Chemistry of Tobacco and Tobacco Smoke

The chemistry of tobacco products and product delivery is extremely complex. To begin with, tobacco in its natural form is made up of more than 3,000 compounds. Cultivation, processing, and manufacture of the tobacco may result in significant chemical variation. A wide variety of tobacco products have been developed, including cigarettes, cigars, bidis, kreteks, dry and moist snuffs, and chewing tobaccos. In addition to differences in their physical composition, these products differ in the form and route of delivery of tobacco constituents (orally or through inhalation), subsequent absorption into the body, and health and other effects.

In the case of smoked products, the burning of the tobacco produces further changes. Cigarette smoke consists of a dynamic mixture of more than 5,000 known chemical compounds. These include both highly volatile gaseous and vapor components (called the gas phase) and larger smoke particles (the particulate phase, often referred to as tar. Some of these compounds have cancer-causing, cardiovascular, respiratory, or other negative health effects. Others may increase the addictive properties of smoking, alter behavioral patterns, or produce additional effects in the brain and central nervous system. Efforts to control both product delivery and composition have resulted in important product changes with significant implications for health and smoking behaviors. However, the function, interaction, and effects of many tobacco-delivered components are still not well understood.

Tobacco Cultivation, Processing, and Product Manufacture

Tobacco is generally distinguished by the curing method used or by the geographic region in which it is grown, each of which may result in important differences in composition, including sugar, nicotine, and nitrogen content. In flue-cured tobaccos, high heat is used to speed the curing process, during which plant starches are converted to sugars and the concentration of acids is increased. The resulting tobacco has high sugar and medium nicotine content and produces smoke that is acidic (low pH) with a light aroma. Air-cured tobaccos include both Burley and Maryland tobaccos. These tobaccos have low sugar content and produce a fuller smoke (higher pH) with more nicotine. Maryland tobacco also continues to burn for a longer period when lit, so that it is less likely to self-extinguish. Sun-cured tobaccos (sometimes called Oriental tobacco) are generally produced in a Mediterranean climate and yield mild, aromatic smoke with low nicotine. Processed tobaccos, including reconstituted tobacco sheet (combined from stems, leaves, and other scraps, along with nontobacco additives) and expanded or puffed tobacco (in which the cellular structure of the leaf is artificially expanded), are also used in cigarette construction and may significantly alter smoke yields of tar and specific smoke components.

Typically, in manufacture of cigarettes and other tobacco products, different tobaccos are blended and used in combination with additives and other design components to determine product characteristics including nicotine content, taste, sensory effects, burn rate, and tobacco or smoke composition. For example, one important aspect of product chemistry is the pH (acidity or basic nature) of the tobacco or smoke. The pH strongly influences the percent of nicotine that is available in the freebase (that is, the more highly volatile, or "unbound") form. Freebase nicotine has a greater impact on sensory nerves in the mouth and throat and facilitates more rapid absorption into the bloodstream in the case of smokeless products, such as oral snuff. It may also increase the speed of absorption from the lungs of cigarette smokers, although this has not been demonstrated experimentally in smokers. The site and rate of absorption are critical determinants of a drug's potential for addiction. Sugars, acids, and other components in the tobacco blend play a critical role in controlling the pH. Likewise, the addition of ammonia or other additives may be used to alter freebase nicotine levels.

Other aspects of product chemistry influence delivery and absorption of constituents. The size of smoke particles may determine how deeply smoke constituents may be carried into the lung. Altering the temperature at which a cigarette burns influences the types of chemical changes that occur in the burning tobacco. Product design features such as paper porosity, ventilation, filtration, and use of additives must be adjusted to control these factors. Products can also undergo chemical changes over time as they sit in storage or on store shelves. A 2001 study of smokeless tobacco demonstrated that simply by sitting on a shelf unrefrigerated for six months, products generated significantly higher levels of tobacco-specific nitrosamines (TSNAs), potent cancer-causing agents (Brunnemann , Qi, and Hoffmann). In cigarettes, more volatile components such as menthol migrate between tobacco, paper, and filter over time, potentially affecting transfer to smoke. Finally, some chemicals can directly influence how other compounds behave in the human body. For example, analysis of menthol use in cigarettes suggests that it may affect the body's absorption of other constituents, alter perception of harshness in the mouth and throat, and increase the smoker's capacity to hold smoke for longer periods within the lungs. A 1991 internal R.J. Reynolds study of smoke irritants found a significant relationship between the addition of a single compound (ethanol) to smoke and consequent smoker perception and behaviors, including reported perception of resistance as well as puff volume and other inhalation measures (Hayes et al. 1991).

Role of Chemical Analysis in Product Design

The complexity of tobacco product composition has led to the development of sophisticated mechanisms to monitor and assess product changes. The only common public measures of product chemistry used by government regulatory agencies are nicotine, tar, and carbon monoxide (a significant gas phase component of smoke). However, manufacturers routinely monitor physical product characteristics such as product weight, length, circumference, density, air ventilation, filtration efficiency, and draft, as well as chemical composition (in both tobacco and smoke) of alkaloids, sugars, ammonia, common additives, and many known toxic and carcinogenic compounds. One role of these analyses is simply to assure that key characteristics of a particular brand are maintained despite variations in growing conditions, agricultural practices, or other factors affecting the finished product. Nicotine content, moisture levels, smoke pH, and other critical blend components are all carefully controlled. This in turn enables manufacturers to minimize product variations over time, as well as across manufacturing and production plants located throughout the world.

Chemical analysis is also used internally to assess competitor products and direct product changes. For example, in the 1980s Brown & Williamson Tobacco Company undertook a series of projects to reverse-engineer the Philip Morris Marlboro brand in an attempt to characterize the factors driving its worldwide success (Wells 1995). They concluded that ammoniation of reconstituted tobacco used in Marlboro resulted in increased smoke pH and free nicotine delivery and produced unique compounds that improved smoke mildness and provided the characteristic Marlboro flavor. This analysis led Brown & Williamson to adopt ammoniation in its own cigarettes.

Chemical analysis has been instrumental in the development of product changes intended to reduce the health consequences of tobacco use. For example, cigarette filters were introduced in the 1950s to reduce harmful constituents in tobacco smoke. However, measures were necessary to determine whether these and other changes were effective in reducing the most harmful chemical compounds. In the 1960s and 1970s, the scientists Dietrich Hoffmann, Ernst Wynder, and others evaluated the smoke produced by cigarettes under different filtered conditions, measuring specific compounds commonly associated with greater health risks, such as ciliatoxic (hydrogen cyanide, acrolein) and cancer-causing agents (nitrosamines, aldehydes, PAHs). In 1989 Hoffmann produced a list of the most harmful known compounds in tobacco smoke, and this list is commonly referred to in discussions of overall product toxicity. Further efforts have been undertaken, both internally by manufacturers as well as independently, to identify means for selective filtration of these components, although with mixed success.

The role of chemical analysis has become even more important as cigarette manufacturers have sought ways to reduce tar and nicotine content while retaining flavor, impact, and other product characteristics. One of the primary areas explored by industry researchers is sensory analysis. The properties of the smoke aerosol, as well as individual smoke components, determine both the body or mouthful of the smoke, as well as the strength or impact on receptors in the mouth and throat, which are critical to consumer perception. In order to provide greater sensory character to lower tar and nicotine delivery cigarettes, industry researchers analyzed dynamics of the smoke aerosol, including the swirl of the smoke produced by different filter and ventilation configurations, and how these affect transfer of smoke particles and consumer sensory perception. Other sensory changes have included increased smoke pH, manipulation of tar/nicotine ratios, and use of additives to increase strength as well as flavor characteristics. Industry research has demonstrated that sensory characteristics may affect consumer satisfaction, inhalation patterns and smoking behaviors, and appeal among specific populations such as youth.

Health and Other Effects of Product Changes

Chemical changes to tobacco and smoke have far-reaching implications for product effects, including health, addiction, and smoking behaviors. A 1995 study of U.S. Tobacco, a smokeless tobacco manufacturer, demonstrated an intentional strategy of graduated product marketing, in which starter products with low nicotine delivery were targeted to new users. The users were then gradually encouraged through advertising and free samples toward products with increasing nicotine delivery until they had adopted Copenhagen, the most addictive product available (Connolly 1995). In a 2002 study of the youth-targeted brand Camel, internal documents revealed the importance of smoothness and mildness as factors in the adoption of cigarette smoking among youth (Wayne and Connolly 2002). Internal documents have also shown that cigarette smoke pH—and consequently free nicotine delivery—were critical factors in the success of cigarette brands such as Marlboro and Kool. Presumably, this was due to the increased sensory character and addictive effects of these products (Hurt and Robertson 1998).

Design changes made to cigarettes in the past decades have resulted in products with reduced delivery of tar and nicotine, as measured by smoking machines. Public perception of these changes is that these products offer reduced health risks. However, research has found that changes in consumer smoking behaviors as a response to the reduction in tar deliveries appear to have undermined the intended benefits of reduced smoke yields. For example, lung cancers have appeared deeper in the lung as smokers have altered behaviors by inhaling more deeply, as well as more frequently. As a result, measures of health outcomes (such as rates of lung cancer) are not reduced as would be expected in relation to product changes. In addition, chemical analysis has demonstrated that some cancer-causing agents in tobacco products have increased even as others have been reduced. For example, design changes over the last 30 years have resulted in a significant reduction in measures of benzo(a)pyrene, but correspond to an increase in the tobacco-specific nitrosamine NNK, another significant cancer-causing agent. Thus, the complexity of the product chemistry has made it much more difficult to accurately assess and control health risks.

See Also Genetic Modification; Toxins.

▌ GEOFFREY FERRIS WAYNE

BIBLIOGRAPHY

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Brunnemann, K., J. Qi, and D. Hoffmann. "Aging of Oral Moist Snuff and the Yields of Tobacco-Specific N-Nitrosamines (TSNA)." 22 June 2001. American Health Foundation, Valhalla, New York. Unpublished report.

Connolly, G. "The Marketing of Nicotine Addiction by One Oral Snuff Manufacturer." Tobacco Control 4 (1995): 73–79.

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bidis thin, hand-rolled cigarettes produced in India. Bidis are often flavored with strawberry or other fruits and are popular with teenagers.

kretek a clove cigarette, originally of Indonesian origin.

tar a residue of tobacco smoke, composed of many chemical substances that are collectively known by this term.

flue-cured tobacco also called Bright Leaf, a variety of leaf tobacco dried (or cured) in air-tight barns using artificial heat. Heat is distributed through a network of pipes, or flues, near the barn floor.

air-cured tobacco leaf tobacco that has been dried naturally without artificial heat.

snuff a form of powdered tobacco, usually flavored, either sniffed into the nose or "dipped," packed between cheek and gum. Snuff was popular in the eighteenth century but had faded to obscurity by the twentieth century.

menthol a form of alcohol imparting a mint flavor to some cigarettes.

alkaloid an organic compound made out of carbon, hydrogen, nitrogen, and sometimes oxygen. Alkaloids have potent effects on the human body. The primary alkaloid in tobacco is nicotine.

ammoniation a process of adding gaseous ammonia during cigarette manufacture to improve smoking and flavor characteristics.

ciliatoxic being harmful to the cilia of the lungs. Cilia are tiny hairlike, structures on the inner surfaces of the lung.