Tobacco smoke is a complex aerosol mixture analyzed in public health for its toxicological impact. Terminology categorizes this smoke based on its origin, which dictates the chemical composition and the nature of exposure. Understanding these distinctions is fundamental to assessing the health risks for both the person using the tobacco product and those in the surrounding environment.
Defining Mainstream Smoke and Its Creation
Mainstream smoke (MSM) is the aerosol drawn through the tobacco product and into the user’s mouth and lungs during puffing. This process involves the user actively inhaling, which pulls air through the burning end and through the column of tobacco. The smoke is generated under conditions of high heat and high oxygen availability as air rushes past the burning material.
This active draw causes the smoke to pass through the entire tobacco rod and the filter, if present, before inhalation. Because it is actively inhaled, mainstream smoke represents the primary source of chemical exposure for the person using the tobacco product.
Key Chemical Components
Mainstream smoke is a dense, complex mixture containing over 4,000 distinct chemical compounds, approximately 60 of which are known carcinogens. The smoke exists as an aerosol, a suspension of fine solid and liquid particles within a gas phase. The visible particulate phase accounts for only about 5% of the cigarette’s total output.
The gas phase includes toxic gases like carbon monoxide (CO), hydrogen cyanide, and nitrogen oxides. CO is a colorless, odorless gas that interferes with the blood’s ability to carry oxygen. The particulate phase, commonly referred to as “tar,” contains substances such as nicotine, phenols, and polycyclic aromatic hydrocarbons (PAHs). Nicotine is responsible for the addictive properties of tobacco. Highly toxic compounds like tobacco-specific nitrosamines (TSNAs) and benzene are also present.
Mainstream Smoke Versus Sidestream Smoke
The difference between mainstream smoke and sidestream smoke (SSM) lies in their generation conditions and chemical composition. Mainstream smoke is generated during the high-temperature, forced-air conditions of a puff. Sidestream smoke is the aerosol that drifts into the air from the smoldering tip of the tobacco product between puffs.
Sidestream smoke is created by smoldering, which occurs at a lower temperature and with less available oxygen compared to the active burn of a puff. This incomplete combustion leads to chemical differences. For many toxic compounds, the concentration is substantially higher in undiluted sidestream smoke than in mainstream smoke. Sidestream smoke can contain up to six times more carbon monoxide and higher levels of ammonia and formaldehyde.
The physical properties also differ: mainstream smoke tends to be more acidic, with a pH ranging from 6.0 to 6.5, while sidestream smoke is more alkaline, averaging around 7.5. This higher alkalinity in sidestream smoke means it contains more unprotonated, or “free,” nicotine, which is more readily absorbed. Sidestream smoke also produces smaller particles that can penetrate deeper into the peripheral regions of the lungs.
Environmental tobacco smoke, often called secondhand smoke, is the mixture to which non-smokers are exposed. This composite is primarily made up of the diluted sidestream smoke that is released from the burning end. It is then mixed with the mainstream smoke that the smoker inhales and subsequently exhales into the air. The sidestream component typically constitutes the majority, around 85%, of the total secondhand smoke found in an enclosed space.