to start smoking if these ads described the act of smoking as “particling.”